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Study of needle ice events at Vancouver, Canada, 1961-1968 1970

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A STUDY OF NEEDLE ICE EVENTS AT VANCOUVER, CANADA 1961 - 1968 by Samuel I. Outcalt A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY to the department of GEOGRAPHY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1970 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r equ i r emen t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag ree tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S tudy . I f u r t h e r ag ree tha t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y pu rposes may be g r an t ed by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada Date 7 %XML 70 A b s t r a c t P r e d i c t i o n o f needle i c e events r e q u i r e s an understanding o f energy and water t r a n s f e r between the atmosphere and the s o i l . A p r o j e c t i n southwestern B r i t i s h Columbia was conducted d u r i n g 1961-1968 f o r the purposes o f (1) c o n s t r u c t i n g a g e n e r a l model of needle i c e growth, (2) c h a r a c t e r i z i n g the p r o c e s s e s which combine t o produce i c e needle events, and (3) e x p l a i n i n g the v a r i a t i o n s i n i c e needle morphology. The problem was approached by (1) comparing environmental c o n d i t i o n s on event.and non-event n i g h t s , (2) i d e n t i f y i n g the h e a t s i n k s which produce s u r f a c e c o o l i n g and f u s i o n , (3) d e t e r m i n i n g the range o f the v a r i a b i l i t y i n s o i l water and heat flow p r o p e r t i e s produced by changing s o i l water content, (4) s t a t i s t i c a l l y a n a l y z i n g the event, non-event r e c o r d , and (5) d e t e r m i n i n g the time dependence o f the components o f the energy- water t r a n s f e r system d u r i n g needle i c e growth. S t a t i s t i c a l and p h y s i c a l a n a l y s i s demonstrated the o v e r r i d i n g c o n t r o l of the thermal and e v a p o r a t i v e heat s i n k s i n needle i c e growth and i n d i c a t e d t h a t the e q u i v a l e n t r a d i a n t temperature o f the n i g h t sky must drop below -15°C b e f o r e a needle i c e event i s p r o b a b l e . F u r t h e r , i t was demonstrated t h a t a f t e r i c e c r y s t a l s form a t the s u r f a c e the magnitude o f the s u r f a c e heat s i n k ( e q u i l i b r i u m s u r f a c e temperature) and the s o i l water c o n t e n t c o n t r o l the depth of the n o r m a l l y f r o z e n s o i l cap above the needles and the s p a t i a l homogeneity of needle growth. The s t a t i s t i c a l s t u d y o f t h e e v e n t r e c o r d p r o d u c e d a s i m p l e dew p o i n t - c l o u d c o v e r e m p i r i c a l model f o r e v e n t p r e d i c t i o n and a l i s t i n g o f f a v o r a b l e - u n f a v o r a b l e c o n d i t i o n s f o r n e e d l e i c e growth. A g e n e r a l model f o r n e e d l e i c e growth was d e v e l o p e d i n d i c a t i n g t h e r e l a t i o n s h i p between s u r f a c e e q u i l i b r i u m t e m p e r a t u r e and s o i l w a ter t e n s i o n on t h e growth and the s p a t i a l t e m p o r a l homogeneity o f n e e d l e growth. C o n t r i b u t i o n s t o g e n e r a l m i c r o m e t e o r o l o g y were made by d e m o n s t r a t i n g t h e u t i l i t y o f combined measurement o f s u r f a c e t e m p e r a t u r e s o i l w a t e r t e n s i o n and s o i l heave i n t h e a n a l y s i s o f s o i l s t r u c t u r a l e v o l u t i o n d u r i n g d i u r n a l f r e e z e - t h a w c y c l e s . S p e c i f i c a l l y , t h e anomalous p o s i t i v e "bump" w h i c h f r e q u e n t l y o c c u r s i n n o c t u r n a l s u r f a c e t e m p e r a t u r e c u r v e s was shown t o be c o i n c i d e n t w i t h a t h e r m a l l y d r i v e n f l u x o f warm s u b s o i l water toward t h e s u r f a c e and a sudden i n c r e a s e i n s o i l w a ter t e n s i o n was d e m o n s t r a t e d t o o c c u r a t t h e ti m e when heave ( i c e s e g r e g a t i began. F i n a l l y s p e c i f i c problems w h i c h appear b o t h t r a c t a b l e and r e w a r d i n g were f o r m u l a t e d f o r f u t u r e i n v e s t i g a t o r s . ACKNOWLEDGEMENTS The a u t h o r w i s h e s t o acknowledge t h e a s s i s t a n c e , a i d and d e t e r m i n a t i o n o f t h e members o f h i s com m i t t e e . These members are : P r o f e s s o r J . R. Mackay, Chairman (Geography) Dr. J a n D e V r i e s ( S o i l S c i e n c e ) Dr. M. Miyake (Oceanography) Dr. J . V. Ross ( G e o l o g y ) Dr. V. C. B r i n k ( P l a n t S c i e n c e ) Dr. G. R. G a t e s (Geography) Dr. W. H. Mathews ( G e o l o g y ) In a d d i t i o n v a l u a b l e c o n s u l t a t i o n was g i v e n by Dr. M. A. M e l t o n ( s t a t i s t i c a l a n a l y s i s ) , D r. P. J . W i l l i a m s ( C a n a d i a n N a t i o n a l R e s e a r c h C o u n c i l - - s o i l p h y s i c s ) , and Mr. J . W. Sayward {U.S. C o l d R e g i o n s R e s e a r c h & E n g i n e e r i n g L a b s - - s o i l p h y s i c s ) . L a s t l y M e s s r s . Don P e a r c e ( P l a n t S c i e n c e F i e l d S t a t i o n ) and Wolfram S c h m i t t ( E n g i n e e r i n g Shops) f o r v a l u a b l e a s s i s t a n c e i n e q u i p m e n t m a i n t e n a n c e and d e s i g n . The a u t h o r i s i n d e b t e d t o t h e U n i v e r s i t y and t h e N a t i o n a l R e s e a r c h C o u n c i l f o r p r o v i d i n g f u n d s f o r t h e p r o j e c t .  TABLE OF CONTENTS CHAPTER PAGE I. INTRODUCTION 1 The Problem 1 D e f i n i t i o n 1 Background 1 Statement of Problem 2 Uniqueness of this Study 2 Importance 2 Organization of Thesis 3 Review of Present Knowledge 4 Needle Ice Studies 4 Atmospheric Processes 4 S o i l Processes 5 Related Topics 6 Forecasting 6 The Choice of the Study Site 6 Summary of F i e l d Procedures 7 I I . THE GENERAL CLIMATOLOGY, NEEDLE ICE RECORD, AND SOIL TEMPERATURE DECLINE PATTERNS DURING THE NEEDLE ICE SEASON . . . . 9 Temperature and P r e c i p i t a t i o n Regimes 9 S o i l Temperature and the Needle Ice Record 10 The Surface Nocturnal Cooling Curve 15 Nocturnal Temperature Patterns 21 I I I . HEAT SOURCES AND SINKS DURING NEEDLE ICE EVENTS 34 Energy Exchange Conditions 34 Radiation Fog 42 i CHAPTER PAGE IV. A PHYSICAL INVESTIGATION OF THE STUDY SITE SOIL . . 46 P a r t i c l e Size D i s t r i b u t i o n and C l a s s i f i c a t i o n 46 S o i l Thermal Properties 47 The Theory of Ice Intrusion 51 The Low Tension Desorption Curve 54 The High Tension Desorption Curve 56 The Big Step Water Expulsion Curve 59 Rapid Expulsion Testing 61 The Unsaturated Hydraulic Conductivity 62 V. THE ANALYSIS OF THE PAST EVENT RECORD 65 The S t a t i s t i c a l Model 65 Radiosonde Data 72 VI. TIME DEPENDENT PROCESSES 77 P o l y c y c l i c and Monocyclic Needle Ice Events 77 Clear Sky Non-Events . 82 The Sequence of Time Dependent Phenomena 85 Temporal Patterns of S o i l Water Tension 88 The S o i l Cap Depth 94 The Equilibrium Temperature Model 96 The Environment of a Late Winter Needle Ice Event . . . . 100 VII. CONCLUSION 112 General Description 112 S p e c i f i c Results and Recommendations 116 The Equilibrium Temperature Model 116 S o i l Water Tension at Nucleation 116 CHAPTER PAGE S o i l Water-Temperature E f f e c t s 117 Time Dependence of the A i r Intrusion Value 117 Time Dependence of Nocturnal Energy Exchange Components 117 Energy Budgets and Process Geomorphology 118 BIBLIOGRAPHY .119 APPENDIX 125 LIST OF TABLES TABLE PAGE I I - l . Monthly Mean A i r Temperature, S o i l Temperature and P r e c i p i t a t i o n 9 II-2. Terzaghi Thermal Advance Estimates 13 II-3. The Needle Ice Event Record 15 II-4. Surface Temperature Data, 4 March 1967 17 II-5. Least Squares Testing of Brunt's Rule 20 I I I - l . Temperature and Humidity 35 III-2. Bowen Ratio Estimates 36 III-3. The Energy Exchange Environment 37 III-4. Austausch Exchange C o e f f i c i e n t Estimates 38 III-5. Austausch C o e f f i c i e n t s and Mean Wind Speeds 38 III-6. Net Radiation and Surface Temperature During the Develop- ment of Radiation Fog on 7 February 1967 43 IV-1. P a r t i c l e Size D i s t r i b u t i o n 46 IV-2. The V a r i a t i o n of S o i l Thermad Properties with S o i l Water Tension at 20°C 49 IV-3. Tension-Water Expulsion 60 IV-4. Unsaturated Hydraulic Conductivity 62 V - l . The Results of K-S Testing 66 V-2. The Results of F-Testing for Homogeneity of Variance . . . 66 V-3. The Results of the T-Test for S i g n i f i c a n t Differences Between Group Variable Means 67 V-4. Variable L i s t i n g 68 V-5. Mahalanobis D 2 Matrix, 64-65 Data 69 V-6. F-Matrix, 64-65 Data 69 TABLE PAGE V-7. C l a s s i f i c a t i o n of the 64-65 Data by Discriminant Analysis 70 V-8. C l a s s i f i c a t i o n of the 66-67 Data 72 V-9. P r e c i p i t a b l e Water (cm.) over Quillayute, Wash., and Port Hardy, B.C., at 0400 P.S.T 73 V.10. A i r Mass Data 1966-1967 75 VI-1. S o i l Heat Flux Toward the Surface, 13 December 1967 . . . 80 VI-2. S o i l Temperatures, 16 December 1967 81 VI-3. Sky Radiant Temperatures . . 84 VI-4. Sequences i n Monocyclic Events, 5-17 February 1968 . . . . 87 VI-5. S o i l Water Tension 89 VI-6. Equilibrium Surface Temperatures, 2200 P.S.T 98 VI-7. Measured Surface Temperatures, 2200 P.S.T 98 VI-8. Equilibrium and Measured Surface Temperatures, Feb. 68 . . 99 VI-9. Energy Exchange Components, 16-17 February 1968 102 VI-10. Estimated Temperatures at 5 cm., 16-17 February 1968 . . . 105 VI-11. Water Flow and S o i l Heave, 16-17 February 1968 106 VII-1. Favorable and Unfavorable Conditions for Needle Growth . . 115 LIST OF FIGURES FIGURE PAGE I - l . Map of Study Site 8 I I - l . The s o i l thermal regime under grass cover at the agrometerological s i t e 12 II-2. S o i l surface cooling, 4-5 March 1967 17 II-3. Accumulated femperature decline as a function of the square root of time, 4 March 1967 19 II-4. Surface temperature and s o i l water tension, 1-2 January 1968 22 .11-5. Surface temperature and s o i l water tension, 31 Dec. 67- 1 Jan. 68 24 II-6. Conditions during a period of variable cloud cover, 2-3 Feb. 1968 . . 25 II-7. Surface temperature traces during needle ice events . . . . 27 II-8. Surface temperature trace, 24-26 January 1968 28 II-9. Surface temperature traces on nights without needle ice events 29 11-10. Surface temperature and s o i l heat f l u x at 0.3 cm. during a night with a nearly balanced thermal r a d i a t i o n flu x , 17-18 Feb. 1968 30 11-11. S o i l temperatures at 0.3 and 3.3 cm. below the surface, 20-21 Nov. 1967 31 11-12. S o i l temperatures on a c l e a r night, 18-19 Nov. 67 32 FIGURE PAGE I I I - l . Austausch-wind speed plot 40 III-2. Conditions during the formation of r a d i a t i o n fog, 7 February 1967 44 IV-1. Schematic diagram of the Porous Plate Extractor . . . . . . . 48 IV-2. The low tension desorption curve . 55 IV-3. The high tension desorption curve 58 IV-4. The hydraulic conductivity-tension curve under isothermal conditions at 20°C 63 V - l . Regions defined by the discriminant analysis of dew point and cloud cover from the 1964-65 data . . 71 VI-1. Monocyclic and p o l y c y c l i c needle ice 79 VI-2. The study s i t e on 26 February 1968 83 VI-3. Net r a d i a t i o n , s o i l heave, surface temperature and s o i l heat f l u x at 0.3 cm. during the needle ice event of 5-6 February 1968 86 VI-4. The time dependent v a r i a t i o n of s o i l temperature isotherms and water tension, 26-27 Feb. 1968 92 VI-5. Heave and s o i l water tension, 6-8 Feb. 1968 93 VI-6. Conditions over bare s o i l at the study s i t e , February 16-17, 1968 101 VI-7. S o i l temperature isotherms, 16-17 Feb. 68 103 VI-8. S o i l heat f l u x toward the surface on 16-17 Feb. 68 . . . . 109 VI-9. The equilibrium temperature- ice i n t r u s i o n model I l l NOTATION FOR PHYSICAL ARGUMENTS ( a l l units c.g.s. unless noted i n text) A austausch c o e f f i c i e n t B net r a d i a t i o n C volumetric heat capacity E volume of expelled water F fusion rate K s v Sverdrup exchange c o e f f i c i e n t K thermal conductivity Kg von Karman constant L sensible heat f l u x P pressure S s o i l heat f l u x T temperature U v e l o c i t y V latent heat f l u x a,m regression parameters e vapor pressure h s o i l heave q s p e c i f i c humidity r c c r i t i c a l pore radius r v heat of vaporization r f heat of fusion t elapsed time x volume f r a c t i o n z distance from surface zo roughness parameter thermal d i f f u s i v i t y h temperature range fi density 0 " Stefan-Boltzman constant. & i n t e r f a c l a l energy s o i l water tension 00 angular v e l o c i t y Subscripts: a a i r 0 surface s s o i l 10 distance from surface 1 i n i t i a l value & ice or organic m mineral w water f frozen x c r i t i c a l value CHAPTER I. INTRODUCTION The Problem 2 D e f i n i t i o n . Needle i c e , v e r t i c a l filaments of ice approximately 1 mm. i n cross section and up to 8 cm. i n length, are formed by the segregation of ice near the ground surface on calm c l e a r "winter" evenings, from late October to early A p r i l , when the s o i l surface has i n i t i a l l y been unfrozen (see F r o n t i s p i e c e ) . Segregation i s the increase i n water (ice) content i n a s o i l layer produced by water migration to the freezing plane during ice formation (Jumikis, 1966). Needle ice i s ref e r r e d to i n the European l i t e r a t u r e as Haarfrost, Haareis, Nedeleis, Stengeleis, Eisfilaments, E f f l o r e s z e n e i s , Barfrost etc... In the United States i t i s usually c a l l e d mushfrost, ice filaments or ice columns. The Swedish term "pipkrake" has enjoyed wide i n t e r n a t i o n a l usage (Hamelin and Cook, 1967; T r o l l , 1944). Background. The general c l i m a t o l o g i c a l conditions, which lead to the growth of needle i c e , have been understood since the late 1930"s because of the exceptional investigations of a group of Japanese school g i r l s ( F u j i t a et. a l . , 1937), who noted the importance of the clear night sky as a major heat sink f o r the s o i l surface and recognized the necessity for unfrozen near-saturated s o i l conditions. The problem has, since that time, been studied quite extensively i n a q u a l i t a t i v e manner, but there have been only l i m i t e d attempts to treat the t o t a l environment of needle ice growth from a quantitative viewpoint (Konko-Tatutaro et. a l . , 1957; Kinosita et. a l . , 1967). 2 Statement of Problem. It i s the purpose of this thesis (1) to construct a general model for needle ice growth, (2) to gain a fundamental knowl- edge of the processes which combine to produce a needle ice event, and (3) to use this knowledge as a veh i c l e for the analysis of the morphology of needle ice growths and t h e i r associated geomorphic processes. Uniqueness of this Study. The work reported i n this study represents the most d e t a i l e d f i e l d examination of natural needle ice growth known to the author. Special attention i s devoted to the p a r t i c u l a r conditions of needle ice growth at the Point Grey Campus of the University of B r i t i s h Columbia i n Vancouver where a unique record of needle ice events has been compiled from 1962 onward. Importance of this Study. Needle ice formation and a b l a t i o n are of i n - terest to both s o i l and plant s c i e n t i s t s because of the downslope, move- ment, ground patterning, and damage to plant materials which often accompany needle ice events. In addition to being an i n t e r e s t i n g variant of the more general segregation ice ( f r o s t heave) and f r o s t warning applied research areas the t h e o r e t i c a l implications of the study are also promis- ing. Due to the r e s t r i c t e d time scale and l i m i t e d v e r t i c a l dimensions of a needle ice event, i t may be possible to gain some insight into quite s i m i l a r , but less accessible, p h y s i c a l processes which have s p a t i a l and temporal dimensions many orders of magnitude larger than needle ice deposits. It i s , therefore, considered that a d e t a i l e d i n v e s t i g a t i o n of the phenomena of needle ice growth may y i e l d numerous benefits through analogy. L a s t l y , an i n v e s t i g a t i o n of this type provides a t r i a l arena for methods of instrumentation and analysis, which can be transferred to r e l a t e d phenomena. 3 Organization of Thesis This thesis i s designed to i l l u s t r a t e new information concerning the process and environment of needle ice growth. The f i r s t chapter i n the body of the manuscript, Chapter I I . , d i s - cusses the broad pattern of the needle ice event record and climatology of the study s i t e at the University of B r i t i s h Columbia i n conjunction with the i n t e r p r e t a t i o n s o i l temperature-time curves produced during nocturnal c o o l i n g . In Chapter I I I . the nature and r e l a t i v e magnitude of the heat sources, including r a d i a t i o n fog, and sinks a v a i l a b l e to the s o i l surface during needle ice growth w i l l be examined. This information w i l l e s t a b l i s h the basic framework for further consideration of atmos- pheric processes during needle ice events. In a l i k e manner, a s i m i l a r framework i s developed for the s o i l i n Chapter IV. which reports the r e s u l t s of laboratory determinations of the p a r t i c l e size d i s t r i b u t i o n , water retention and flow properties, and the thermal c h a r a c t e r i s t i c s of the s o i l at the study s i t e . Having provided this background data, the past even record i s examined s t a t i s t i c a l l y to discover the broad environmental conditions which lead to needle ice events and to produce a model for event p r e d i c t i o n (Chapter V.). Chapter VI. i s a discussion of the p r i n c i p a l r e s u l t s of f i e l d obser- vations c a r r i e d out during the needle ice season of 1967-1968. In this chapter some of the components of the needle ice growth system are ex- amined i n d e t a i l and a general model for needle ice growth i s developed. The work i n t h i s chapter leads d i r e c t l y to the conclusion and recommenda- tions which are the f i n a l items i n the body of the t h e s i s . 4 Review of Present Knowledge Needle Ice Studies. As stated e a r l i e r , the main facts of needle ice growth were demonstrated by a group of Japanese school g i r l s ( F u j i t a et. a l . , 1937). These basic facts of needle ice growth are as follows: (1) the water source for the ice needles i s the s o i l ; (2) the major heat sink during s o i l freezing i s the c l e a r - c o l d night sky; (3) the rate of needle growth i s c o n t r o l l e d by the a v a i l a b i l i t y of water at the freezing plane; (4) steep s o i l temperature gradients increase growth rates; (5) the most susceptible s o i l s are loosely packed low density members of the loam group. The most recent and comprehensive treatment of the needle ice problem i s that of Sayward (1966), who grew needle ice i n the laboratory to test the heave s u s c e p t i b i l i t y of various s o i l s and the influence of additives on the ice segregation process. This work i s of considerable i n t e r e s t because f i e l d observation and small scale freezing tests were demonstrated to be the most r e l i a b l e methods of t e s t i n g the needle ice s u s c e p t i b i l i t y of s o i l s . These authors indicate that the following sequence i s necessary for a needle ice event: (1) s o i l surface unfrozen; (2) the formation of an ade- quate heat sink to cool the s o i l surface; (3) under-cooling the s o i l sur- face to the nucleation temperature of the s o i l water; (4) the s t a b i l i z a t i o n of the freezing plane near the s o i l surface as a r e s u l t of ice segregation. Atmospheric Processes. The f i r s t three items are e s s e n t i a l l y c l i m a t i c a l l y c o n t r o l l e d conditions which have been studied under such headings as "energy exchange during the night hours a t . . . " or "nocturnal energy balance 5 at...etc. These studies of nocturnal energy budget, during periods of various i n t e n s i t i e s of r a d i a t i o n cooling, are reported by Geiger (1965) and Sutton (1953); however, none specify simultaneous needle ice growth or the absence of i t . For this reason, the measurement of the energy transfer environment during needle ice growth as a means of lo c a t i n g and estimating the r e l a t i v e magnitude of heat sources and sinks was given a high p r i o r i t y i n the author's f i e l d program. S p e c i f i c investigations of r a d i a t i o n fog, a feedback mechanism which l i m i t s the strength of r a d i a t i o n cooling, have been c a r r i e d out experiment- a l l y and t h e o r e t i c a l l y by Davis (1957) i n the eastern United States and empirically by Belhouse (1961) at Vancouver International A i r p o r t . Sutton (1953) reported numerous physical and empirical models for the p r e d i c t i o n of night minimum temperatures. S o i l Processes. The fourth item i s e s s e n t i a l l y the formation and s t a b i l i - z ation of an active plane of ice segregation near the s o i l surface. The mechanism of ice segregation has been studied by numerous investigators both i n theory (Takagi, 1965) and experimentally (Williams, 1968). An evaluation of the e f f e c t s of s o i l texture on ice structure was c a r r i e d out by Byrnes (1951) and a study of the influence of the freezing rate on f r o s t action reported by Penner (1960). Gradwell (1963) has recently completed a study of nocturnal heat losses from s o i l s having varied bulk d e n s i t i e s . An excellent review of current thinking and research on the problem was presented by Penner (1960). Williams (1966a) demonstrated that s o i l pore geometry and water tension at the freezing plane w i l l determine i f ice se g r e g a t i o n , ( s t a b i l i z a t i o n of the freezing plane) or normal freezing, (continued descent of the freezing plane) s h a l l occur at a given l e v e l i n 6 the s o i l . For t h i s reason the measurement of s o i l water tension was i n - cluded i n the author's f i e l d program. Related Topics. Other topics such as the crystallography of needle ice (Steinemann, 1953) and the possible role of needle ice i n the martian wave of darkening (Otterman and Browner, 1966) have been under study i n recent years. Records of the geomorphic e f f e c t s of needle ice growth i n New Zealand have been compiled by Gradwell (1960,1954,1955) and Soons (1967). The e f f e c t s of needle ice formation on a g r i c u l t u r a l lands i n southwestern B r i t i s h Columbia has been treated by Brink et. a l . (1967). Forecasting. The only paper which considers the forecasting of needle ice events was that of Konko-Tatutaro et. a l . (1967), which evaluated a i r and s o i l temperature patterns p r i o r to needle ice formation i n Japan. For this reason the author assigned a high p r i o r i t y to an attempt to develop and test a s t a t i s t i c a l model for event p r e d i c t i o n using the unique needle ice event record which was available to him at the University of B r i t i s h Columbia. The Choice of the Study Site Because of the past h i s t o r y (Brink et. a l . , 1967) of needle ice inves- tigations at the u n i v e r s i t y agrometeorological s i t e , a secondary weather s t a t i o n gathering data for a g r i c u l t u r a l users, a bare f i e l d (winter) ad- jacent to the meteorological l o t was designated as the major study s i t e for t h i s i n v e s t i g a t i o n (see map, F i g . I - l . ) . The author's recorders and support equipment were stored i n the Apiary and e l e c t r i c a l lines run to the sensors at the study s i t e . Regular agrometeorological data signals from the meteorological l o t are recorded and sampled on the second f l o o r of the Feed Bui l d i n g . 7 Summary of F i e l d Procedures The study of the past event record led the author to recognize the necessity for experimental work on needle ice growth. I n i t i a l l y small scale freezing tests were attempted i n a large home freezing unit but numerous d i f f i c u l t i e s were encountered i n attempts to f a i t h f u l l y duplicate natural boundary conditions. It was soon evident that a d d i t i o n a l data must come from the f i e l d measurement of s o i l and atmospheric conditions during natural needle ice events. Thus, the recording equipment, de- scribed i n the appendix, was set i n operation at the study s i t e when either the weather o f f i c e was p r e d i c t i n g a sub-freezing night minimum or the author's s t a t i s t i c a l model indicated that i n t e r e s t i n g r e s u l t s might be obtained (near events e t c . . . ) . A considerable amount of the f i e l d instrumentation was of a prototype nature and thus, somewhat u n r e l i a b l e . Even the commercially manufactured equipment was d e l i c a t e and subject to frequent malfunctions. For these reasons i t was d i f f i c u l t to obtain complete data sets for any single event. However, using a knowledge of instrument response to natural processes i t was possible to gain an adequate picture of an event even with malfunctions and l i m i t e d recorders. Once the major d e t a i l s of the time dependent pro- cesses were known, the f i e l d experiments (reported i n Chapter VI.) were designed to explore s p e c i f i c r e l a t i o n s h i p s . Thus, i n the 1967-1968 season i t was possible to concentrate on the measurement of the s o i l water tension- heave-soil temperature variable set and gain increasing insight into the conditions which determine the course of events, needle ice or normal freezing, on a f r o s t night. F i g . I - l , Map of study s i t e . CHAPTER II. THE GENERAL CLIMATOLOGY, NEEDLE ICE EVENT RECORD, AND SOIL TEMPERATURE DECLINE PATTERNS DURING THE NEEDLE ICE SEASON The purpose of this chapter i s to f a m i l i a r i z e the reader with the general climatology, and the needle ice event record at the study s i t e . In addition, nocturnal temperature decline patterns w i l l be examined to set the stage f o r the further analysis of needle ice events. The Temperature and P r e c i p i t a t i o n Regimes Monthly means of a i r temperature and p r e c i p i t a t i o n were calculated for the 1961-65 i n t e r v a l from the Agrometeorological Data Summaries. These data are presented below i n tabular form. TABLE II-1. MONTHLY MEAN AIR TEMPERATURE, SOIL TEMPERATURE AND PRECIPITATION Month A i r Temp.(°C.) Precip.(mm.) S o i l Temp, at 1 cm.(°C.) January 3.3 167 3.2 February 5.7 160 5.6 March 5.8 94 7.2 A p r i l 8.7 69 11.6 May 10.9 57 15.6 June 15.0 35 20.5 10 TABLE I I - l . (continued) Month A i r Temp.(°C.) Precip.(mm.) S o i l Temp, at July 17.0 45 21.9 August 17.1 63 20.9 September 14.2 59 16.9 October 10.9 145 12.3 November 6.7 176 7.0 December 3.9 195 4.1 It should be mentioned that the agrometeorological s i t e was moved a quarter of a mile during the time i n t e r v a l . However, as the general trend i s of int e r e s t here, no attempt was made to correct these data for the s i t e change. The main point i s that the cold season i s also the wet season at Vancouver. This i s extremely important for needle ice growth since this condition indicates that, i f winter season nocturnal cooling can bring the s o i l surface to the ice nucleation point, the s o i l may contain adequate water for the ice needle growth. S o i l Temperature and the Needle Ice Record The s o i l layer within 2 centimeters of the surface i s of sp e c i a l i n - terest i n investigations of needle i c e . Three general conditions must be f u l f i l l e d , i f one i s to expect needle ice growth. The near surface layer must be unfrozen and near f i e l d capacity, and the s o i l surface temperature must be within the nocturnal cooling range of ice nucleation tempera- ture (approximately -2°G.). At this time the s o i l temperature at the 1 centimeter depth w i l l be considered (see Table I I - l . ) . These measure- ments are not t o t a l l y representative of conditions at the study s i t e as 11 the probes i n the meteorological l o t are under grass cover. Note that during the winter months the mean temperature of s o i l near the surface i s within 10°Celsius of the ice point. The mean monthly s o i l temperatures are plo t t e d as functions of depth and time i n Fig.II-1. to i l l u s t r a t e two features of the s o i l thermal environment at the study s i t e . The f i r s t being the greater r e l a t i v e i n - tensi t y of the summer heat wave i n comparison to the winter cold wave. This difference i s due to the increased winter cloud cover which produces a "continental type" summer and a "maritime type" winter thermal regime at Vancouver. The second feature i l l u s t r a t e d i n Fig.II-1. i s the heat reservoir that i s available to the s o i l surface during the infrequent winter cold- c l e a r s p e l l s . R e c all that the mean monthly s o i l thermal data are i l l u s - trated here and that the mean winter day i s overcast and r e l a t i v e l y warm i n comparison with col d - c l e a r s p e l l s . Thus, the cold- c l e a r days during which needle ice events occur are days with unusually steep thermal gradients between the heat res e r v o i r at depth and the surface, which i s subjected to a large diurnal r a d i a t i o n f l u x range under these conditions. The thermal d i f f u s i v i t y of the s o i l at the study s i t e was estimated from the annual amplitude of s o i l temperature at depths of 1, 10, 20, 50, 100, and 150 centimeters. This was done using the following r e l a t i o n s h i p ( S e l l e r s , 1965). Eq.II-1. a = UL f Z r Z 2 f 2 [ln.(8,/S 2) J  13 These computations indicate a mean annual thermal d i f f u s i v i t y of approxi- -4 2 —1 mately 45x10 cmt.sec'V'C Iheicomputed diffusiMtyLiMisimilaihtto/.tthe.flahoratory test values which were obtained by a r t i f i c i a l heat pulse t r i a l s (see Chapter IV.)- The thermal d i f f u s i v i t y of the s o i l can be employed using Equation II-2. to estimate the rate at which a thermal disturbance (.e.-.g., the e f f e c t s of a winter col d - c l e a r s p e l l ) w i l l advance through the s o i l . (Terzaghi, 1952). Eq.II-2. z = y i 2 a t Using the e a r l i e r estimate of s o i l thermal d i f f u s i v i t y (45x10 ^'crn^sec ^.) the r e s u l t s are as follows. TABLE II-2. TERZAGHI THERMAL ADVANCE ESTIMATES length of disturbance penetration depth (hours) (cm.) 1/2 10 2 20 133 50 52 100 115 (5 days) 150 These data indicate that i t would be approximately f i v e days before a thermal disturbance could reach the 1.5 meter l e v e l . Thus, during the Vancouver winter, cold-clear weather s p e l l s of less than a week would be expected to create rather steep temperature gradients near the surface. This condition i s known to favor needle ice formation (see Chapter I.) and may produce a thermally driven water flow directed toward the surface 14 (see Chapter VI.)» The cloudy-rainy winter weather i s a major factor i n the growth of needle ice during the infrequent col d - c l e a r s p e l l s . The normally cloudy-rainy weather keeps the s o i l unfrozen, by l i m i t i n g noc- turnal r a d i a t i o n cooling, and maintains the s o i l water content near f i e l d capacity, which by d e f i n i t i o n occurs a f t e r three days of i n f i l t r a t i o n following a soaking r a i n . Droughts of three consecutive days are i n f r e - quent during the Vancouver winter. The record of needle ice events at the s i t e i s presented i n Table II-3. These data were c o l l e c t e d by the observer, Mr. Don Pearce, at the agrometeorological s i t e during the morning (0815 P a c i f i c Std. Time (P.S.T.)) observation series and include both needle ice and frozen ground events. The monthly mean number of events and the standard deviations from these means are also presented using integer values. It should be remembered that there i s a tendency for such a record to underestimate the number of annual events due to the early morning ablation of short needles and night melting following increasing cloud cover. There are also cases on record (December 1967) when the normally frozen s o i l layer above p o l y c y c l i c needle ice would support a man on foot giving no hint of the needles below. 15 TABLE II-3. . THE NEEDLE ICE EVENT RECORD Oct. Nov. Dec. Jan. Feb. Mar. Total 1961-62 1 10 9 1 10 6 37 1962-63 0 1 6 19 0 8 34 1963-64 0 3 8 2 14 1 28 1964-65 0 4 6 0 5 7 22 1965-66 0 2 3 2 4 4 15 1966-67 0 2 2 2 7 6 19 Mo. Mean 0 4 6 4 7 5 Std. Dev. 1 3 3 7 5 3 The pattern i s one of a general high event frequency i n December and February with lower frequencies i n October, November, March and January. The standard deviation of the monthly values peakes i n January. This may r e s u l t from the infrequent outbursts of A r c t i c a i r which freezes the ground to depth on some occasions and produces snow cover, preventing needle ice growth or discovery, at other times. The event season i s l i m i t e d by the s o i l temperature r i s i n g above the nocturnal cooling range to the nucleation point. The Surface Nocturnal Cooling Curve A necessary condition for needle ice formation i s ice nucleation which occurs a f t e r the s o i l surface has been undercooled to approximately 2° Celsius below the freezing point. Thus, the time dependence of surface temperature during nocturnal r a d i a t i o n cooling i s a major factor i n determining the i n t e n s i t y of needle ice events. A test case w i l l be employed to i l l u s t r a t e the general form of the cooling curve. 16 During the early evening hours of 4 March 1967 net radi a t i o n , surface temperature and s o i l heat flux at the 2 centimeter depth were recorded i n the Apiary at the study s i t e . The portion of the temperature-time trace during the period before nucleation was of in t e r e s t as the decline rate provided a test of Brunt's parabolic model for r a d i a t i o n cooling (Brunt, 1934). The formula i s as follows. Eq. II-3. * i l a T- ̂  2 Bp f t iTT CsJCx s *Note the heat f l u x sign convention: (+•) toward the surface, (-). away from the surface. The temperature time data gathered on th i s night are presented i n Table II-4. and plotted as Figure II-2. TABLE II-4. SURFACE TEMPERATURE DATA, 4 MARCH 1967 Time ( P a c i f i c Std.) Surf. Temp (°c.) (Ti-T)o t (10 3 sec.) t (10 1 sec 1700 9.4 0.0 0.0 0.0 1800 4.9 4.5 3.6 6.0 1900 2.8 6.6 7.2 8.5 2000 2.0 7.5 10.8 10.4 2100 1.0 8.4 14.4 12.0 2200 0.0 9.4 18.0 13.4 17 Fig 0II " 2 o S o i l surface cooling ? 4°5 March 19670 (a°b) Brunt cooling s (b) nucleation 9 (b=c) undercooling, (c) s o i l water freezing pto S (c=d) needle i c e growths, (d~e) warming by beam radiation© 18 During the measurement series the mean net r a d i a t i o n f l u x was -.100 l y . minT''" and the mean value of the s o i l heat f l u x at the 2 c e n t i - meter depth was .037 l y . min.''". A plo t of the r e l a t i o n s h i p of the temper- ature decline to the square root of elapsed time i s presented as Fig.II-3. The standard error of th i s graphical s o l u t i o n was approximately one-half of a degree Celsius or about the probable p r e c i s i o n of the temperature determination. This r e s u l t indicates that the Brunt model f i t s natural conditions on one occasion,but further v e r i f i c a t i o n i s necessary. The r u l e states that on r e l a t i v e l y calm nights with stable cloud cover the surface cooling should be proportional to the square root of the elapsed time since the s o i l heat f l u x became directed toward the surface. A further test of the general v a l i d i t y of the rule, using data from a twelve night period (5-17 February 1968) when the sky was known to be cle a r , was c a r r i e d out. Data taken at the study s i t e indicated that the s o i l heat f l u x becomes directed toward the surface at approximately 1600 hours P a c i f i c Standard Time (P.S.T.) during t h i s observation period. Therefore, that time was selected as the i n i t i a l time used i n the calcu- l a t i o n s . Points (temperature i n °C. and time i n hours) along the decline curve were used to determine the degree to which the measured surface temperatures f i t t e d Brunt's r u l e . The method of curve f i t t i n g was that of least squares i n which a l i n e a r equation of best f i t was calculated from the observed data. The equation, an empirical form of the rule, has the following form: Eq.II-4. (Ti-T) = a+ mjt 19 0 20 40 60 80 100 120 140 y/~T (sec.)1/2 FigoII~3° Accumulated temperature decline as a function of the square root of time 9 4 March 1967c The l i n e a r equation i s (T^-T) = o073 t o 20 The regression c o e f f i c i e n t (a) i s t r i v i a l as i t w i l l l i e somewhere i n the region of the 1600 hours temperature but the c o e f f i c i e n t (m) indicates the "steepness" of the temperature decline and i s thus an in d i c a t i o n of the e f f i c i e n c y of the cooling environment. Points at one hour i n t e r v a l s between 1600 P.S.T. and the time of nucleation were used i n the analysis. The c o e f f i c i e n t s of determination were high i n a l l cases i n d i c a t i n g that between 97 and 1007o of the v a r i a t i o n i n temperature could be treated as square root of time dependent. However, the standard error of the regression l i n e y i e l d s a se n s i t i v e i n d i c a t i o n of the "goodness of f i t " i n each ease and, thus, offers a test of the rule's v a l i d i t y . These re s u l t s are presented i n Table I I - 5 . TABLE I I - 5 . LEAST SQUARES TESTING OF BRUNT'S RULE' Date, Feb. 1968 Slope (m) of Equation Std. Dev.(°C.) 5-6 - 2 . 7 1 0.59 6-7 - 2 . 5 7 0 .36 7-8 - 3 . 8 8 0.37 8-9 - 3 . 8 6 0 .38 9'rlO - 3 . 1 7 0 .44 10-11 - 3 . 1 1 0 .63 11-12 - 3 . 8 2 0 .38 12-13 - 4 . 5 1 0 .52 13-14 - 3 . 8 4 0 .83 14-15 - 4 . 5 4 0 .61 15-16 - 4 . 6 8 0 .86 16-17 - 5 . 0 0 0 .14 21 Note that the mean value of the standard deviations was 0.543.0.17 °Celsius. This r e s u l t indicates the general v a l i d i t y of Brunt's rule during c l e a r weather. Some p o t e n t i a l sources of v a r i a t i o n s i n standard deviation and curve slope (m) are r a d i a t i o n fog formation, changing wind speed ( l e v e l of turbulent exchange), dew formation, and s o i l water migra- t i o n toward the surface (see Chapter VI.). These r e s u l t s also indicate a wide range of surface cooling rates even during r e l a t i v e l y calm-clear To- nights . Unfortunately the occurrence of weather conditions with the "sta- b i l i t y " of the data reported above i s infrequent as w i l l be demonstrated by the examination of several temperature traces i n the next section of t h i s chapter. Nocturnal Temperature Patterns As noted e a r l i e r , the major environmental condition i n i t i a t i n g a needle ice event i s the cooling of an unfrozen s o i l surface to a temper- ature below the freezing point of the s o i l water to the ice nucleation temperature. After nucleation the growth of needle ice w i l l depend upon s o i l water supply and the maintenance of heat sinks above the s o i l surface. A t y p i c a l trace of surface temperature and s o i l water tension (see Chap- ters IV. and VI.) for the needle ice event of 1-2 January 1968 i s presented i n Figure II-4. Note that shortly a f t e r nucleation the increasing s o i l water tension, measured i n the 4-6 cm. depth range, indicates that migra- t i o n to the freezing plane (i c e segregation, needle ice growth) has started. The question i s then: In general what weather conditions favor r e l a t i v e l y 12 10 8 6 4 2 0 ( / _ A — — . . 1 \ 10 - 2 5 mm. n - ice. obs / \ nucleot ion / r i , 100 80 60 4 0 0> X , to - 2 1200 1600 2 0 0 0 0 0 0 0 0 4 0 0 FigoII=4a Surface temperature (T 0) and s o i l water tension {Xs)9 see chapter IV.y 1=2 January 1968e 0800 1200 t (P.S.T.) 23 rapid nocturnal temperature declines to sub-freezing levels where f i r s t nucleation and l a t e r freezing and segregation (needle ice growth) are possible? Some insig h t into the physics of the temperature decline can be gained through the examination of s o i l surface temperature traces on nights which produced needle ice i n comparison with nights when events did not occur. An i n s t r u c t i v e trace i s that of the "New Years" event of 1968 shown i n Figure II-5. It w i l l be noted that on that evening the temperature decline during the early hours was s l i g h t and that the surface temperature o s c i l l a t e d between 334°Celsius u n t i l s h o r t l y a f t e r midnight when a steep decline began. The i n i t i a t i o n of the temperature decline coincided with c l e a r i n g skies and the decreasing l e v e l of thermal r a d i a t i o n from the sky hemisphere which would r e s u l t from the decreasing cloud cover. The s o i l water tension trace shows a sudden jump at approximately 0730 P.S.T. which may indicate a b r i e f period of ice segregation, although no needle ice was observed during a v i s i t to the s i t e just before noon. Further evidence of the rapid response of the s o i l surface temperature and heat f l u x (disc at approximately 3mm., see appendix) to changes i n the thermal r a d i a t i o n f l u x produced by c l e a r i n g skies i s i l l u s t r a t e d on the trace of 2-3 February 1968 when no event occurred. The passage of a "clear spot" i n the sky produced arrapid temperature drop just p r i o r to 0400 P.S.T. (see Figure II-6.). Before this drop occurred the s o i l heat f l u x was near zero ( a c t u a l l y a small f l u x directed toward the surface) i n d i c a t - ing that the turbulent exchange was nearly balancing the thermal r a d i a t i o n loss at the surface. The protracted shallow decline of surface tempera-  E co -40 - 20 0 20 8 S 6 1200 / V w s 7 Y ) V" B J l • \ clear j —̂ 1600 40 20 " i _ >» 0 6 CD -20 -40 -60 2000 0000 0400 0800 t(RS.T) FigsII»6e Conditions during a period of variable cloud cover s 2-3 Feb.1968. The s o i l heat f l u x (s) and net radiation (B) are plotted with .surface temperature„ to 26 cure i n the period between 2000 and 0300 P.S.T. supports this i n t e r p r e t a - t i o n . Three surface temperature traces during needle ice events i n February 1968 are presented i n Fig.II-7. to demonstrate the persistence of the parabolic decline form during c l e a r and p a r t l y cloudy periods and to i l l u s t r a t e the e f f e c t of variable cloud cover and/or turbulent exchange rates on the late evening portion of the 15-16 February 1968 trace. The surface temperature conditions during the period from the 2 4 t h through the 25 of January 1968 are shown i n Fig.II-8. to demonstrate conditions which nearly produced needle ice events. Three addi t i o n a l traces are presented i n Fig.II-9. for periods when no events occurred. Of these traces, that of surface temperature on 30-31 December 1967 appears to have been the r e s u l t of dense cloud cover conditions whereas the traces of 15-16 January 1968 and 16-17 January 1968 seem to r e f l e c t c l e a r or p a r t l y cloudy conditions i n the l a t e afternoon followed by variable c l o u d i - ness and/or turbulent f l u x o s c i l l a t i o n i n the l a t e r portion of the night, (see Chapter I I I . ) . The e f f e c t of variable turbulent f l u x , presumably due to wind v e l o c i t y v a r i a t i o n , i s demonstrated by the s o i l surface temperature and heat f l u x record of 17-18 February 1968 (see Fig.11-10.). It should be noted that the net r a d i a t i o n balance during the 1430-0730 P.S.T. period was zero and thus, the general temperature r i s e i n the i n t e r v a l must be due to increasing turbulent energy input, mainly a sensible heat f l u x from the a i r toward the ground surface. This e f f e c t would l i k e l y r e s u l t from increasing wind v e l o c i t i e s . \\ \\ \ /> , // / il \ <'' / f I 1 / 'i 1 r 1 1 • \ \ \ 1 / / ! / A T \ / — > > C y ^ z — 1 \ ii \ > ^ / 'i i 1 2 0 0 1 6 0 0 2 0 0 0 0 0 0 0 0 4 0 0 0 8 0 0 1 2 0 0 t(RS.T.) Fig©II°7o Surface temperature traces during needle i c e events 0 7=8 Febe 19689 13=14 Feb* 1968 (both clear) 15=16 Febo 1968 ( p a r t l y cloudy) ^ l-°6 - 2 f\ nucleation Y J j^*N mm. ice c m s t 1 Obs. . 1 1 no ic e obs| 1 1200 1800 0000 0600 1200 1800 0000 0 6 0 0 1200 t ( R S . T . ) Figei:!>8o Surface temperature trace 9 24=26 January 1968 0 CO \ \ • < \ \ * \.\ J — ^ , J — I 1600 2000 0000 0400 0800 HRS.T.) F i g 0 I I - 9 . Surface temperature traces on nights without needle i c e eventso 30-31 Dec«67» • 15-16 Jan 368 s 16=17 Jan„68. 21 1 1 1 1 1600 2000 0000 0400 0800 t (P.S.T.) F i g o I I ^ l O o Surface temperature and s o i l heat f l u x at 0 o 3 oma during a night with a nearly balanced thermal radiation f l u x , 1 7 ° l 8 Febol968 0 6 / N />.' V ^ — 3 . 3 c m . c m . ^ ^ _ s 2000 0000 0400 0800 HP.S.T.) F i g o I I - l l e S o i l temperatures at Q°3 and 3o3 cm» below the surfaces,;20=21 Mov01967o 10 8 6 4 2 \ \\ / 3.3 cm. // / / 0.3 cm. ^ «... J i 1 1 • ' 0 4 0 0 0800 t( P.S.T,) 1600 2000 0 0 0 0 F i S o I I ° 1 2 o S o i l temperatures on a clear night 9 18=19 Nov067 33 The record of 20-21 November 19b7 i s presented i n Fig.11-11. to demonstrate the termination of a"temperature decline due to increasing cloud-cover. At 0430 P.S.T. on November 21 cloud cover from the west was moving over a r e s i d e n t i a l area s i x miles east of the study s i t e at a time when f r o s t had already formed on the l o c a l roof tops. F i n a l l y , the record of s o i l temperature at two levels on 18-19 November 1967 i s included as Fig.11-12. to i l l u s t r a t e the l i m i t i n g e f f e c t s of an i n i t i a l l y high s o i l temperature under c l e a r conditions and to show the curious bulge i n the temperature decline curves which occurred at about 2300 P.S.T. This " b l i p " i n the curves i s a common occurrence i n tempera- ture decline curves and w i l l be discussed i n Chapter VI. In summary, i t would appear that the v a r i a t i o n i n cloud cover has a primary r o l e i n l i m i t i n g the frequency of needle ice events at Vancouver, whereas turbulent exchange e f f e c t s are secondary. The question of how the components of the surface energy exchange mechanism operate on needle ice nights w i l l be explored i n the next chapter. A second point of in t e r e s t i s the v a r i f i c a t i o n of the u t i l i t y of the parabolic cooling curve predicted by the Brunt model. L a s t l y , Vancouver's winter cloudy-rainy weather pattern, broken by infrequent clear-cold-dry atmospheric conditions, was demonstrated as the major seasonal c l i m a t i c factor leading to needle ice growth. 34 CHAPTER I I I . HEAT SOURCES AND SINKS DURING NEEDLE ICE EVENTS In Chapter I I . the nature of the nocturnal temperature decline curve was discussed as a major element i n the growth of needle i c e . In t h i s chapter the magnitude of the heat sources, including r a d i a t i o n fog, and sinks which control the rate of nocturnal surface cooling and needle ice growth w i l l be examined. Energy Exchange Conditions During four nights i n March 1967 net r a d i a t i o n and s o i l heat f l u x were recorded at the study s i t e . In addition, s o i l surface temperature and a i r temperature 46 centimeters above the surface were also recorded (further information on instrumentation and measurement i s included as an appendix). Measurements of two of the energy exchange components, net r a d i a t i o n (B) and s o i l heat f l u x (S), were available from the recorder traces. The problem was then to estimate the values of the other two components of surface energy transfer, evaporation-condensation (V) and the sensible heat f l u x ( L ) . The method used i n the c a l c u l a t i o n of these values was the Bowen r a t i o (Geiger, 1965). This r a t i o i s as follows: E q . I I I - l . (L/V)= 0.65 [(To-T a) / (So-g a)] where: temperature (T,°C.) and vapor pressure (e, mb.). The data required for computing the Bowen r a t i o were obtained from s l i n g psychrometer p r o f i l e measurements. The following data was taken 35 during c l e a r - c o l d evenings. TABLE I I I - l . TEMPERATURE .AND HUMIDITY Relative Vapor Dew Time Height Humidity Pressure Point Temp. 1967 (P.S.T.) (cm.) a) (mb.) (°c.) (°c.) 2 March, 2300-2330 132 75 5.68 -1.3 3.1 46 80 5.80 -0.6 2.4 0 (100) (5.91) (-0.5) -0.5 3 March, 1900-1930 132 79 6.98 1.7 5.2 46 86 7.76 3.5 5.5 0 (100) (7.24) (2.4) 2.4* 12 March, 1900-1930 132 78 6.10 -0.1 3.4 46 82 6.26 0.3 3.3 0 (100) (6.67) (1.2) 1.2 24 March, 2000-2130 150 88 6.67 2.1 3.0 15 95 7.17 1.9 2.4 0 (•100) (6.16) (0.1) (0.1)* * The s o i l surface 1 temperature i s below the dew point of the middle measurement l e v e l . ( ) Saturation vapor pressure at surface temperature i s assumed. In a l l p r o f i l e s , the r e l a t i v e humidity and vapor pressure increase as the s o i l surface i s approached. There i s also a temperature inversion between the top l e v e l and the s o i l surface i n a l l the p r o f i l e s . On the 3 March p r o f i l e the inversion appears quite shallow as the i n f l e c t i o n point is located between the s o i l surface and the upper measurement l e v e l . F i e l d observations of s o i l water tension and t r i a l water balance computations (Thornthwaite and Mather, 1957) indicate that the near surface layers of the study s i t e s o i l are normally within the f i e l d capacity range of s o i l water tensions, 100-500 cm. of water, during the needle ice season. During 36 most of this period the s o i l surface may approximate a free water surface. Thus, saturation vapor pressure i s assumed at the surface. The Bowen r a t i o was calculated using three measurement i n t e r v a l combinations; upper- surface, middle-surface and upper-middle. The r e s u l t s are as follows: TABLE III-2. BOWEN RATIO ESTIMATES Measurement Period Computation Estimate of 1967 (P.S.T.) Levels (cm.) Bowen Ratio 2 March/2300-2330 0,132 -10.2 0, 46 -17.2 46,132 - 3.8 3 March/1900-1930 0,132 -V7.i0 0, 46 3.9 46,132 - 0.3* 12 March/1900-1930 0,132 - 2.5 0, 46 - 3.3 46,132 - 0.4* 24 March/2000-2130 0,150 3.7 0, 15 1.5 15,150 - 0.8* The method of the Bowen r a t i o i s known to break down as the r a t i o approaches a value of minus one; from either d i r e c t i o n ( S e l l e r s , 1965). For t h i s reason a l l Bowen r a t i o estimates between minus two and zero are considered u n f i t for further use i n computation. These are indicated by an a s t e r i s k (*) i n the table above. Notice that those " u n f i t " values occur i n three out of four cases when the upper-middle measurement levels are used i n the computation. It was, therefore, decided to continue using only the upper-surface and middle-surface estimates of the Bowen r a t i o i n computing the values of the sensible (L) and evaporation-condensation (V) 37 heat fluxes using the method described i n Geiger (1965). From the law of energy conservation i t i s known that; Eq.III-2. S + B + L + V * 0 As (L/V) has been calculated and (S) and (Bj are recorded, i t i s possible to estimate the magnitudes of (L) and (V). Eq.III-3. v = - ( S + B ) [1.+ (L/V)] Eq.III-4. T - - (L/V) ' (S+B) ~ [1. + (L/V)] The r e s u l t s of these c a l c u l a t i o n s are presented i n Table III-3. TABLE III-3. THE ENERGY EXCHANGE ENVIRONMENT (two l e v e l means) 2 March 67 3 March 67 12 March 67 24 March 67 2300-2330 1900-1930 1900-1930 2000-2130 B (mly./min.) -105 -105 -110 - 92 S (" ") 55 50 500 55 L (" ") 53 49 93 26 V (" ") - 03 06 - 33 11 Wind Speed (meters/sec.) 0.6 1.3 2.9 0.7 Cloud Cover 0/10 0/10 0/10 3/10 Next Morning (0700-0730 P.S.T.): Cloud Cover 0/10 0/10 1/10 10/10 Needle Ice ( c m . ) l 1/2 1 none Note: The best needle ice growth occurred on nights without dew. 38 Afte r c a l c u l a t i n g the sensible and latent heat flows by the Bowen r a t i o i t was possible to compute the austausch exchange c o e f f i c i e n t (the product of a i r density and eddy d i f f u s i v i t y , see Eq.III-10, I I I - l l . ) for the measurement in t e r v a l s at two l e v e l s . The re s u l t s are as follows: TABLE III-4. AUSTAUSCH EXCHANGE COEFFICIENT ESTIMATES Austausch Coeff. Date Time (P.S.T.) Level (cm.) (g. cmT secT ) 2 March 67/2300-2330 23 .093 66 .147 3 March 67/1900-1930 23 .040 66 .208 12 March 67/1900-1930 23 .122 66 .408 24 March 67/2000-2130 08 .016 75 .104 During the four measurement periods i t was possible to compute austausch c o e f f i c i e n t s at two l e v e l s . The c o e f f i c i e n t value midway between the surface and instrument shelter l e v e l (1.2 meters) i s valuable when using standard weather screen data. These values at a height of 60 cm. were determined graphically by p l o t t i n g log height against log austausch as demonstrated by Geiger (1965). Mean wind v e l o c i t i e s and austausch values are tabulated i n Table III-5. TABLE III-5. AUSTAUSCH COEFFICIENTS AND MEAN WIND SPEEDS A . n f e . c m : 1 ^ : 1 ^ M e a r l W i n d S P e e d Test Period (meters/sec. ) 2 March 67/2300-2330 .140 0.58 3 March 67/1900-1930 .180 1.35 12 March 67/1900-1930 .385 2.91 24 March 67/2000-2130 .087 0.67 39 The AgQ c o e f f i c i e n t s calculated from the Bowen r a t i o estimates were plotted i n Figure I I I - l . The four values and the o r i g i n of the graph were used i n a graphical f i t t i n g process. The l i n e of best f i t i s t i e d to the -4 -1 -1 o r i g i n as the austausch coeff icientty of s t i l l a i r i s near 2x10 g.cm. sec. (Geiger, 1965). Using only the o r i g i n and the three points which l i e close to a stra i g h t l i n e , a l i n e a r wind rule was developed to y i e l d a rough estimate of the austausch c o e f f i c i e n t at a height of 60 cm. above the surface (g.cm.''"sec. ̂ ) from the mean wind v e l o c i t y at an elevation of 120 cm. (meters sec.^"). Eq. III-5. 5 A 6 0 = 134x10" • U 1 2 0 Considering the adiabatic p r o f i l e assumption and other l i m i t i n g assump- tions, further refinement of th i s form i s unwarranted at th i s time (see Light, 1943; P a s q u i l l , 1949; Scott, 1964 and Sverdrup, 1936). Hubley (1957) developed a si m i l a r l i n e a r r e l a t i o n s h i p between the exchange co- e f f i c i e n t and a reference l e v e l wind v e l o c i t y for the inversion conditions which are t y p i c a l of a melting g l a c i e r surface and p a r t i a l l y analogous to a wet s o i l surface during r a d i a t i o n cooling. In this approach the rough- ness parameter (ZQ) and the varia t i o n s due to non-adiabatic flow are treated empirically, from an analysis of net ra d i a t i o n , s o i l heat flux, and temperature-humidity gradients above the surface. The rel a t i o n s h i p s between t h i s method of analysis and the exchange c o e f f i c i e n t of Sverdrup (Kg^ and the von Karman constant (Ko) under adiabatic conditions, when measurements of reference l e v e l wind v e l o c i t y , a i r temperature and humidity are c a r r i e d out at a height ( Z ) above the surface and surface temperature and humidity are known, are i l l u s t r a t e d i n Equation III-6. 40 ( ) date March 1967. 41 Eq. III-6. (BO+SQ) Z f> K o U 2 [c(T-T 0 )+r v (q r q 0 )] [in. Z/^J = K S VU 2 The analysis of;the March :1967 data, y i e l d the following computation forms: Eq. III-7. where: wind speed (U, meters sec. \ temperature (T, ° C ) , vapor pressure (e, mb.) and heat f l u x values (mly./min.). These computation forms were checked with Bowen r a t i o estimates of turbulent transfer l e v e l s . This test indicated a standard error of e s t i - mate of approximately 1 8 mly. min.''" or about the same p r e c i s i o n as the s o i l heat f l u x and net r a d i a t i o n recorders. It i s therefore, probable, that even under i d e a l nocturnal conditions (e.g., wet surface and deep inversion) energy transfer estimates only approach a p r e c i s i o n of 10-207o i n estimating the r e l a t i v e magnitude of the heat flux components. The cases producing the largest v a r i a t i o n s were those of 2 March 67 when there appears to have been an increase i n the roughness length due to air' flow over the Apiary (see F i g . I - l . ) ; and the latent heat f l u x of 3 March 67 when the vapor pressure p r o f i l e was strongly bow shaped between the surface and 120 cm. This l i n e a r wind form w i l l be employed i n Chapter VI. as an aid i n developing an equilibrium temperature model. Having con- structed a rough model for estimating the magnitude of turbulent heat f l u x L 6 0 = 16 U 1 2 0 ( T 1 2 0 - T o ) Eq.. III-8. V60 - 25 U 1 2 0 (e , 120 _e ;o) 42 at the study s i t e i t i s now time to consider a major feedback mechanism which often terminates surface cooling and acts as an ad d i t i o n a l heat source. Radiation Fog During c l e a r and near-clear nights when the a i r i s r e l a t i v e l y s t i l l and conditions are favorable for needle ice formation, the a i r near the screen l e v e l often reaches the dewpoint temperature and produces a deep r a d i a t i o n fog. The occurrence of nocturnal r a d i a t i o n fogs has been i n - vestigated at Vancouver International A i r p o r t and an empirical p r e d i c t i o n model was developed (Belhouse, 1961). F i e l d studies of r a d i a t i o n fog formation and thermal r a d i a t i o n f l u x divergence near the surface have been made by Davis (1957) and Funk (1960, 1962). An example of the e f f e c t s of one of these events at Vancouver follows. On the evening of 7 February 1967 a bead thermistor and an "economical net radiometer" were placed over a bare s o i l surface i n a r e s i d e n t i a l back- yard. The radiometer was e s s e n t i a l l y the model described by Swan et. a l . (1961) with only one layer of polyethene f i l m s h i e l d i n g . The s o i l surface thermistor resistance was determined with a Wheatstone n u l l i n g bridge (see appendix) and the net r a d i a t i o n computed from the counters of a t o t a l i z e r c i r c u i t s i m i l a r to that described by Goodell (1962). During the measurement series there was some condensation on the sh i e l d i n g of the upper radiometer unit which was wiped dry at the end of each 30 minute measurement i n t e r v a l . After c o r r e c t i n g for radiometer lag, approximately 15 minutes, i t was possible to compute the r a d i a t i o n temperatures of the ground surface and the sky hemisphere. In these computations the emis- 43 s i v i t y of both the ground surface and the sky hemisphere was assumed to be unity. It should be noted that at t h i s s i t e the sky hemisphere i s com- posed p a r t i a l l y of hot buildings and trees. The re s u l t s of these measure- ments are presented as half-hour mean values centered on the indicated time i n Table III-6. TABLE III-6. NET RADIATION AND SURFACE TEMPERATURE DURING THE DEVELOPMENT OF RADIATION FOG ON 7 FEBRUARY 1967. Time (P.S.T.) Surface Temp. ( ° c . ) Net Radiation (ly./min.) Sky Temp. ( ° c . ) Cloud Cover Cloud Type 1815 1.7 -.042 -4.7 2/10 stratus 1845 1.3 -.049 -6.3 2/10 stratus 1915 1.1 -.047 -6.2 4/10 fog 1945 0.8 -.047 -6.5 4/10 fog 2015 0.6 -.040 -5.7 4/10 fog 2045 0.3 -.045 -6.8 4/10 fog 2115 0.1 -.038 -6.8 5/10 fog 2145 0.6 -.031 -4.2 5/10 fog 2215 1.3 -.034 -3.9 6/10 fog 2245 1.3 -.031 -3.4 6/10 fog A.graphical p l o t of the temporal v a r i a t i o n of the ground surface temperature, with the r a d i a t i o n temperature of the sky hemisphere, i s pre- sented i n Figure II-2. to i l l u s t r a t e the rapid response of surface temper- ature to the deepening r a d i a t i o n fog. It would appear that on clear-calm nights, when the a i r temperature reaches the dew point, r a d i a t i o n fog can b u i l d to great depths and, with the aid of the sensible heat flux, which 0 \ sky 1800 2000 2 2 0 0 t(RS.T.) F i g e I I I - 2 s Conditions during the formation of radiation f o g s 7 February 19o7» The fog noticeably deepened just after 2100 P.S.T. 45 i s always p o s i t i v e under inversion conditions, can terminate the cooling process and re-warm the s o i l surface. This re-warming i s possible as the r a d i a t i o n fog blanket i s occupying a warm portion of the atmosphere. The fog base, a r a d i a t i n g surface, may be several degrees warmer than the s o i l surface e s p e c i a l l y i f the dew point at the screen l e v e l and higher i s well above the ice point. In summary, increased i n s i g h t was gained i n three areas from the material presented i n t h i s chapter. F i r s t , the r e l a t i v e magnitude of the components of surface energy exchange during needle ice growth were docu- mented and the major heat sinks, evaporation and the night sky, available to the s o i l surface were located. Thus, from the energy exchange viewpoint an i d e a l needle ice night would be one on which both the thermal radiant and latent heat flows were directed away from the s o i l surface and were operating at optimum e f f i c i e n c y . These conditions can be produced i n nature by a clear-dry - c o l d atmosphere above a warm-wet s o i l surface. The larger the contrast between atmospheric temperature and humidity, i n com- parison to s o i l surface temperature and humidity, the greater the e f f i c i e n c y of the radiation-evaporation cooling mechanism i s and the more reduced the p r o b a b i l i t y of re-warming due to either r a d i a t i o n fog or dew formation i s . Second, the r e l a t i o n s h i p between wind speed and turbulent exchange w i l l permit rough estimates of the sensible and latent heat flux levels using wind v e l o c i t y , temperature and humidity data. Third, the e f f i c i e n c y of r a d i a t i o n fog as a terminal feedback mechanism was demonstrated. 46 CHAPTER IV. A PHYSICAL INVESTIGATION OF THE STUDY SITE SOIL In the e a r l i e r chapters the discussion centered around the climato- l o g i c a l conditions which lead to needle ice events. However, one major aspect of the environment was lar g e l y neglected; the s o i l . The geometry of the s o i l matrix and the p a r t i c l e size d i s t r i b u t i o n have long been recognized as indices of the " f r o s t heave s u s c e p t i b i l i t y " of a s o i l sample. In addition, the hydraulic conductivity and water retention properties have a considerable influence upon the processes which r e s u l t from s o i l c ooling. In this chapter the p a r t i c l e s i z e d i s t r i b u t i o n , water retention and flow properties, and the thermal c h a r a c t e r i s t i c s of the study s i t e s o i l w i l l be considered. P a r t i c l e Size D i s t r i b u t i o n and C l a s s i f i c a t i o n . P a r t i c l e s i z e d i s t r i b u t i o n i n a sample of the study s i t e s o i l was investigated using a combination of the hydrometer and dry sieving tech- niques (Day, 1965). The r e s u l t s of t h i s i n v e s t i g a t i o n are presented i n Table IV-1. TABLE IV-1. PARTICLE SIZE DISTRIBUTION (U.S.D.A.) C l a s s i f i c a t i o n CLAY SILT Very Fine Si Fine VND Medium Coarse 7o of sample by weight 5 20 4 14 45 12 E f f e c t i v e diameter 2 50 100 200 500 2000 (microns) 47 The c l a s s i f i c a t i o n of the s o i l i s a sandy loam according to the United States Department of Agriculture ( c i r c a . 1950) c l a s s i f i c a t i o n of s o i l s by texture. It i s also of importance to note that the bulk of the sample (75%) i s i n the sand size range. The sample i s extremely effervescent when hydrogen peroxide i s added i n d i c a t i n g the presence of organic matter. The presence of organic matter and, perhaps, some swelling clay minerals of the montmorillonite group (Yong and Warkentin, 1966) can also be inf e r r e d by the tendency of the a i r dried material to swell when water i s added to the sample. S o i l Thermal Properties Estimates of the s o i l thermal d i f f u s i v i t y were obtained from temper- ature measurements at depths of 0.2, 1.5 and 3.0 centimeters i n the sample. A heat pulse was produced by two i n f r a r e d lamps located 60 centimeters above the sample and co n t r o l l e d by a variable transformer. The data analysis was performed using the heat pulse method (Van Wijk and Derksen, 1966). These tests were c a r r i e d out at several points i n the low water tension range. The s o i l water tension was co n t r o l l e d by means of a hang- ing water column connected to the s o i l sample through a semi-permeable membrane (Richards, 1941, 1948) as i l l u s t r a t e d i n Figure IV-1. The duration of the heat pulse was lim i t e d between three and s i x hundred seconds r e s t r i c t i n g the magnitude of the surface temperature disburbance to approximately 10°Celsius. Under these conditions, of evaporation at the surface and temperature increasing toward the surface, the tension and thermal gradient components of s o i l water flow are opposed and tend ^positive air pressure air porous plate water at atm pressure s collector Fig.IV-1• Schematic diagram of the Porous Plate Extractor. 49 to cancel each other (Cary, 1965, 1966). Estimates of the volumetric heat capacity (computed from the low tension desorption curve with the aid of Equation IV-1.) and the re s u l t s of the heat pulse d i f f u s i v i t y t e s t s , at test tensions, are presented i n Table IV-2. TABLE IV-2. THE VARIATION OF SOIL THERMAL PROPERTIES WITH SOIL WATER TENSION AT 20°C. Tension Heat Pulse D i f f u s i v i t y Volumetric Heat Capacity (cm.H20) (cm? secT1 x 10 4) ( c a l . cmT3 °C~.1) 5 34 .50 40 56* .47 73 52 .44 91 30 .43 151 32 .37 * s t r u c t u r a l change due to "cracking". The v a r i a t i o n of s o i l thermal properties as a function of s o i l water content i s discussed by De Vries i n Van Wijk (1966). There i s a pronounced tendency for the volumetric heat capacity to decrease r e g u l a r l y with decreasing water content (increasing tension). De Vries (1966) gives the following method for estimating the volumetric heat capacity (C s) of a s o i l from the volume fr a c t i o n s of water (Xw), organic matter (X Dr) and mineral s o i l ( X m ) . Eq. IV-1 C s = X w + 0.60 X o r-h 0.46 Xjj, 50 This expression was used to estimate the volumetric heat capacities from the low tension desorption curve. In these estimates the organic f r a c t i o n was lumped with the mineral fraction, as the independent organic content was not measured. D i f f u s i v i t y (°^) i s related to volumetric heat capacity ( C , ) and conductivity ( K , ) i n the following manner. Eq.IV-2. «C s s K / C It would be expected that decreasing the s o i l water content would produce a gradual decline i n the thermal d i f f u s i v i t y i f the conductivity remained constant. However, De Vries ( i n Van Wijk, 1966) indicates that conductivity i s known to decline slowly with decreasing water content i n a quartz sand. The net e f f e c t should then be a gradual decrease i n thermal d i f f u s i v i t y , including the conductivity e f f e c t s , with increasing s o i l water tension. The v a r i a t i o n from this pattern, at a tension of 40 cm.^O i n the experimental data, i s quite probably due to s t r u c t u r a l changes i n the s o i l sample. This condition makes estimates of volumetric heat capacity using Equation IV-1. only approximate, due to v a r i a t i o n s i n bulk density. In any case, these values give reasonable computation estimates i n general agreement with the estimate of the mean annual thermal d i f - f u s i v i t y beneath the agrometeorological s i t e which was a depth weighted -4 2 - 1 mean of 45 x 10 cm. sec. for the s o i l column between the surface and 150 centimeters. The e f f e c t of s o i l water tension on the nocturnal cooling rate can be estimated by the s u b s t i t u t i o n of the test values of the thermal pro- perties l i s t e d i n Table IV-1. into Brunt's cooling rule (Equation II-3.). The cooling rate, expressed as temperature divided by the square root of 51 time, i s inversely proportional to the thermal property of the s o i l near the surface ( C s ' \ OCs ). The thermal property varies d i r e c t l y with water content. Thus, i n general, the drier a s o i l the more quickly i t w i l l cool. The maximum and minimum values of the thermal property derived from the experimental data occur at tensions of 40 and 151 cm. of water. Their r a t i o i s approximately 1.7 which i s similar to the ratio-between the max- imum and minimum empirical cooling slopes (m) i n Table II-5. This value which was derived from f i e l d temperature decline data on similar clear- calm nights i s approximately 1.8. This result suggests that with stable energy exchange conditions the entire v a r i a t i o n i n observed cooling rates could be produced by variations i n near surface s o i l water tensions only slightly greater than the experimental range. In this study the experimental values of the thermal properties of the s o i l , as a function of s o i l water tension, w i l l be used as guides i n selecting computation values of s o i l thermal properties at the study s i t e . Where no conclusions can be drawn about the anticipated value of s o i l 3 —1 o water tension the computation values are selected as 1 x 10 cly^.secf or cm. C -'•> —3 2 —1 foraconduetjLvity and SOx^iOg• semi"osecifruforLdiffusivity. The Theory of Ice Intrusion It i s known that some s o i l s are more susceptible to "frost heaving" produced by ice segregation ( i . e . the increased water content of the frozen s o i l due to water migration toward the freezing plane during s o i l freezing) than others. Extremely coarse s o i l s i n the sand range and dense clays are usually not "heave susceptible," whereas, medium textured s o i l s ( i . e . the 52 loams) are often highly susceptible. One notable attempt to re l a t e the s e n s i t i v i t y of a s o i l to the ice segregation mechanism through the s o i l pore geometry was made by Williams (1966a). Following Williams, a i r w i l l replace water i n a c a p i l l a r y when: Eq.IV-3. Pa-Pw £ (2<Taw) / r c In addition, the r a t i o of the i n t e r f a c i a l energies between ice-water and air-water (C7aw/^iw) i s a dimensionless r a t i o equal to approximately .42 (Williams, 1966a). In the f i e l d problem, at the study s i t e , the tensiometer (see appendix) measures the difference between atmospheric pressure and the pressure (here a tension) i n the s o i l water. Thus, i n the needle ice system, when the e f f e c t of overburden pressure i s n e g l i g i b l e , the a i r i n t r u s i o n tension can be stated as: Eq.IV-4. _ ' <r- 4 T > 2 in J- awx — — aw S t i l l following Williams (1966a), ice i n t r u s i o n near the s o i l surface should occur when the s o i l water tension at the freezing plane i s greater than .42^T a w x. This statement of the ice segregation problem, which quite neatly s p e c i f i e s the conditions under which an ice front w i l l advance forming a concrete i c e - s o i l structure ( i . e . s o i l water frozen i n s i t u without segregation) or remain stationary at a fi x e d l e v e l producing an ice segregation (needle ice or ice lens), i s the one r e s u l t from a consid- erable t h e o r e t i c a l and experimental programme (Byrnes, 1951; Everett, 1961; Penner, 1960, 1963); Sayward, 1966; Takagi, 1965; Tanuma, 1967 and Williams, 1963, 1966a-b). These workers have indicated that the freezing rate and 53 the s o i l water content exert a measurable influence upon the ice form and s o i l structure. The needle ice problem i s a test case for the a p p l i c a t i o n of this ice in t r u s i o n model. As a s o i l sample i s moved along a desorption curve by applying a stepped pressure to i t , preferably increasing the a i r pressure above the sample i n an apparatus of the type described by Richards (1941, 1948) or used by Williams (1966a), water w i l l be expelled from the sample. Keeping the water beneath the sample at atmospheric pressure prevents the gradual d i f f u s i o n of a i r out of the water i n the s o i l and into the reser- v o i r i n the base of the apparatus. This design consideration was employed by Dr. J . De Vries (Dept. of S o i l Science, U.B.C.) i n helping the author carry out desorption tests on the study s i t e s o i l (see Fig.IV-1.). The water expelled at each pressure step i s c o l l e c t e d and weighed. An abrupt increase i n the volume of expelled water should be noted when the a i r in t r u s i o n tension l i e s within the pressure step. The c r i t i c a l tension for ice i n t r u s i o n iwx) i - s then known to be approximately 42% of th i s value. The occurrence of a pronounced a i r i n t r u s i o n value depends upon the frequency d i s t r i b u t i o n of the e f f e c t i v e pore diameters ( i . e . the pore diameter that matches retention properties of the pore population i n a f i n i t e volume of s o i l ) i n the sample under study. A s o i l sample with a uniform pore si z e d i s t r i b u t i o n ( i . e . a l l diameters having equal frequency over a tension range) may not be expected to show a pronounced a i r i n t r u s i o n value on desorption. This does not indicate that the theory does not hold for t h i s case, but rather, that i t s t i l l operates at the scale of i n d i v i d u a l pores. This condition, however, does make the a p p l i c a t i o n of the theory untractable. This consideration may, i n f a c t , be the t h e o r e t i c a l v a r i f i c a - 54 t i o n of the observed phenomena that s o i l s , a f t e r slow freezing at temper- atures just below the ice point, are more susceptible to ice segretation than before freezing. Freezing, e s p e c i a l l y with ice segregation, i s known to loosen the s o i l structure (Baver, 1965). Therefore, a production of l o c a l i n e q u a l i t i e s i n the pore si z e d i s t r i b u t i o n near the surface i s ina ferred. The Low Tension Desorption Curve An apparatus of the type described by Richards (1941, 1948) was used with a b l o t t e r paper-glass bead membrane, A i r pressure was applied to the top of the sample to determine the low tension desorption properties of the study s i t e s o i l (see Fig.IV-2.). The s o i l was weighed, mixed with de- aired water, and puddled to a bulk density of .74 g.cm. before the pres- sure was applied. These manipulations permitted the volumetric water con- tent to be recovered as the s o i l was i n i t i a l l y a i r dried, with a known water content, and a running account of the sample water budget maintained during the tes t . Pressure increments of 10 cm.H20 were applied and a time period of at least s i x hours was allowed f o r the sample to a t t a i n e q u i l i b - rium before the next step. This run was terminated by the f a i l u r e of the b l o t t i n g paper membrane at the 130 cm.H.20 (96 mm. Hg.) step. These data are presented i n Fig.IV-2. and i l l u s t r a t e two facts about the sample. (1) The pores emptied by tensions i n the test range have an extremely uni- form e f f e c t i v e radius d i s t r i b u t i o n and (2) i f a pronounced a i r i n t r u s i o n value does e x i s t i t i s above 120 cm.HfO (88 mm.Hg.). By deduction, i f (2) i s v a l i d one should expect ice i n t r u s i o n at some tension above 50 cm.^O. (37 mm.Hg.). This deduction i s i n good agree- x 36 6 0 80 T(mm.Hg.) F i g . I V - 2 o The low tension desorption curve. (O ) test points. 56 merit with f i e l d observations as the f i e l d tensiometer (see appendix) buried at a depth of approximately 5 cm. recorded tensions of 100 to 120 mm.Hg. when needle ice was growing on the nights of 6-7 and 7-8 February 1968. The actual tension just below the freezing plane must have been even higher due to s o i l desiccation on freezing. It i s , therefore, ex- pected that i f a pronounced a i r i n t r u s i o n value e x i s t s i n the laboratory sample i t w i l l be located i n the tension region abovd 290 mm.Hg. (400 cm.^O.). It should be noted that the laboratory tests were c a r r i e d out on disturbed samples and that the pore si z e d i s t r i b u t i o n s which de- termine the f i e l d conditions are i n an extremely shallow layer within 2 cm. of the s o i l surface. Thus, laboratory studies can only be expected to produce an approximation of the natural conditions. Conversely, the study s i t e i s i n a plo t which i s r e g u l a r l y disturbed by plowing and other agrarian a c t i v i t i e s and thus, the v i o l e n t disturbances produced by labor- atory manipulation may not greatly v i o l a t e r e a l i t y i n the bulk sample. The High Tension Desorption Curve Due to the implications ofcthe f i e l d measurements of s o i l water ten- sion during needle ice growth, discussed i n the previous section, i t seemed advisable to run a desorption curve covering higher tensions i n the realm of 300 mm.Hg. The experimental arrangement was i d e n t i c a l to that used for the low tension test except that a porous ceramic plate was used as the semi- 3 permeable membrane. A 344 cm. sample of the study s i t e s o i l was saturated with de-aired water and allowed to reach an equilibrium water content 57 (27.27o by volume) at a tension of 100 mm.Hg. over a period of 24 hours. It was possible to compute water content by volume as the i n i t i a l volume of water added to the sample was known and the volume of expelled water was measured at each step. The sample bulk density was approximately -3 0.95 g.cm. . It w i l l be noted that due to differences i n geometry and methods of bringing the samples to an i n i t i a l equilibrium, that i n the region of overlap (projecting the low tension curve to 100 mm.Hg.\ between the low and high tension desorption curves the volumetric water contents are s l i g h t l y d i f f e r e n t . The high tension sample contains approximately 1% more water by volume at 100 mm.Hg. This value has a sign which indicates that an a i r i n t r u s i o n point was not missed i n the region between the two curves (88-100 mm.Hg.). Pressure increments of 20 mm.Hg. were applied over'one day periods. This equilibrium time was necessary because of the r e l a t i v e l y high impedence of the ceramic p l a t e . The r e s u l t s of th i s test are i l l u s t r a t e d i n Figure IV-3. The r e s u l t i n g curve indicated, as i n the low tension test, a r e l a t i v e l y uniform pore size d i s t r i b u t i o n and no pronounced a i r i n t r u s i o n value. This outcome further strengthens the author's opinion that i n the needle ice system the operational a i r i n t r u s i o n value i s not a bulk pro- perty of the s o i l sample and cannot be treated as i t i s i n the deeper ice lens system (Williams, 1966a). Instead, i t i s a property of the upper 0r2 cm. near surface of the s o i l s (sandy-loans, loamy-sands) which are susceptible to needle ice formation. The pore - p a r t i c l e geometry of thi s near surface zone i s i n a constant s t r u c t u r a l f l u x during the needle ice season not only due to f r o s t phenomena, but also due to the e f f e c t s of r a i n splash and sheet wash, etc. Fig.IV°»3° The high tension Resorption curve e 59 These conditions limit the range of experimental methods available for specifying the c r i t i c a l ice intrusion tension in the needle ice system. Two possible methods of attack are (1) the application of air intrusion tests to thin (1-2 cm.) slices of surface s o i l samples expelling the water in situ and (2) continued observations of so i l water tension during nat- ural freezing events. Both of these approaches require the development of new or modified instrumentation. The f i r s t would require a device sim- i l a r to that used by Williams (1966a), but modified to permit the intro- duction of undisturbed f i e l d samples. The second would u t i l i z e an ex- tremely small recording tensiometer which could be placed within 1 cm. of the s o i l surface. The Big Step Water Expulsion Curve A test of the water expulsion-tension properties of the study site s o i l was run using the same apparatus and s o i l sample used in the high tension desorption test. The sample was allowed to take up water for a 72 hour period at a tension of 0 mm.Hg. The sample was then subjected to a 100 mm.Hg. tension increase at 24 hour intervals. Before each increased tension step the accumulated amount of water expelled from the sample was determined. The results are listed in Table IV-3. 60 TABLE IV-3.. TENSION-WATER EXPULSION Tension Accumulated Expelled Water (mm.Hg.) 0 0.0 100 68.4 200 80.4 300 86.6 400 91.0 These re s u l t s were plotted on l i n e a r graph paper transforming both tension and expelled water volume to t h e i r natural logarithms. The r e l a - tionship between these variables i s l i s t e d i n Equation IV-5. the tension-expulsion r e l a t i o n s h i p i s well represented by an exponential function which produces a curve without an "S" type i n f l e c t i o n exhibited by s o i l s having pronounced a i r i n t r u s i o n values (Williams, 1967). These data further substantiate the i n t e r p r e t a t i o n of the preceding sections and, i n addition, indicate the absence of an "S" type expulsion curve. I n f l e c - tions of the "S" type are quite probably uncommon i n well packed disturbed samples with approximately 757» of t h e i r p a r t i c l e s i n the sand te x t u r a l range. Eq.IV-5 = .21 lnX+ ln 3.3 In E The expelled water volumes estimated from this graphical f i t were with i n the realm of possible experimental error (1 cm. ) i n d i c a t i n g that 61 Rapid Expulsion Testing Williams(1966a) tested the study s i t e s o i l i n his a i r i n t r u s i o n ap- paratus at the S o i l Mechanics Section of the D i v i s i o n of Building Research, National Research Council, Ottawa. His unit uses a much shorter i n t e r v a l between tension steps (0.5 - 1.0 min.) and does not e s t a b l i s h the desorp- t i o n curve of the s o i l . The re s u l t s indicated a larger v a r i a t i o n i n a i r -2 i n t r u s i o n values of .15, .21, .25 and .05 Kg.cm. . The l a s t r e s u l t of the series was considered "anomalous" and the remaining values were considered to have "larger divergence i n the three r e s u l t s than normally found". This divergence was at t r i b u t e d by Williams to the wide grain s i z e d i s t r i b u t i o n and the problem of obtaining uniform samples. This author would add that i n addition these r e s u l t s indicate differences i n pore size d i s t r i b u t i o n r e s u l t i n g from v a r i a t i o n s i n bulk density and geometry within ("uniform") samples during laboratory manipulation. Ice i n t r u s i o n , according to Williams' reports, i s to be expected i n the tension region of 46-86 mm.Hg. The Williams method of te s t i n g i s more p r a c t i c a l than the long equilibrium desorption methods used by the author for obtaining a i r i n t r u s i o n values as the method i s much f a s t e r . However, the divergence of re s u l t s and the proximity of Williams' values to tension measurements at a depth of 5 cm. during f i e l d needle ice growth (100-120 mm.Hg.) indicate the realm of the c r i t i c a l ice i n t r u s i o n value for the study s i t e s o i l i s highly v a r i a b l e and i n the realm of 100 mm.Hg. 62 The Unsaturated Hydraulic Conductivity The unsaturated hydraulic conductivity of the s o i l was determined by applying a i r pressure to the sample and varying the head across the sample, which was 2.5 cm. thick. The cross section area of the porous membranes, . 2 between which the sample was placed, was 136 cm..® The re s u l t s of this test are presented i n Table IV-4. TABLE IV-4. UNSATURATED HYDRAULIC CONDUCTIVITY S o i l Water Conductivity Tension ^ _^ (mm.Hg.) (10 cm. sec. ) 0 990 22 740 49 82 88 16 141* shrinking 4 It should be remembered that this t e s t does hot duplicate the f i e l d s i t u a t i o n as the experiment was.run under isothermal conditions at a tem- perature of 20°C. When hydraulic conductivity i s p l o t t e d as a function of tension on semi-log paper (see Fig.IV-4.) the steepest portion of the curve i s i n the region of 50 mm.Hg. and no sharp changes i n slope are evident at higher tensions where the e f f e c t s of a i r i n t r u s i o n might be expected. It i s i n t e r e s t i n g that three orders of magnitude were covered i n the 0-141 mm.Hg. tension range. This suggests that at some point the i n a b i l i t y of a desorbing s o i l to transmit water at the fusion rate to the freezing ^ \ \ \ \ \ \ \ vJ » I V V \ \ \ \ \ • X A , - A X 0 50 1 0 0 150 T (mm.Hg.) Fig.IV-4. The hydraulic conductivity-tension curve under isothermal conditions at 20°G. (•) sample points. 64 plane should produce dessic a t i o n and ice i n t r u s i o n . It was impossible to follow this l i n e of i n v e s t i g a t i o n due to the absence of f i e l d data on water table depths and tension gradients during needle ice formation. In summary, the material i n this chapter reports' some of the physical c h a r a c t e r i s t i c s of the study s i t e s o i l and examines the ice i n t r u s i o n model as a method for estimating the e f f e c t s of variable s o i l water content (tension) on needle ice growth. It i s found that this approach i n practice i s complicated by the f a c t that ice i n t r u s i o n i n the near surface s o i l (0=2 cm.), where segregation occurs during needle ice growth, i s dependent on both s o i l water tension and s o i l structure ( e f f e c t i v e pore radius). As structure and tension near the surface are time dependent i n nature, labor- atory tests which destroy near surface structure were found to be of l i m i t e d value i n the analysis of needle ice growth. The r e s u l t s , reported i n Chapter VI., of s o i l tension measurements at the study s i t e which were made i n conjunction with post-freezing observations of s o i l conditions pro- vider a more e f f i c i e n t means of examining tension e f f e c t s on the needle ice system. As the highest measured values of s o i l water tension occurred during needle ice growth; i t i s reasonable to believe that tension i s only a l i m i t i n g factor at the study s i t e during unusually dry periods within the needle ice season and at better drained s i t e s i n the Vancouver area. How- ever, at the study s i t e s o i l water tension gradients appear to have a pronounced e f f e c t upon the depth of the normally frozen s o i l layer or cap which rides atop needle ice growth (see Chapter IV.) and the nocturnal cooling rate. 65 CHAPTER V. THE ANALYSIS OF THE PAST EVENT RECORD At the agrometeorological s i t e adjoining the study s i t e a unique record of needle ice events has been compiled covering the period from 1964 onward (the record a c t u a l l y begins i n 1961 at another s i t e on the University of B r i t i s h Columbia Campus). It i s the purpose of this chapter to analyse this record to discover i f a d d i t i o n a l information about the nature of needle ice events i s forthcoming from a s t a t i s t i c a l analysis of routine meteorological data. The S t a t i s t i c a l Model In an attempt to produce a model that could be used at the study s i t e to estimate the p r o b a b i l i t y of a needle ice event, i t was decided to develop a s t a t i s t i c a l model based upon an examination of the 1615 P.S.T. a i r temperature, r e l a t i v e humidity, dewpoint temperature and the nocturnal wind v e l o c i t y and cloud cover. The data used was the record of a l l months when needle ice events occurred during the period from 1964 through 1967. These data were divided into two groups, the data c o l l e c t e d during 1964-65 and that c o l l e c t e d during 1966-67. These data were tested using the Kolmogorov-Smirnov test to examine the p r o b a b i l i t y that the data belong to a known d i s t r i b u t i o n (Siegel, 1956). The re s u l t s are as follows: 66 TABLE V - l . THE RESULTS OF K-S TESTING Hypothesis Result of Hypothesis Testing The 64-65 variables i n the non-event group are normally d i s t r i b u t e d . The 64-65 variables i n the event group are normally d i s t r i b u t e d . The 64-65 event variables are a sample from the d i s t r i b u t i o n of non-events. Reject at the .05 l e v e l for cloud cover. Accept at the .05 l e v e l a l l other v a r i a b l e s . Accept at the .05 l e v e l a l l v a r i a b l e s . Accept at the .05 l e v e l a l l varia b l e s . These test show that a l l the variables appear to have reasonably normal d i s t r i b u t i o n s about t h e i r means, with the exception of cloud cover which i s skewed to the high value side of the mean i n the non-event group. The variables i n the 64-65 event and non-event groups were also tested for homogeneity of variance using the F - d i s t r i b u t i o n (Alder and Rossler, 1962) with the following r e s u l t s . TABLE V-2. RESULTS OF F-TESTING FOR HOMOGENEITY OF VARIANCE Variable Results at 57» Level a i r temperature r e l a t i v e humidity dew point wind speed cloud cover not homogenous not homogenous homogenous homogenous homogenous 67 Because of these r e s u l t s , i t was necessary to set up the following hypothe- s i s to test the variable means for s i g n i f i c a n t differences between the event and non-event groups. The assumption was made that the population para- meters were defined by the 64-65 non-event group and then the hypothesis was tested that the event group variables were random samples from this popula- t i o n . The r e s u l t s of a T-test for s i g n i f i c a n t differences i n the variable means of the two groups are as follows (Alder and Rossler, 1961): TABLE V-3. THE RESULTS OF THE T-TEST FOR SIGNIFICANT DIFFERENCES BETWEEN GROUP VARIABLE MEANS 64-65 non-event 64-65 event Variable Means {Variance) Means (Variance) T-test Results at the .001 l e v e l a i r temp.* (°F.) r e l a t i v e humidity* (%) dew point (°F) wind speed (m.p.h.) Cloud cover (tenths) 41.9 (52.7) 84.8 (148) 37.4 (59) 3.33 (2.9) 6.7 (10.1) *variance non-homogenous 39.7 (21.1) 72.0 (345) 30.6 (59) 2.54 (2.9) 3.1 (10.1) Reject Reject Reject Reject Reject Thus, there are s t a t i s t i c a l l y s i g n i f i c a n t differences i n the mean values of variables measured on event and non-event nights. As one would expect, the a i r temperatures are cooler on event evenings at 1615 i n d i - c a t i n g more rapid surface cooling under c l e a r skies. In addition to the reduced cloud cover on event nights, the atmosphere i s also d r i e r as 68 shown by the lower r e l a t i v e humidity and depressed dewpoint. It should be noted that these are p r e c i s e l y the variables used i n empirical e s t i - mates of the thermal r a d i a t i o n balance (Sutton, 1953) and that a r e l a t i v e l y dry, cloudless atmosphere, with reduced wind, presents excellent conditions for the rapid cooling of the s o i l surface (Vei'tch, 1960). The variance of a i r temperature i s somewhat less and the variance of r e l a t i v e humidity i s greater on event nights, whereas the variances of the other variables appear s i m i l a r . Thus, by showing a near normal distributionsand s i g n i f i - cant differences among variable means in. the event and non-event groups the stage has been set for an attempt to produce a s t a t i s t i c a l model for event p r e d i c t i o n . A l l pairs of variables were examined using discriminant analysis to determine which of these v a r i a b l e pairs gave the best i n d i c a t i o n ( d i s - crimination) of needle ice events i n the 64-65 data. The variable symbols are l i s t e d below: TABLE V-4. VARIABLE LISTING Symbol Meaning Units T 1615 a i r temperature °F. RH 1615 r e l a t i v e humidity % DP 1615 dew point temperature °F. W nocturnal wind speed m.p.h. N nocturnal cloud cover tenths The a p p l i c a t i o n of the BMD 04M discriminant 'analysis programme, which developed by the Health Sciences Computing F a c i l i t y at the University of C a l i f o r n i a , to these data yielded two matrices which are of i n t e r e s t i n 69 evaluating the s e n s i t i v i t y of the var i a b l e pairs for discrimination. 2 These are the Mahalanobis D s t a t i s t i c , which i s an estimate of the pro- b a b i l i t y of m i s c l a s s i f i c a t i o n , and the F - s t a t i s t i c , which indicates i f there i s a s i g n i f i c a n t difference between the mean values of the discrim- inant function between groups (Freese, 1964). Due to the c h a r a c t e r i s t i c s of these s t a t i s t i c s , t h e i r maximum values should indicate the best v a r i - able combination for discrimination. TABLE V-5. MAHALANOBIS D 2 MATRIX, 64-65 DATA T RH DP W N T 1.02 1.13 .32 1.80 RH 1.07 .97 1.60 DP -- .93 2.13 W -- 1.38 N TABLE V-6. F- MATRIX, 64-65 DATA T RH DP W N T 10.8 11.9 3.4 19.1 RH 11.3 10.3 16.9 DP -- 9.9 22.5 W ._ 14.7 N It w i l l be noted that the most se n s i t i v e variable pair i s the dew point - cloud cover combination, which indicates the overriding influence of thermal back r a d i a t i o n i n needle ice and f r o s t forecasting (Vietch, 70 1960). In this case, the F - s t a t i s t i c i s s i g n i f i c a n t at the .001 l e v e l . 2 The p r o b a b i l i t y m i s c l a s s i f i c a t i o n estimated from the Mahalanobis D s t a t i s t i c i s approximately 23%. These re s u l t s are consistent with the program output which y i e l d s the following r e s u l t s . TABLE V-7. CLASSIFICATION OF THE 64-65 DATA BY DISCRIMINANT ANALYSIS Events Non-Events Sum Correct Group 19 109 128 Incorrect group 6 35 41 sum 25 144 169 The actual m i s c l a s s i f i c a t i o n was 41/169 or approximately 24%, which indicates that the assumption that D/2 can be treated as a standard normal deviate appears j u s t i f i e d i n t h i s case, even though i t w i l l be r e c a l l e d that the cloud cover variable f a i l e d the K-S test f o r normality at the 5%, l e v e l . This most se n s i t i v e two var i a b l e equation i s as follows: Eq.V-1. Discriminant Function derived from the 64-65 data. L = -.00068 DP -.00228 N+.03438 event region when L> 0 Eq.V-2. P l o t t i n g form of the Discriminant Function (see Fig.V-1.) DP- -3.35 N+50.6 The 1966-67 data were run using the event region defined by the analysis of the 1964-65 data with the following r e s u l t : 71 60 ^ 50 o a. a 4 0 30 2 0 10 < jvent reg ion 20 4 0 60 80 100 N (%) Fig sV« = l o Regions defined by the discriminant analysis of dew point (DP) and cloud cover (N) from the 1964=65 data e 72 TABLE V-8. CLASSIFICATION OF THE 66-67 DATA. Events Non-Events Sum Correct Group 23 180 203 Incorrect Group 8 30 38 Sum 31 210 241 The t o t a l m i s c l a s s i f i c a t i o n of items was 38/241 or approximately 16% which was better than the c l a s s i f i c a t i o n for the o r i g i n a l data. In ad- d i t i o n , the m i s c l a s s i f i c a t i o n of events was 8/31 or approximately 26% i n the anticipated realm, whereas, the m i s c l a s s i f i c a t i o n of non-events was 30/210 or approximately 147o, which, though not anticipated, must be put down to good luck at this stage. The s t a t i s t i c a l model indicated that thermal r a d i a t i o n from the sky hemisphere was a major factor i n l i m i t i n g the frequency of needle ice events. Because of th i s s i t u a t i o n the amount of p r e c i p i t a b l e water i n the atmosphere i s a c r i t i c a l factor which must exert an overriding influence on the timing of needle ice events due to the e f f e c t s of atmospheric back r a d i a t i o n . A short analysis of p r e c i p i t a b l e water depth at Quillayute, Washing- ton, and Port Hardy, B r i t i s h Columbia, was c a r r i e d out over a four month period (November 1966 - February 1967) using punch card data from the United States Weather Bureau and the Canadian Meteorological Branch. A com- puter program was written to y i e l d the depth of the p r e c i p i t a b l e water column between the surface and the 400 mb. level.. . Canadian Department Radiosonde Data TABLE V-9. PRECIPITABLE WATER (CM.) OVER QUILLAYUTE, WASH., AND PORT HARDY, B Nov. , 66 Quillayute Port Hardy 1 1.02 2.37 2 0.97 1.08 3 1.61 1.87 4 1.26 1.52 5 1.82 (0.96) 6 0.65 0.67 7 0.90 0.97 8 (0.76) (1.01) 9 1.73 10 0.57 0.59 11 0.75 12 0.84 1.00 13 1.21 0.79 14 1.24 1.15 15 1.05 1.07 16 1.10 0.79 17 1.16 0.95 18 1.52 19 (1.67) 1.59 20 1.22 1.26 21 1.20 22 0.96 1.09 23 0.85 24 (1.41) 1.57 25 (1.70) 26 0.76 0.88 27 1.37 1.22 28 (1.24) 0.65 29 1.55 0.77 30 1.35 0.89 31 needle ice event. ( , AT 0400 P.S.T. Dec. 66 Quillayute Port Hardy (1.53) 1.18 1.24 1.13 1.28 0.89 1.07 1.07 0.93 0.77 1.11 (0.44) 0.85 0.48 0.64 0.55 0.95 0.95 1.57 1.28 1.82 1.54 1.44 1.01 1.65 1.20 1.04 1.08 1.76 1.63 1.69 1.09 1.65 1.42 (1.77) (1.22) (1.73) (1.74) 0.90 0.93 1.08 0.65 0.88 1.17 0.91 1.24 1.12 0.78 0.61 0.74 (0.64) 0.78 1.30 1.40 1.63 1.63 0.99 1.35 ) neglect i n analysis. TABLE V-9 (Cont.) PRECIPITABLE WATER (CM.) OVER QUILLAYUTE, WASH., AND PORT HARDY, Jan. 67 Quillayute Port Hardy 1 0.84 0.92 2 1.42 1.41 3 0.92 0.91 4 1.11 0.67 5 0.56 0.51 6 0.90* 0.94 7 1.31* 1.30 8 (1.72) 0.75 9 1.09 1.58 10 1.01 1.61 11 0.92 0.97 12 0.98 1.00 13 1.17 0.91 14 1.50 . 1.51 15 (1.54) (0.85) 16 0.85 17 0.82 0.91 18 1.20 1.22 19 . 1.34 1.25 20 1.14 1.01 21 0.80 0.80 22 0.82 0.75 23 0.86 0.77 24 0.70 0.58 25 0.92 0.54 26 1.20 0.91 27 1.62 1.19 28 1.45 1.26 29 1.53 1.10 30 0.78 1.54 31 0.69 0.53 B.C., AT 0400 P.S.T. Dec. 66 Quillayute Port Hardy (1.15) (1.08) (1.45) (1.56) 1.64 0.88 1.81 0.72 0.80 0.88 1.15 1.16 0.93 0.99 0.84 1.30 1.33 0.94 0.97 1.43 (1.29) 1.27 (1.13) 0.87 0.79 0.69 0.70 (0.88) (0.79) (0.99) (0.95) 1.63 1.56 0.93 0.90 0.62 0.81 0.67 1.11 0.81 0.89 0.79 1.11 1.37 0.90 0.83 0.87 1.06 1.05 1.17 0.89 1.30 (0.89) needle ice event. ( ) neglect i n analysis. * snow on surface at Vancouver. 75 of Transport synoptic charts prepared for the same observation time as the radiosonde launch (0400 P.S.T.) were examined to determine i f air/mass conditions at the radiosonde s i t e s were obviously d i f f e r e n t from Vancouver. The r e s u l t s of these computations are presented i n Table V-9. It w i l l be noted that there i s a general tendency for needle ice events to occur when the depth of the p r e c i p i t a b l e water column declines. It i s also i n t e r e s t i n g to note that needle ice events quite frequently occur toward the end of a low p r e c i p i t a b l e water s p e l l a f t e r there has presumably been s u f f i c i e n t time for the s o i l surface to cool to a temper- ature near the ice point. Synoptic conditions during the eight needle ice event groups i n this period were examined on United States Weather Bureau d a i l y charts to determine i f rec u r r i n g patterns leading to needle ice events could be recognized. The r e s u l t s of t h i s i n v e s t i g a t i o n are shown i n Table V-10. TABLE V-10. AIR MASS DATA 1966-67 Event Date A i r Mass Type* C i r c u l a t i o n over Vancouver 10-11 Nov. GA Front near/ P a c i f i c High SW quad. High 26 Nov. P a c i f i c High NE, NW quad. High 8-9 Dec. cA Front near SE quad. Low 24-25 Jan. cA Front near NE quad. High 31 Jan. P a c i f i c High center High 7 Feb. P a c i f i c High NE, NW quad. High 13-14 Feb. P a c i f i c High NE quad. High 18-21 Feb. cP over Vancouver NE, NW quad. High *cP,cA (continental polar and a r c t i c a i r masses) 76 Although this record i s b r i e f i t was possible to recognize two d i s - t i n c t patterns which lead to needle ice events. (1) Those events during which Vancouver was located i n the northeast to northwest quadrants pf a P a c i f i c High as i t crossed the coast moving eastward and (2) the invasion of A r c t i c or Polar a i r into the Fraser Lowland. In addition, there appear to be frequent combinations of these types. This occurs when the lower Fraser Valley i s either located i n a limb or northern sector of a P a c i f i c High and a front of A r c t i c / P o l a r continental a i r i s located within 50-100 a i r miles northeast of Vancouver. Under t h i s l a t t e r condition i t i s im- possible to determine the d i s t r i b u t i o n of cold continental a i r i n the rugge topography of southwestern B r i t i s h Columbia without a more detai l e d study. An examination of the computer output for the s t a t i s t i c a l model during this test period indicated that t h i s model was quite s e n s i t i v e to pre- c i p i t a b l e water depth v a r i a t i o n s and that the f a i l u r e i n event pre d i c t i o n , when events were predicted but did not occur, resulted from r e a l reduc- tions i n p r e c i p i t a b l e water depths over Vancouver. This indicates that p r e d i c t i o n f a i l u r e i s l a r g e l y the r e s u l t of near surface and s o i l condi- tions and not a model f a i l u r e i n estimating gross thermal r a d i a t i o n condi- tions, excluding r a d i a t i o n fog, from l i m i t e d cloud cover data. In summary, a two variable discriminant function was developed from the standard weather data. This model was correct i n placing a night i n the event or non-event group better than 757» of the time. In addition the model was demonstrated to be quite s e n s i t i v e to r e a l v a r i a t i o n s i n precip- i t a b l e water depth over Vancouver. This analysis i l l u s t r a t e s the overrid- ing influence of the gross thermal r a d i a t i o n environment on empirical models and r e a l event frequency. Model f a i l u r e must l a r g e l y r e s u l t from v a r i a t i o n s i n the near surface micrometeorological environment, which i s the topic of the next chapter. 77 CHAPTER VI. TIME DEPENDENT PROCESSES The e a r l i e r chapters have described the nature of some processes active on event nights and the environmental conditions which accompanied needle ice growth. These observations i n conjunction with the examination of the past event record demonstrated the necessity for an exploration of the nature and i n t e r a c t i o n of the time dependent processes of water and energy transfer near the s o i l surface. This chapter w i l l explore some of these processes with an increased emphasis upon the dynamic nature and i n t e r a c t i o n of the phenomena under in v e s t i g a t i o n . P o l y c y c l i c and Monocyclic Needle Ice Events Two d i s t i n c t types of needle ice growth were recognized, during the 1967-68 observations, those produced by a single cycle of gr'owth and those which continued to grow for several days. The condition" which determines whether a growth w i l l be monocyclic or p o l y c y c l i c i s a complex function of t e r r a i n and climate. The observed cases of p o l y c y c l i c growth were on a north facing water saturated road bank next to a forest margin and i n a flower bed on the northern side of a four foot wooden fence. These two s i t e s had one major feature i n common. Both were located where they were ' shaded from d i r e c t beam r a d i a t i o n during most of the daylight hours. The e f f e c t of this shading i s e a s i l y demonstrated by shading the net radiometer at the study s i t e . This net radiometer was shaded from beam ra d i a t i o n between 1345 and 1350 P.S.T. on the afternoon of 10 March 1968. The r e s u l t was a change i n the net r a d i a t i o n flux from 290 mly./min. 78 directed toward the surface to a value of 97 mly./min. directed away from the surface toward the cloudless sky. The r a d i a t i o n balance at shaded s i t e s i s determined by the thermal r a d i a t i o n budget and the absorption of d i f f u s e hemispherical short wave radi a t i o n , which i s usually assumed to have a value i n the neighborhood of 10% of the absorbed beam ra d i a t i o n . As a further i l l u s t r a t i o n , i t was noticed that while the needle ice at the back fence flower bed was often p o l y c y c l i c , a s i m i l a r flower bed next to the south wall of the house, exposed to beam ra d i a t i o n , was producing a sequence of monocyclic growths which completely melted during the day- l i g h t hours. It would seem that an i d e a l s i t e for the development of large p o l y c y c l i c deposits would be one with a p l e n t i f u l water supply i n a p o s i t i o n shielded from beam r a d i a t i o n during the cold season, namely those conditions which ex i s t at north facing wet bank s i t e s . Photographs of monocyclic and p o l y c y c l i c growths are presented i n Figure VI-1. The new (youngest) layers of the compound growths are at the base, i n contact with the s o i l which i s the source of water for :growth ( F u j i t a et. a l . , 1937). There may be an exception to this rule when impermeable frozen ground thaws at the surface during the day and ice needles form during the following evening. The diurnal layers of a p o l y c y c l i c deposit are usually separated by a t h i n layer of ice which runs h o r i z o n t a l l y between the v e r t i c a l l y oriented needles of d i f f e r e n t ages and i s sub-parallel to the s o i l surface. This layer i s believed to form during the daylight hours when energy sources at the surface diminish the heat flux through the needle ice and the thermal gradient i n the needle ice i s not s u f f i c i e n t to transport a l l of the heat a r r i v i n g from the warm s o i l layers at depth. As a r e s u l t melting occurs  80 at the base of the needles. This hypothesis i s substantiated by s o i l heat f l u x measurements made during the extended period of needle ice growth which ran from 10 December 1967 through 21 December 1967, when p o l y c y c l i c growth was occurring beneath a concrete s o i l - i c e crust at the study s i t e . During the daylight hours the upper s o i l heat f l u x disc (pre-heave p o s i t i o n at a depth of 0.3 cm.) was showing no heat flux, i n d i c a t i n g that i t had become entrapped i n isothermal melting needle i c e j the lower disc (pre- heave depth 3.3 cm.) was recording a heat flow directed toward the surface and the base of the needles. Values recorded during a t y p i c a l period of this type are presented i n Table VI-1. TABLE VI-1. SQiEL HEAT FLUX TOWARD THE SURFACE, 13 DECEMBER 1967. Time (P.S.T.) Upper Heat Flux (mly./min.) Lower Heat Flux (mly./min.) 0800 1000 1200 1400 1600 1800 15 00 00 00 00 27 11 11 11 11 11 11 Other measurements i n the 1967-68 series showed that even during ex- tended p o l y c y c l i c events the surface temperature frequently climbed above the ice point, i n d i c a t i n g that the needle ice was melting from both above, due to heat imput from the atmosphere and solar r a d i a t i o n , and below, by conduction of heat from the warmer s o i l layers at depth. Typical temper- ature data of this type are shown i n Table VI-2. using the o r i g i n a l probe 81 depths as p o s i t i o n i n d i c a t o r s . Only the surface probe p o s i t i o n and the 3 cm. spacing between the buried probes, which are attached to the ends of copper tubes set i n a perspex slab; are r e l i a b l e due to heave. TABLE VI-2. SOIL TEMPERATURES, 16 DECEMBER 1967. Time S o i l Temperatures (°C.) (P.S.T.) 0.0 cm. 0.3 cm. 3.3 . 0800 -0.8 0.0 0.5 1000 2.8 0.3 0.4 1200 5.1 1.7 0.4 1400 1.6 1.6 0.5 1600 0.0 0.0 0.5 1800 -0.6 0.1 0.5 2000 -0.7 0.1 0.5 2200 -1.3 -0.2 0.5 These data demonstrate the ablation of needle ice from both above and below. Further, these data indicate that at the time of measurement the probe o r i g i n a l l y buried at a depth of 0.3 cm. was near the base of the needle ice when the surface began to thaw at midday. The lower probe i s w ell below the needles i n a s o i l layer which i s well insulated from surface thermal disturbances by the diurnal growth and ablation of the ice needles above. The downs lope movement of material overlaying needle ice growths and the d i s t i n c t i v e ground surface patterns r e s u l t i n g from needle ice ablation may be produced largely by the s t r u c t u r a l f a i l u r e of the needles at the base due to basal melting and not a gradual melting commencing from the 82 top. It should be noted that, although the destructive e f f e c t s of needle ice growth on plant materials have been a topic of extended discussion, these same materials below the needle ice layer are protected from both sub freezing temperatures and large diurnal temperature extremes by the growth and ablation of the ice needles above them. Clear Sky Non-Events There are occasions when, on a cle a r or near clear night during the needle ice season, nucleation f a i l s to occur and the s o i l surface tempera- ture becomes quasi-stable a few degrees above the realm of the nucleation temperature (-2°C). This occurred during the night of 26-27 February 1968. A photograph of the study s i t e on that evening i s presented as Figure VI-2. The cloud cover varied between 10% and 307», but the surface temperature never undercooled to the nucleation point. Cloud cover condi- tions were quite s i m i l a r to the event nights of 16-17 February 1968 and 15-16 February 1968. This problem may be attacked using the equilibrium temperature model described i n K r e i t h (1967), which i n e f f e c t states that the heat load across a surface w i l l be balanced at some equilibrium temperature (To)• In the needle ice problem there are two thermal c o n d u c t i v i t i e s (Ka,Ks) and two "distances from the surface ( Z a , Z s ) . It i s known that the thermal, conduc- t i v i t y of the a i r increases with wind v e l o c i t y and that the thermal*, con- d u c t i v i t y of the s o i l i s a complex function of s o i l structure and water content. The a i r (T a) and s o i l (T s) temperatures at some distance from the surface are a d d i t i o n a l components of the problem. The thermal radia- 83 Fig.VT-2. Hie study site on 26 February 1968 84 ti o n balance (^[Tgi^y - T 0 ] ) i s the remaining major component of the noc- turnal energy transfer system, neglecting latent heat to simplify the problem. The s i m p l i f i e d nocturnal energy conservation equation may be expressed i n the form of Equation VI-1. Eq. VI-1. 'Tsky " <rTo + ^s. (T s - T 0 ) + Ka ( T a-T 0) ̂  0 ^a The t h i r d term, i n Equation VI-1., i s known from measurements i n March of 1967 to have values i n the realm of 52 mly./min. on clear nights with the fourth term ranging between 26 and 96 mly./min. under the same condi- tions. It i s also known that as a prerequisite f o r needle ice growth, the ground surface temperature must descend to the region of -2°C. It i s thus possible to use this value (-2°C.) i n computing the second term. There- fore, needle ice events can be expected when the sky hemisphere has a radiant temperature between -15 and -20°C. (or below). This hypothesis i s i n agreement with Stoll-Hardy radiometer (see appendix) measurements on the three nights i n question (see Table VI-3.). TABLE VI-3. SKY RADIANT TEMPERATURES Date Radiant Temperature of Sky Hemisphere (°C.) Outcome 15- 16 February 1968 16- 17 February 1968 26-27 February 1968 •23 •21 • 9 needle ice needle ice no event 85 The Sequence of Time Dependent Phenomena It was discovered that c e r t a i n important sequences during needle ice events could be e a s i l y recognized from the environmental conditions recorded at the study s i t e . These sequences are i l l u s t r a t e d by data taken during the needle ice event of 5-6 February 1968 (see Figure VI-3.). The same time sequence occurred on each of the following eleven nights producing a twelve event s e r i e s . The events occurred during a period of clear weather, Monocyclic needle ice formed and was completely melted during each diurnal cycle. Total melting i s indicated by the return of the heave meter (see appendix) to the o r i g i n a l s t a r t i n g p o s i t i o n and v a r i f i e d by a sudden i n - crease i n the heat flow across the upper d i s c . Further evidence of the complete melting of a l l ice n u c l e i , which indicates monocyclic events, i s the diurnal r e g i s t r a t i o n of an ice nucleation thermal "snap back" (see Jumikis, 1966) on both the surface temperature trace and the s o i l heat flux record. The twelve event record i s presented i n Table VI-4. It w i l l be noted that several features of these events are regular during t h i s twelve day cl e a r weather s p e l l , namely the net r a d i a t i o n balance times are close. Further, there i s considerable r e g u l a r i t y i n the times when the s o i l heat f l u x turned toward the surface i n the late a f t e r - noon, although t h i s event i s a function of turbulent exchange magnitudes and d i r e c t i o n s , i n addition to the net r a d i a t i o n balance. The events which follow the s o i l heat flux balance show a wide scatter i n t h e i r times of occurrence as the energy transfer environment i s determined on clear nights by va r i a t i o n s i n the thermal properties of both the s o i l (e.g. heat capacity varies with water content) and the atmosphere (e.g. eddy d i f - 1 / J • 1600 2000 0000 0< \00 0800 12 t{ P.S.T.) FigeVI - 3 e Net radiation (B), s o i l heave, surface temperature ( T Q ) , and s o i l heat f l u x (S) at 0»3 cnu during the needle ice event of 5=6 February 1968. TABLE VI-4. SEQUENCES IN MONOCYCLIC EVENTS, 5 Net Rad. Neg. SHF Pos. S o i l Surf, cools to 0°C. Ice Nucleation (temp.) Ice Nucleation (SHF) S o i l Heave Begins Net Rad. Pos. Surface warms to 0°C. S o i l Heave Max. SHF st a r t s steep climb Surf. temp.starts steep climb SHF Neg. Heave decline stops 5-6 6-7 7-8 8-9 1615 1615 1600 1610 1605 1550 1615 1620 2015 2145 1955 2050 2100 2315 2105 2240 2100 2305 2050 2240 2215 0000 2330 0045 0835 0835 0840 0835 0830 0915 0900 0915 0845 0940 0915 0920 1130 1110 1040 1015 1130 1105 1050 1015 1140 1120 1120 1040 1120 1110 1110 1110 17 FEBRUARY 1968 (P.S.T.) 9-10 10-11 11-12 12-13 13-14 14-15 15-16 16-17 1610 1600 1615 1610 1610 1610 1615 1615 1620 1615 1630 1645 1630 1615 1620 1625 2145 2220 2040 2210 2110 1835 1805 1815 2340 2355 2155 2255 0040 2040 1855 1940 2330 0000 2150 2250 0045 2045 1855 1940 0230 0315 0020 0310 0400 2315 2230 2350 0835 0845 0830 0840 0845 0840 0845 0800 0925 1000 0915 0915 0920 0930 0925 0825 0850 0915 0945 0935 0940 0945 1010 0940 0900 0930 0930 0920 0945 1015 1030 1115 0950 1000 0940 0930 0940 0930 0925 0950 1010 0945 0940 0950 1020 1040 1145 1010 1020 1115 1030 1035 1130 1150 1515 88 f u s i v i t y varies with wind v e l o c i t y ) . Between nucleation and the time at which noticeable heave begins; the s o i l - i c e cap, which w i l l ride atop the growing needles, forms. The depth of this cap i s dependent upon the sur- face s o i l heat flux and s o i l water conditions just below the surface. During this period heave i s l i m i t e d to a maximum value of 10% of the pro- duct of the depth of the freezing plane, and the s o i l water content. The most s u r p r i s i n g item i s the temporal r e g u l a r i t y of the completion of needle ice ablation during t h i s measurement s e r i e s . It has been noted by Mackay (1966) that the lineations on bare s o i l surfaces that appear during the midday, on days following needle ice events, have approximately the same o r i e n t a t i o n as the shadows from the sun at 1000 P.S.T. However, T r o l l (1944) believed these features to be the products of wind action. This measurement series indicates that this time i s p r e c i s e l y centered on the period of needle ice ablation. Thus, the process which re s u l t s i n the li n e a t i o n s would be due to needle ice ablation although the precise process mechanics are obscure. Temporal Patterns of S o i l Water Tension During the 1967-68 observations; c f s o i l water tension was measured at a depth of 5 cm. using the tensiometer described i n the appendix. Two rather persistent time dependent patterns appeared i n these data which are of importance i n the needle ice problem. These are the reduction of s o i l water tension during the late afternoon hours and the tension increase associated with ice segregation. Both phenomena occur during clear weather i n the needle ice season. 89 On the cl e a r sunny afternoons which characterize needle ice days during the cold season the s o i l water tension r i s e s i n the early afternoon and then declines r a p i d l y shortly a f t e r the s o i l heat flux.turns toward the surface. Records taken during the cl e a r period of 28-29 February 1968 w i l l demonstrate this pattern which has been given considerable attention by Cary (1965, 1966). TABLE VI-5. SOIL WATER TENSION Time S o i l Water Tension Date (P.S.T.) (mm.Hg.) 28 February 68 0900 50 28 February 68 1400 82 28 February 68 1600 79 28 February 68 1715 77 29 February 68 0830 52 The pattern appears to be produced by evaporation early i n the a f t e r - noon. At th i s time, the thermal and water content gradient components of s o i l water flow are opposed and the evaporation rate i s so great that there i s a progressive decrease i n the s o i l water content as the surface i s approached. This produces a tension increase at the measurement l e v e l , due to an advancing drying front. Later i n the afternoon the thermal gradient near the s o i l surface becomes negative and s o i l temperature i n - creases with depth, causing both the thermal and water content flow com- ponents to be directed toward the surface. A s o i l water tension decline i s frequently recorded shortly a f t e r t h i s event. This tension decline would appear to represent the e f f e c t of the thermal component of the s o i l water flow overriding evaporation losses i n the presence of continued flow 90 down the water content gradient, as the evaporation rate decreases with surface cooling. A further c h a r a c t e r i s t i c of t h i s pattern i s i l l u s t r a t e d i n Fig.VI-4., which i s derived from data taken on the c l e a r non-event night of 26-27 February 1968. It w i l l be noted that the tension decrease was at f i r s t rapid, s t a b i l i z i n g at about 2000 hours, and slowly d e c l i n i n g there- a f t e r . This pattern i s believed to r e s u l t from the increase i n measure- ment l e v e l enthalpy produced by the v e r t i c a l flow of warm water from lower s o i l layers to the measurement l e v e l . The thermal gradient which i n i t i - ated the flow is weakened by the a r r i v a l of warm water from the lower s o i l zones which are under the influence of the diurnal heat wave. It would appear that the slope break i n the tension-time curve at 2000 hours re- sulted from both a reduction i n the thermal gradient and the response of the s o i l water f l u x rate to the reduced nocturnal evaporation rate, once the e f f e c t i v e conduit diameter has been increased by the decline i n s o i l water tension (see Fig.IV-4.). The e f f e c t of water flow i n reducing the rate of cooling i n the s o i l layers beneath growing needle ice was noted by Kinosita et. a l . (1967) who calculated the v e r t i c a l v e l o c i t y of s o i l water flow to the freezing plane from the notable reduction i n subsurface cooling rates. The major importance of the thermal gradient component of v e r t i c a l s o i l water flow i n the needle ice problem i s that this process, i n e f f e c t , wets and prepares the near surface s o i l for a needle ice event because freezing point depression and, perhaps, ice nucleation, (Palmer, 1967), are dependent upon s o i l water tension. Of prime importance, needle ice growth v e l o c i t y i s strongly dependent on s o i l water tension and a low water 91 tension environment i s necessary for the i n i t i a t i o n of needle growth (ic e segregation, a more complete discussion i n Chapter IV.)- Further, at low tensions the hydraulic conductivity of the s o i l i s increased; f a c i l i t a t - ing water flow to the freezing plane. To maintain needle growth s u f f i - cient water must reach the freezing plane to match the fusion rate. If this condition i s not s a t i s f i e d , the s o i l below the f r e e z i n g plane dries, tension increases and ice i n t r u s i o n follows. The second phenomena, a tension increase during ice segregation, i s w e l l documented, but infrequently measured i n the f i e l d (Williams, 1966a). As ice segregation begins there i s a reduction i n the s o i l water content of the unfrozen region beneath the freezing plane producing a tension gradient which drives water v e r t i c a l l y toward the freezing plane. The pl o t of s o i l heave and water tension on two growth-ablation cycles, 6-8 February 1968, (see Fig.VI-5.) demonstrates the range of diurnal s o i l water tension v a r i a t i o n s during a twelve day sequence of monocyclic needle ice events. This diurnal sequence produced low water tensions a f t e r abla- t i o n . It should be noted from this record that s o i l heave begins just a f t e r the tension increase which indicates dessication due to ice segrega- ti o n above the water tension measurement l e v e l at a depth of 5 cm. During the time period between nucleation and s o i l heave, the freezing plane i s assumed to be advancing downward, forming the concrete s o i l - i c e cap structure without segregation. A major fac t i s that the highest tensions recorded at the 5 cm. depth were recorded during needle ice growth. This indicates that the water content ( s o i l water tension) gradient does p r o h i b i t ice segregation when there i s s u f f i c i e n t time for the freezing plane to descend below the upper most s o i l layers into the region 2 cm. below the surface. However, above E u N 10 20 5 ^ J \ 1600 2000 0 0 0 0 t (P.S.T.) Fig.VT-4, Th© time dependent variation of soil temperature isotherms (°G«) and water tension, 26-2? Feb. 1968, 2000 0000 0400 0800 J200 1600 2000 0000 0400 0800 1200 t (P.S.T.) F i g o V I ° 5 ® Heave and s o i l water tension^ 6=8 Feb 81968 0 94 the measurement l e v e l there must frequently be steep tension gradients produced by evaporation e s p e c i a l l y during periods when the s o i l thermal gradient i s weak and there i s no prolonged pre-soaking flow of s o i l water toward the surface p r i o r to nucleation. On several occasions when the f i e l d surface at the study s i t e was "hard frozen" to.a depth of approxi- mately 1 cm. nearly 2 cm. of needle growth was observed at nearby wet roadbank s i t e s . However, i f the freezing plane had descended to s l i g h t l y greater depths (2-5 cm.) lower water tensions would be encountered with increased proximity to the water table and segregation should begin. E v i - dence of a s i m i l a r phenomenon w a s recorded during the extended p o l y c y c l i c r H needle ice event of 10-21 December 1967 and noted on both the 13 and 15 of that month. On these occasions a s o l i d mass of concrete frozen s o i l 5-10 mm. i n depth was overlying p o l y c y c l i c needle i c e . The surface was of s u f f i c i e n t strength to support a man on foot. At these observa- tions the needles at the base of p o l y c y c l i c growth were "hard frozen" to the underlying s o i l increasing the bearing strength of the overlying materials. This structure resulted from the i n a b i l i t y of the s o i l water fl u x to the freezing plane to match the fusion rate followed by dessica- t i b n below the needles and ice i n t r u s i o n . Note that i n a p o l y c y c l i c growth diurnal ablation i s incomplete and as a r e s u l t the t o t a l amount of s o i l water " t i e d up" i n needle ice i s much greater than i n diurnal mono- c y c l i c growth series i n which the same water i s e s s e n t i a l l y cycled through ii. phase changes. The S o i l Cap Depth The frozen s o i l cap which rides atop growing needle ice has now been introduced. It i s of i n t e r e s t to estimate the depth of t h i s cap given the following assumptions: 95 (1) Assume the unfrozen and frozen s o i l s have roughly equivalent thermal c o n d u c t i v i t i e s (1x10 3cly...sec\. ̂  cm. °C. (2) Assume a mean surface temperature of -2°C. during the formation of the cap, from an examination of f i e l d data. (3) Assume the range of time for cap formation to be 1-4 hours i n agree- ment with the data taken during 5-17 February 1968. (4) From laboratory t e s t i n g i t i s known that ice i n t r u s i o n should not occur at s o i l water tensions less than 100 mm.Hg. The desorption curves (see Chapter IV.) indicate that a reasonable computation value for volume f r a c - t i o n of s o i l water under these conditions would be .25. (5) The f i n a l assumption i s that the Stefan s o l u t i o n for s o i l f reezing depth i s v a l i d i n t h i s case (Jumikis, 1966). This s o l u t i o n s p e c i f i e s that there must be no ice segregation at the freezing plane and that the heat used i n cooling the s o i l i n the frozen zone i s n e g l i g i b l e compared to the heat expended i n fusion. The development of the problem i s presented by Jumikis (1966) and using the thesis notation i s as follows: The cap could be expected to be between 8-17 mm. i n depth a f t e r 1-4 hours of growth. This figure i s i n general agreement with the heights of the caps (1-2 cm.) on samples c o l l e c t e d during the December 1967 poly- c y c l i c event and indicates the general u t i l i t y of the Stefan s o l u t i o n i n the period of ice i n t r u s i o n between nucleation and segregation. I t should also be noted that the cap depth varies inversely with the square root of the s o i l water content once the ice i n t r u s i o n tension i s exceeded. This Eq.VI-2 96 indicates that a study of cap depths, and the c l i m a t o l o g i c a l conditions which preceded t h e i r formation, may be a rewarding object for future i n v e s t i g a t i o n . In summary, the s o i l cap indicates that water tensions i n that zone were above the c r i t i c a l value for ice segregation. The base of the cap i s p r e c i s e l y the l e v e l at which the v e r t i c a l l y descending freezing plane f i r s t encountered s o i l water tensions low enough for segregation to begin, under the condition that s o i l water content decreases from the water table to the s o i l surface. Further the evidence, so interpreted, indicates the presence of a strong water tension gradient just below the s o i l surface and, by inference, evaporation i n the near surface s o i l layers during and p r i o r to needle ice growth. This hypothesis i s i n agreement with both macro and micro scale studies of the c l i m a t o l o g i c a l environment of needle ice events. F i n a l l y , the temporal v a r i a t i o n between nucleation and notice- able heave i n the records of the monocyclic event sequence (see Table VI-4.) must represent i n t e r v a l s of normal freezing (ice intrusion) which produce the s o i l - i c e cap and indicate the range of influence exerted by water ten- sion gradients near the s o i l surface under late winter conditions. The Equilibrium Temperature Model A further analysis of the three c l e a r nights during February 1968, when cloud cover conditions and wind speeds were quite s i m i l a r , was at- tempted using an equilibrium temperature model which employed estimates of the values of the sensible and latent heat flux using the li n e a r wind model (see Chapter I I I . ) . The r e s u l t i n g computation form of this equation, i s presented as Equation VI-3. (reference Eq.VI-1.). 9*7 Eq. VI - 3 . < r ( T s k y + 273.15) 4 " CT(T 0* 273.15) 4 + .0005 ( T s 2 - T G) + .022 ( T a 5 5 - T G) -U 9 0+.036 ( e a 5 5 - eo<5)>-U90 = 0 where: U(meters/sec.), T(°C.)> e(mb.) It was also possible to treat the surface vapor pressure as a function of surface temperature only, i f the assumption i s made that the surface water i s held at a low tension. This assumption i s i n agreement with f i e l d observations of s o i l water tension p r i o r to needle ice growth. Thus, i f the surface vapor pressure i s assumed to be at the saturation vapor pres- sure at the surface temperature, i t i s possible to approximate this r e l a - tionship over the -10 to 10°C. temperature range by a simple logarithmic expression. Eq.VI-4. l o g 1 0 e 0 = 9.361 - [2341 / ( T Q + 273.15)] The sky hemisphere temperatures used i n the c a l c u l a t i o n s were taken using the Stoll-Hardy radiometer i n the manner described i n the appendix. The expression i n Equation VI - 4 . , may be rearranged grouping a l l the terms which are functions of the surface temperature on the l e f t side. Eq.VI-4 0-(T o + 273.15) 4 + .0005 T Q+ [.022 T o +.036 e 0 ] -U 9 0 = C T ( T s k y + 273.15) 4 + .0005 T s 2 + [v022?.Ta55 + v036 ea551 -U 9 0 It i s apparent that the l e f t side of the expression i s a function of surface temperature and wind speed only,., and thus, can be computed for incremented steps of wind speed and surface temperature. Tables of these 98' l e f t side values (ly./min.) were produced f o r wind v e l o c i t i e s ranging from 0-5 meters per second using 0.1 meter per second steps. Each table fo r a f i x e d wind v e l o c i t y contained l e f t side values for -10 to 10 °C. temperature range. These tables were prepared using the f a c i l i t i e s of the University's Computing Centre. It i s then possible to estimate the equi- librium surface temperature by s u b s t i t u t i n g the measured values i n the r i g h t side of the expression and then to consult the table prepared for matching wind v e l o c i t y conditions. The res u l t s for the three nights are as follows: using data taken at 2200 hours. TABLE VI-6. EQUILIBRIUM SURFACE TEMPERATURES, 2200 P.S.T. Date Temperature (°C.) Outcome 15 February 68 -3.3 needle ice 16 February 68 -3.2 needle ice 26 February 68 2.8 no event The measured surface temperatures during these same nights are reported i n Table VI-7. TABLE VI-7. MEASURED SURFACE TEMPERATURES, 2200 P.S.T. Date Temperature ( fC. ) Ground Condition 15 February 68 -1.0 j" concrete ice j 16 February 68 -1.4 concrete ice 26 February 68 4.1 unfrozen 9§ It should be restated that the computation values used i n the r i g h t side of the expression were i n no way derived from the measured surface temperature (e.g. the r a d i a t i o n temperature of the sky was determined by methods independent of surface temperature and net r a d i a t i o n ) . The equilibrium temperature values appear to be i n reasonable agreement with the measured surface temperatures. This can be demonstrated further by examining the time dependent values of equilibrium surface temperature i n comparison to Stoll-Hardy radiometer and thermistor probe measurements of the surface temperature on the two nights of 16 and 26 February 1968. TABLE VI-8. EQUILIBRIUM AND MEASURED SURFACE TEMPERATURES, FEB. 68 Equilibrium* Thermistor Radiation Time Temperature Temperature Temperature Date (P.S.T.) ( P Q (°C.) (°C.) 16 1800 -1.6? 0.4 -2.3 16 2000 -3.5 -0.4 -0.3 16 2200 -3.1 -1.4 -1.0 16 2400 -3.4 -1.1 -1.1 26 1800 6.5 7.5 5.2 26 2000 5.4 5.9 4.4 26 2200 2.8 4.0 5.4 *T-testing showed no s i g n i f i c a n t departure from measured values at the 5% l e v e l . In conclusion, the equilibrium temperature model appears to be a powerful t o o l f o r l i n k i n g f r o s t events to the c l i m a t i c environment (e.g. during the two event nights reported i n Table VI-6. the estimated equi- librium temperature was below the ice nucleation temperature of approxi- 100 mately -2 C ) . However, more data i s necessary to evaluate the time de- pendent behavior of the model. The Environment of a Late Winter Needle Ice Event During the night of 16-17 February 1968 manual measurements of s o i l and a i r temperatures were c a r r i e d out at the study s i t e . These data, when combined with the recorded data, gave a deta i l e d picture of the needle ice event. Temperatures at 120 cm., 55 cm., and the surface, as well as the a i r dew point, wind speed, and cloud cover during the observation period are i l l u s t r a t e d i n Fig.VI-6. Note that the dew point i s normally above the surface temperature i n d i c a t i n g that evaporation predominates during t h i s event, which i s consistant with the pattern of e a r l i e r measurements (see Chapter I I I . ) . Only for a short time i n t e r v a l from 0045 to 0330 P.S.T. i s there any i n d i c a t i o n of condensation at the surface. Another i n t e r e s t i n g item i s the s t a b i l i t y of the a i r temperature gradient and the wind v e l o c i t y during the period of needle growth. Using the equilibrium temperature model, i t i s possible to use wind v e l o c i t y , temperature and humidity data, with net r a d i a t i o n records to estimate the temporal v a r i a - t i o n of the s o i l surface heat flux, treated as a r e s i d u a l . These data are reported i n Table VI-9. where p o s i t i v e values indicate heat f l u x toward the surface from either above or below. 101 10 T3 O - C O o 0400 0800 t (P.S.T.) Fig»YI-6» Conditions over bare s o i l at the study s i t e , February l6«=17s 1968, Subscripts refer to distance (cnu) above surfacea 102 TABLE VI-9. ENERGY EXCHANGE COMPONENTS, 16-17 FEBRUARY 1968 Net Sensible Latent S o i l Surface Time Radiation Heat Flux Heat Flux Heat Flux (P.S.T.) (mly./min.) (mly. /min.) (mly. /min. ) (mly. /min. ) 1600 53 - 53 -131 131 1800 -106 80 - 54 80 2000 - 88 26 - 12 74 2200 - 88 45 - 5 48 0000 - 88 51 - 17 54 0200 - 79 43 12 24 0400 - 62 24 - 2 40 0600 - 72 68 - 22 26 0800 00 36 - 8 | 28 It i s known that these values present only a "rough sketch" of the actual energy transfer environment due to the use of an unventilated net radiometer, the l i n e a r wind model, and changing s o i l thermal properties, etc. However, the estimated pattern of the directions and r e l a t i v e magnitudes of the components of energy transfer i s temporally consistent with observations of dew formation and subsurface temperature. The pat- tern i s one of a diminishing rate of heat flow toward the s o i l surface. The subsurface isotherms for this period are presented i n Fig.VI-7. These data i l l u s t r a t e the descent of the diurnal heat pulse as i t produces a strong temperature gradient near the surface during the early evening houj?s and also i l l u s t r a t e the e f f e c t s of s o i l water flow toward the sur- face i n the upper 10 cm. of the s o i l . The weakening of the s o i l thermal gradient through time, which i s consistent with both the predicted heat 103 1600 2000 0000 0400 0800 t(P.S.T.) Fig.VI - 7 . S o i l temperature isotherms (°C), 16-17 Feb.68, 104 f l u x and needle ice growth below the former surface, i s worthy of further consideration. The presence of both a thermal and a tension induced water flow to the freezing plane during needle growth was discussed e a r l i e r . With the aid of the data now under examination i t w i l l be possible to explore the e f f e c t s of these water flows upon the time de r i v a t i v e of s o i l tempera- ture at the 5 cm. l e v e l . A general equation for the v a r i a t i o n of tem- perature at a point with time, i n which the c a l c u l a t i o n value of thermal d i f f u s i v i t y i s assumed to have been measured under conditions where the e f f e c t s of s o i l water flow were eit h e r minimal or canceled due to p e r i o d i c i t y , can be written following Kinosita et. a l . (1967): Eq.VI-6. dT / a t - ocCa2T/aza)+\Jw(u/cs)(9T/5»z) The laboratory and mean annual s o i l d i f f u s i v i t y were examaned and a -3 "oi "-" computation value of 5x10 cm. '/sec", discussed e a r l i e r , was used i n the computations of the f i n i t e difference form of Eq.VT.6. discussed by Kreith (1967). An estimate of the volumetric heat capacity of the s o i l at a -3 -1 tension of 120 cm.H^O (see Chapter IV.) of .38 cal.cm. °C7 was used i n these c a l c u l a t i o n s . Eq.VI-7. AT 5/At=C([(T 0 - 2T 5+ T 1 0 ) / ( A Z ) 2 ] In the computations the i n i t i a l surface- temperature was assumed to be 0°C. a f t e r nucleation at 1940 P.S.T. The r e s u l t s are presented i n Table VI-10. 105 TABLE VI-10. ESTIMATED TEMPERATURES AT 5 CM., 16-17 FEBRUARY 1968. Time Interval (P.S.T.) Temperature Change Predicted (°c.) Temperature Change Obs.* (°c.) P r e d i c t i o n Corrections ( ° C ) 1600-1800 -1.3 -1.2 0.1 1800-2000 -6.2 -2.1 4.1 2000-2200 -3.3 -1.3 2.0 2200-0000 -1.3 0.2 1.5 0000-0200 -1.9 -0.4 1.5 0200-0400 -1.2 -0.1 1.1 0400-0600 -1.2 -0.3 0.9 0600-0800 -0.7 -0.2 0.5 *T-testing showed s i g n i f i c a n t differences at the .5% l e v e l . The c o r r e c t i o n which must be applied to the predicted temperatures to obtain the observed values must represent the thermal e f f e c t s of ver- t i c a l s o i l water flow which can be expressed i n a f i n i t e difference form as follows ( r e f . Eqs. VI-6., VI-7.): Eq.VI-8. A T 5 / A t = ;(C W/C S) [ ( T 1 0 - T o ) / A Z] U W When this equation i s solved f o r flow v e l o c i t y i n units of millimeters perMiour the r e s u l t s are compared to s o i l heave rates as follows: 106 TABLE VI-11. WATER FLOW AND SOIL HEAVE, 16-17 FEBRUARY 1968, Estimated Water Flow S o i l Heave Time V e l o c i t y at 5 cm. Ve l o c i t y (P.S.T.) (mm./hr.) (mm./hr.) 1600-1800 1 0.0 1800-2000 20 0.0 2000-2200 9 0.0 2200-0000 9 0.0 0000-0200 9 1.4 0200-0400 7 1.3 0400-0600 7 1.4 0600-0800 4 1.0 Recognizing noise sources r e s u l t i n g from water f l u x divergence, s o i l geometry, evaporation etc., the order of magnitude and time dependent response of water flow and heave v e l o c i t y appear to be s i g - n i f i c a n t although experimentally p r i m i t i v e . I f the r e l a t i v e magnitude of the v e l o c i t y estimates i s accepted, and the assumption i s made that the water flow conduit area i s constant through time, the early evening water f l u x down the temperature gradient has twice the magnitude of the flow maintained by desiccation at the freezing plane under the r e l a t i v e l y high tension gradients during needle ice growth. The flux of warm water to the s o i l surface may be the mechanism which produces a p o s i t i v e bump just p r i o r to undercooling i n s o i l surface temperature traces (see Figs.II-2., 11-12.) and breaks the smooth parabolic noctural tem- perature decline produced by r a d i a t i o n cooling. It i s i n t e r e s t i n g to 107 note that t h i s same bump appears i n Funk's data (1962, F i g . 3 . ) and may be a universal phenomena associated with the r a d i a t i o n cooling of moist s o i l surfaces. These data also indicate the importance of water content conditions i n the near surface s o i l layers. The highest water flow v e l o c i t y observed during growth was i n the realm of 1 cm./hr. It follows that i f growth continued for 10 hours water from the 10 cm. layer would be a r r i v i n g at the freezing plane when growth stopped. It can, therefore, be assumed that the growth environment for needle ice i s dependent upon s o i l water tension and the pore geometry of the upper 10 cm. of the s o i l and that even within t h i s layer dependence r a p i d l y decreases with depth. I n i t i a l estimates of the energy used i n needle growth (fusion rate) were ca l c u l a t e d using the nucleartion "snap back" of the upper heat flux disc as an ind i c a t o r of the rate at which energy was withdrawn for fusion. The d i u r n a l heave recorded (see appendix) during the twelve event sequence, 5-17 February 1968, was also used i n the problem. The s o i l heat flux snap back (ly./min.) was m u l t i p l i e d by the time (min.) between nucleation and the time of the i n f l e c t i o n point i n the s o i l heave record when ab l a t i o n begins ( i . e . maximum s o i l heave, see F i g . V I - 3 . ) . Estimates of the t o t a l fusion heat were between 5 and 13 l y . during these twelve nights. These values divided by 72 l y . estimate the height of pure ice (cm./cm.). It follows that t h i s figure divided by the peak heave height i s an estimate of the bulk density of needle i c e . The r e s u l t of these c a l c u l a t i o n s for the twelve night period was an estimated needle ice bulk density of 0.15— .01 g.cm. . Two independent checks on th i s value were furnished by the determination of bulk density by evaporating melt 10 "8 yater from a 225 cm. monocyclic needle i c e - s o i l sample c o l l e c t e d on _3 26 November 1967 which yielded a value of 0.17 g.cm. and four published measurements (Brink et. a l . , 1967) which showed a mean bulk density of -3 -3 0.19 g.cm. and a range of 0.27 to 0.12 g.cm. for the loosely packed sandy loam at the study s i t e . Here 0.17 g.cm. i s selected as a computa- t i o n value for needle ice bulk density at the study s i t e . If bulk density i s known or estimated, i t i s then possible to estimate the mean fusion rate from the heave rate when ice segregation?, i s taking place. The f o l - lowing computation method was used: Eq.VI-9. F s 72 X i h t " 1 The fusion rate computed by the method outlined above i s plotted with the estimated heat flux through the s o i l surface (from Table VI-9.) i n Figure VI-8. In these computations i t was assumed that the freezing rate p r i o r to s o i l heave was equal to the post heaving rate. This pattern of heat f l u x v a r i a t i o n i s i n good agreement with s o i l temperature data on this night (compare Figs. VI-7. and VI-8.). During the early evening there i s a rather large heat f l u x toward the surface which decelerates i n a roughly parabolic manner as the evening progresses. At the time of nucleation, 1940 P.S.T., the temperature gradient i n near surface zone i s s t i l l rather steep (see Fig.VI-7.) and cooling i n the near surface zone continues u n t i l shortly a f t e r the time when s o i l heaving begins, 2350 P.S.T.. At t h i s time there i s a noticeable warming i n the upper 10 cm. of the s o i l due to the flow of water from deeper (warmer) layers toward the freezing plane. From the beginning of ice segregation (needle growth) nearly a l l of the heat l o s t at the surface represents Fig 0VT»8 a S o i l heat f l u x toward the surface on 16=17 Feb0689 Notes the crossed area indicates the heat expended i n fusions HOC fusion at the base of the growing needles. The remaining heat loss main- tains a s l i g h t decrease i n the surface temperature which tends to preserve a quasi-stable heat f l u x across the growing needles as t h e i r height increases. In summary, the purpose of the observations reported i n this chapter i s the demonstration of some d r i v i n g and feedback mechanisms active i n the needle ice growth system. The two major mechanisms i n the system can be combined into a general equilibrium temperature-ice i n t r u s i o n model for needle ice growth. These mechanisms and feedback, to and from the sur- rounding environment, constitute a general model for needle growth i n unfrozen heave susceptible s o i l s . This model, which i s presented as Figure V I - 9 a l s o demonstrates the sequence of events which produce some of the d i s t i n c t i v e forms of needle ice morphology (e.g. c y c l i c deposits and caps). The model i s developed by asking three questions about the needle ice environment. Sustained needle growth, at an instant, i s indicated by three sequential yes answers. Steps i n the evolution of a needle ice event are completed by escaping from no loops as time passes. However, needle growth may be terminated by a negative (no) condition at any step. The model i s v a l i d for one instant at a point on the s o i l surface (step #1.) and the freez i n g plane (steps #2.&3.). It should be remembered that a f t e r needle growth begins a sudden increase i n the heat f l u x from the freezing plane to the surface can increase s o i l water tension. Thus, there i s an addi- t i o n a l feedback loop from the energy exchange environment at the s o i l sur- face (equilibriunr,;temperature) to the f i n a l question. In i t s present form the model cannot be used as a p r e d i c t i n g device but does accurately specify the state of the needle ice growth environment. T nucleation< yes (1) IS THE SURFACE EQUILIBRIUM TEMPERATURE - BELOW -2°Co? no *no nucleation- thermal water flow during s o i l cooling (2) IS THE \ SOIL WATER TENSION AT needle i c e ̂  yes —THE FREEZING . growth begins PLANE BELOW THE ICE INTRUSION VALUE ? .no- 1 | normal freezing- (3) IS THE SOIL WATER FLUX TO THE -yes-< FREEZING PLANE • MATCHING THE FUSION RATE « continued needle ice growth -*-no- increasing tension Fig0VX<=9o The equilibrium temperature- i c e in t r u s i o n modelo 112 CHAPTER VII. CONCLUSION General Description The p r i n c i p a l contribution of this thesis i s the analysis of needle ice growth under natural conditions and the development of an equilibrium temperature-soil water tension model to explain and pin-point combinations of environmental factors which produce needle i c e . In addition, the tabu- l a t i o n i n s t a t i s t i c a l form of the unique s i x year record of needle ice events c o l l e c t e d by Mr. Don Pearce at the un i v e r s i t y ' s agrometeorological s t a t i o n and analysed by the author i s (to the author's knowledge) the long- est record of diurnal ground surface conditions to be compiled and analysed i n North America. S p e c i f i c a l l y i t i s shown above, i n the equilibrium temperature model, that when micrometeorological conditions (e.g. sky radiant temperature, wind speed, temperature and humidity gradients, etc.) combine i n such a manner as to y i e l d a surface temperature less than -2°C. (approx.) ice nucleation w i l l occur. A f t e r nucleation there i s a r i s e i n temperature, due to fusion e f f e c t s , to the freezing temperature of the s o i l water which i s just below 0°C. Then, the course of s o i l freez- ing i s determined by the s o i l pore size, d i s t r i b u t i o n and s o i l water ten- sion at the freezing plane. If the s o i l water tension i s below a c r i t i c a l value, determined by the pore geometry, at the freezing plane, the descent of the freezing plane w i l l stop and ice segregation w i l l begin. If the s o i l water tension i s above the c r i t i c a l value (estimated between 80-150 mm.Hg. at the study s i t e ) the freezing plane w i l l continue to descend and 113 s o i l water w i l l be frozen i n s i t u ( i . e . normal f r e e z i n g ) . Once the f r e e z - i n g plane has been s t a b i l i z e d v e r t i c a l l y and segregation ( i . e . needle growth) s t a r t e d , the maintenance of the process i s c o n t r o l l e d by the a b i l i t y of the unfrozen s o i l to transport water upwards to the f r e e z i n g plane commensurate w i t h the f u s i o n r a t e ( i . e . the s o i l heat f l u x through the i c e needles and s o i l cap), which i s determined p r i m a r i l y by the energy t r a n s f e r environment at the s o i l s u r f a ce. The mechanisms which draw water to the f r e e z i n g plane are water flow along both a tension and a thermal gra d i e n t . The ten s i o n gradient i s produced by d e s i c c a t i o n due to i c e f o r - mation at the f r e e z i n g plane. The thermal gradient f l u x i s upwards from the warm unfrozen subsurface zone to the c o l d f r e e z i n g plane. I f e i t h e r the f u s i o n r a t e increases because of c o o l i n g c o n d i t i o n s above the surface or the s o i l p e r m e a b i l i t y decreases due to a decreasing supply of s o i l water and i n c r e a s i n g s o i l water tension, water w i l l not a r r i v e at the f r e e z i n g plane i n s u f f i c i e n t q u a n t i t y to keep pace w i t h the f u s i o n r a t e . Therefore, a f u r t h e r d e s i c c a t i o n on the unfrozen side of the plane w i l l occur. I f t h i s process i s continued the s o i l water t e n s i o n both at and below the f r e e z i n g plane w i l l increase and e v e n t u a l l y exceed the c r i t i c a l i c e i n t r u s i o n value. When t h i s occurs, the f r e e z i n g plane w i l l descend, thus, t e r m i n a t i n g needle i c e and l e a v i n g the needles frozen to the s o i l at t h e i r bases. The bump which f r e q u e n t l y i s observed as the surface temperature b r i e f l y increases or s t a b i l i z e s during c l e a r sky r a d i a t i o n c o o l i n g , i s probably the r e s u l t of a thermally d r i v e n s o i l water flow upwards from the warmer s o i l at depth. This a d d i t i o n of water to the near surface zone lowers the s o i l water t e n s i o n and a l s o favors e a r l y needle i c e growth by minimizing the depth to which normal f r e e z i n g w i l l occur. 114 The s o i l cap which rides atop the needles indicates that during normal needle ice weather conditions xrx the s o i l water tension i n the upper 0-2 centimeters of the s o i l i s above the ice i n t r u s i o n value. Under these conditions, both s t a t i s t i c a l studies of past events and energy ex- change measurements indicate that evaporation normally occurs at the s o i l surface j u s t p r i o r to nucleation, the freezing plane then descends through increasingly wetter ( i . e . lower tension) s o i l layers u n t i l the depth of the c r i t i c a l i ce i n t r u s i o n tension i s reached and segregation begins. In short, the presence of the normally frozen s o i l cap i s further evidence that the major difference between needle ice night energy exchange and conditions on non-event nights i s that, i n addition to higher values of the net thermal r a d i a t i o n f l u x from the surface to the sky, evaporation on event nights i s the r u l e . This a d d i t i o n a l heat sink may speed nucleation but leaves the s o i l cap as evidence of increased s o i l water tension due to evaporation p r i o r to nucleation. The v a r i e t y of micrometeorological influences exerted by slope and exposure at a s i t e are i l l u s t r a t e d by the concurrent examination of poly- c y c l i c and monocyclic growth o£ needle i c e . At s i t e s with shaded north facing exposures p o l y c y c l i c growths are common because needle ice ablation was incomplete during the daylight hours due to the absence of beam rad-. i a t i o n as a heat source. It was demonstrated that the net r a d i a t i o n balance was already negative at shaded s i t e s during the early afternoon hours on clear mid winter days. Observations at wetter road-bank s i t e s w i t h i n 100 meters of the study s i t e demonstrated that on some nights (espe- c i a l l y when nucleation occurred a f t e r midnight) when the study s i t e s o i l was normally frozen, the road-banks where the water table intersected the surface were producing up to 2 centimeters of needle i c e . 115 Further, the formation of r a d i a t i o n fog was demonstrated to have a major r o l e i n a l t e r i n g the equivalent sky temperature. On nights when r a d i a t i o n fog formed, the increased sky temperature was shown to ac t u a l l y re-warm the surface. F i n a l l y , i t i s possible to b r i e f l y summarize the experience gained by the s t a t i s t i c a l analysis of the past event record and f i e l d observa- tions during both event and non-event nights by l i s t i n g favorable and unfavorable conditions for needle ice growth at Vancouver with some guide- l i n e values. This l i s t i n g appears as Table VII-1. TABLE VII-1. FAVORABLE AND UNFAVORABLE CONDITIONS FOR NEEDLE GROWTH Weather at 1630 P.S.T. Favorable Unfavorable Radiant Sky Temperature: less than -20 C. Wind Speed: less than 1.2 meters/sec. Cloud Cover: Dew Point: A i r Temperature: S o i l : Texture: Water: Structure: S i t e : Horizon: Vegetation: Slope and Exposure: less than 3/10. less than -2°C. less than 4°C. intermediate near saturation loose at surface unobstructed none (bare) shadowed, north greater than -15°C. greater than 1.5 m./sec greater than 7/10. greater than 3°C. greater than 6°C. pure clay or sand below f i e l d capacity compact at surface obstructed trees and tu r f unshadowed, south 116 Specific Results and Recommendations In summary, the thesis results have indicated that there are certain lines which should be followed by future investigators to further advance the understanding of the needle ice problem to a state of increased p r e d i c t a b i l i t y and process knowledge. The Equilibrium Temperature Model: As the equilibrium temperature model is e n t i r e l y physically based, i t should be possible to improve upon i t and make i t an extremely valuable model for both general frost and needle ice damage forecasting.. I t i s suggested that detailed radiation thermometer measurements of sky and s o i l surface temperatures with p a r a l l e l determina- tions of wind v e l o c i t y at a reference height, s o i l - a i r temperature and humidity be used to establish the relationship between the turbulent ex- change co e f f i c i e n t and reference l e v e l wind v e l o c i t y to produce a " s i t e model" for regular frost forecasting from late afternoon and early evening data. S o i l Water Tension at Nucleation: The research indicated that the s o i l - i c e cap depth was largely determined by the s o i l water tension-depth gradient at nucleation and that at some c r i t i c a l l y high tension needle ice would not form, but rather s o i l water would be frozen i n pore spaces i n s i t u ( i . e . normal freezing). Therefore, the production of an extended measure- ment series y i e l d i n g information about s o i l water tension at nucleation and the s o i l structure after freezing would provide a means of correlating structure with tension. By this means, the realm of c r i t i c a l tension beyond which normal s o i l freezing occurs could be specified at a s i t e . 117. S o i l Water-Temperature E f f e c t s : It was noted i n near saturated s o i l s , that water flow down thermal gradients during surface cooling and toward the freezing plane during needle ice growth produced a recognizable e f f e c t upon sub-surface temperature patterns through time. A d e t a i l e d f i e l d study of these e f f e c t s could f u r n i s h yet another method of i n v e s t i g a t i n g the magnitude of water f l u x i n wet s o i l s . Extreme refinement of this ap- proach might lead to a method of evaporation estimation from below the surface and should fu r n i s h increased p r e c i s i o n i n estimating the proba- b i l i t y of ice segregation at a s i t e . Time Dependence of the A i r Intrusion Value: An analysis of the v a r i a t i o n of the c r i t i c a l a i r i n t r u s i o n value through time as the surface s o i l structure ( i . e . frequency d i s t r i b u t i o n of pore diameters) i s modified by changing weather (e.g. swelling and cracking during wet and dry periods) would define the range of surface s t r u c t u r a l states which a f f e c t the out- come ( i . e . frozen structure) of f r o s t events. This knowledge would i n - crease the p r e c i s i o n of needle ice forecasting as the a i r i n t r u s i o n value i s known to be an index of s o i l s u s c e p t i b i l i t y to ice segregation. Time Dependence of Nocturnal Energy Exchange Components: Net ra d i a t i o n , a i r temperature and humidity, and wind v e l o c i t y are t r a d i t i o n a l l y treated as independent variables which i n concert determine the heat loss at the s o i l surface and, thus, the rate of surface temperature declines. These inve s t i g a t i o n s , however, indicate a strong interdependence among these variables which r e s u l t s i n a continuous spectrum of environmental states which at some point favor needle ice formation. The feedback mechanisms 118 which lead to r a d i a t i o n fog and s o i l water flow down thermal gradients were demonstrated i n the the s i s . As the i n v e s t i g a t i o n of the needle ice process becomes more sophis- t i c a t e d , i t w i l l become necessary to treat these feedback mechanisms i n models which predict t h e i r time dependent behavior or indicate t h e i r pro- b a b i l i t y . In a continued i n v e s t i g a t i o n the c h a r a c t e r i s t i c s of the time dependence and feedback between environmental variables should be ex- plored i n greater d e t a i l . This approach could provide a means for l i m i t - ing the input data necessary i n the equilibrium temperature model to a degree which w i l l make i t tractable as a normal forecasting t o o l . Energy Budgets and Process Geomorphology During the past decade energy budget climatology has become an i n - creasingly prominant research area for workers i n many f i e l d s . Lacking a s p e c i f i c object of a p p l i c a t i o n , these e f f o r t s are ei t h e r channeled into instrument development technology or into a rather demanding sophisticated book-keeping, which often appears to have as i t s object the v e r i f i c a t i o n , ad infinitum, of the energy conservation law. Examples of applied studies i n a g r i c u l t u r e are the current i n v e s t i - gations of evapotranspiration and f r o s t damage which both employ energy exchange.climatology as a major t o o l . In geomorphic research the applica- t i o n of energy budget methods i s less frequent. 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(1968), Properties and Behaviour of Freezing S o i l s : Norges Geotekniske I n s t i t u t t , Publications NR72, 119 pp. Yong, R. N. (1966), Warkentin, B. P., Introduction to S o i l Behavior: Macmillan Series i n C i v i l Engineering. 125 APPENDIX: MEASUREMENT Temperature Measurement: I. Thermistors 1. Probes: Y. S. I. "400" Series bead thermistors. Yellow Springs Instrument Co., Yellow Springs, Ohio. 2. Bridge: Wheatstone bridges with f i x e d arms of 7.5K ohms and va r i a b l e arms (0-10K ohms and 0-100K ohms) were constructed. These bridges are c a l i b r a t e d by f i x i n g the d i a l potentiometer and a decade resistance at 7355 ohms (0°C.) and then adjusting a small potentiometer i n the opposite arm of the c i r c u i t u n t i l no out of balance current was registered by the galvanometer. A IK ohm r e s i s t o r i s placed i n series with the bridge power source (a 1.5V. Dry C e l l ) , when the units are used for continuous recording, to lessen the s e l f heating of the probes. Manual out of balance measurements are made with a Simpson 25-0-25 M. A meter. 3. Recording: The bridge out of balance currents are placed across the galvanometer of Rustrak recorders (25-0-25At A and 0-50AL. A) and the d e f l e c t i o n i s c a l i b r a t e d by using a resistance decade i n the place of the probe and the manufacturers resistance-temperature f i g u r e s . The probe interchangeability i s l i s t e d as 2 .2°C. w'ithih^therbemperature range of concern. 126 4. Accuracy: Manual readings of the thermistor probes should be accurate to + .05°C. when the probes have been ice bathed and c a r e f u l attention i s given to s e l f heating e f f e c t s of the probes. Recordings are assumed to have an absolute accuracy i n the realm of i ,.3°C. over the c a l i b r a t e d range (15 to - 1 5 ° C ) . The rates of temperature change, however, have a much higher p r e c i s i o n over l i m i t e d temperature ranges. 5. Switching: I t was possible to record the temperature at several l e v e l s i n the s o i l when a small r e l a x a t i o n o s c i l l a t o r was used to tri g g e r a twelve point stepping switch to which one side of the probe leads wa<sz connected. I I . Thermocouples 1. Probes: Copper-Constantan thermocouples. 2. Reading: The e.m.f. r e s u l t i n g from the temperature d i f f e r - ence between a thermocouple i n the s o i l and one i n an ice water bath was read on a Leeds and Northrup Potentiometer. 3. Accuracy: The absolute accuracy of these readings i s be- lie v e d to be i n the realm of + .4°C, whereas the r e l a t i v e accuracy i s assumed to be somewhat higher ( i . 2 ° C ) . 4. Switching: S o i l thermocouples were switched using an eleven p o s i t i o n rotary switch (Cu). The copper junctions were soldered and the common eonstantan junction was wound. The enti r e switching unit was en- cased i n a f i b e r g l a s s jacket i n an attempt to l i m i t the magnitude of thermal gradients i n the switch. 127 Radiation Measurement: I. Net Radiation A Thornthwaite Net Radiometer (unventilated) was used to monitor net r a d i a t i o n at approximately 1/2 meter above the s o i l surface to reduce problems of fl u x divergence (see Funk (1960, 1962)). The s i g n a l from the radiometer was recorded on a Thornthwaite Net Radiation Recorder and l a t e r on a Thornthwaite Microvolt Recorder, which allowed use of nearly the f u l l scale i n the 300 XuV. range. II . D i r e c t i o n a l Radiation A Stoll-Hardy Radiometer (Williamson Development Co.) with a red f i l t e r over the sensing cone was used to detect differences i n thermal r a d i a t i o n l e v e l s . The "x" scale was c a l i b r a t e d to cover energy d i f f e r - ences of 0-0.3 l y . min. ^. It was hoped that t h i s type of c a l i b r a t i o n would eliminate the errors encountered when the instrument i s not used at i t s c a l i b r a t i o n temperature. The mean sky hemisphere temperature was computed using the nine d i r e c t i o n a l shots with the sensing head, which has a 20° f i e l d of view. The mean sky hemisphere energy difference was com- puted by mul t i p l y i n g the zenith reading, the mean of the four 45° i n c l i n e d readings taken at the four points of the compass, and the mean of four readings i n c l i n e d approximately 10° from the horizontal at the four points of the compass by geometric weighting f a c t o r s . These factors were propor- t i o n a l to the portion of the t o t a l sky hemisphere which the readings re- present. The boundaries of these zones measured from the horizontal were i n c l i n e d h o r i z o n t a l (0-27.5°), 45° i n c l i n e d (27.5° -67.5°) and zenith 128 (67.5° -90°). The appropriate weight factors were .46, .46 and .08. The sky hemisphere r a d i a t i o n temperature was then obtained by subtracting the mean energy difference from the black body function of the c a l i b r a t i o n cube temperature and consulting black body tables to evaluate the sky hemisphere radiant temperature. The same procedure was used i n evaluating the radiant temperature of the s o i l surface. Net r a d i a t i o n values meas- ured during the 15-17 Feb. 1968 series were on the average .02 l y . min. \ lower than the same item recorded by the Thornthwaite system. This d i s - crepancy could a r i s e through c a l i b r a t i o n d r i f t , zero d r i f t and sampling errors i n either or both of the instrument systems. No attempt was made to further evaluate the source of t h i s discrepancy which i s obscured by the a d d i t i o n a l problem of comparing continuous with spot time measurements. Accuracy: I t i s d i f f i c u l t to determine the absolute accuracy of these measurements. The information above indicates that i t would be unwise to attach an absolute accuracy of greater than 20% to the range of nocturnal r a d i a t i o n f l u x values. However, due to the design of the Thornthwaite radiometer the zero value i s quite r e l i a b l e , i f frequent checks are made of the zero c a l i b r a t i o n point on the recorder. S o i l Heat Flux Measurement: S o i l Heat Flux Discs and Recorders Thornthwaite s o i l heat f l u x discs were used with the same manu- facturer's s o i l heat f l u x recorder and microvolt recorders. The use of the l a t t e r i n the study allowed the u t i l i z a t i o n of nearly the en t i r e chart width when the 1000 JCV. range was employed and provided a se n s i t i v e i n d i c a t o r of the sensor output time v a r i a t i o n s . This instrument system 129 has been the subject of much discussion, as i t s design and c a l i b r a t i o n contain several r e s t r i c t i v e assumptions (Portman, D. et. a l . i n Lettau and Davidson (1957) p. 17-79). A test was c a r r i e d out i n the S o i l Physics Laboratory at the University of B r i t i s h Columbia to determine the e f f e c t of the heat f l u x disc on s o i l thermal conditions. A bead thermistor was placed at the same l e v e l (2 cm.) i n the s o i l as a s i m i l a r bead underlying a heat flux d i s c . The moisture tension i n the s o i l sample from the needle ice study s i t e was held constant by a 60 cm. hanging column of water and a heat pulse was delivered to the sample by means of two i n f r a r e d lamps. The temperature r i s e and decline was always i n agreement to t .3°C. which i s the realm of accuracy of the recording system. While t h i s does not eliminate the prob- lem, this r e s u l t indicates that measured heat f l u x values may be expected to be i n the correct realm. During the 1967-68 measurement ser i e s , a disc was placed quite close to the s o i l surface (approx. 3 mm). As the heat flux value normally increases r a p i d l y when the surface i s approached from below, t h i s sensor arrangement should have produced values near to the magnitude of the sur- face f l u x . At this stage i t i s impossible to evaluate the accuracy or even the p r e c i s i o n of these measurements except within the rather narrow confines of design assumptions due to the perpetual v a r i a t i o n of moisture and s t r u c t u r a l conditions i n the s o i l near the surface ( i n fa c t the object under i n v e s t i g a t i o n ) . However, the measurements are of great value i n detecting the times of ice nucleation, heat f l u x d i r e c t i o n a l change, and sudden changes i n heat f l u x rates that c o r r e l a t e with other observed phenomena. 130 S o i l Heave Measurement: I. The Freyman Meter ( b u i l t by S. Freyman, former student i n the Dept. of Plant Sci./U.B.C.) This instrument can best be described as a mechanical thermograph- type drum on which the pen arm i s attached v e r t i c a l l y to the s o i l surface by a drinking straw. The instrument has several d e f i c i e n c i e s . (1) It shields the s o i l beneath i t from a considerable portion of the coldest area of the night sky near zenith. (2) the heave-trace r a t i o i s non-linear and d i f f i c u l t to evaluate due to leaning of the straw once heaving has begun. (3) There i s s u f f i c i e n t drag on the pen to prevent the meltout of needle ice from being recorded. While these l i m i t a t i o n s do e x i s t the unit does give a considerable amount of information at an extremely low cost. The l i m i t a - tions of t h i s unit led to the construction of the unit described below. II . The Schmitt Heave Meter (named for W. Schmitt, C i v i l Engineer- ing Shop, U.B.C, who helped design and then b u i l t the instrument). E s s e n t i a l l y a resistance s l i d e wire device with which the s o i l sur- face i s d i r e c t l y linked mechanically to a wiper: The wiper s l i d e s along the resistance wire which was taken from a discarded v a r i a b l e potentio- meter and straightened. S o i l heave i s d i r e c t l y translated to a resistance v a r i a t i o n which i s converted i n a temperature measurement bridge to an out-of-balance current which i s then recorded. The un i t , as constructed, converts 5 cm. of v e r t i c a l displacement to a f u l l scale d e f l e c t i o n of a 0-50 jUU, A Rustrak recorder and, thus, makes surface elevation v a r i a t i o n s 131 as small as 1 mm. e a s i l y detectable. The unit has almost no drag so the meltout i s also recorded. A photo of the unit i s included i n F i g . A - l . , w/here (1) i s the wiper, (2) connection to the resistance wire, (3) linkage of wiper to heave arm, (4) top of heave arm, (5) foot which rides on s o i l surface. Measurement of Water Tension: The De Vries Tensiometer (as b u i l t by Dr. J . De V r i e s / Dept. of S o i l Sci./U.B.C.) This instrument i s e s s e n t i a l l y a mercury manometer'wwhich i s i n hydraulic contact with the s o i l water. In a l l respects, except the mode of this contact, i t i s s i m i l a r to other units for water tension measure- ment. In t h i s unit the contact between the s o i l water and the water i n the tensiometer system, which i s de-aired i n the laboratory before the f i e l d i n s t a l l a t i o n , i s maintained through a paste of 29 At, s p h e r i c a l glass beads placed i n a p l a s t i c holder and connected to the manometer through a sheet of fi n e porus nylon ( m i l l i p o r e membrane). The system has a low impedance and i s useable to a tension of approximately 150 mm.Hg. which i s the a i r i n t r u s i o n value of the composite tensiometer membrane. The unit i s shown i n Figure A-2. where (1) i s the manometer stem, (2) i s the mer- cury reservoir, (3) i s the point where the sensing head i s buried i n the s o i l centered on a depth of 5 cm. The glass bead holder i s mounted ver- t i c a l l y and has contact dimensions of approximately 2x2 cm. and, thus, gives a mean tension reading for the 3-5 cm. depth zone i n the s o i l . This basic unit can be made into a recording device by using the mercury as a s l i d i n g contact on a resistance wire i n the same manner   134 as the wiper on the Schmitt heave meter. This arrangement i s i l l u s t r a t e d i n F i g . A-3. which shows the s l i d e wire mounted inside the manometer stem and the two e l e c t r i c a l c l i p s which bring the bridge voltage into contact with both the mercury reservoir (lower c l i p ) and the resistance s l i d e wire by way of a b o l t i n the perspex manometer stem (upper c l i p ) . This v a r i a b l e resistance i s made an arm of a wheatstone bridge, and as such, the out-of-balance current becomes a non-linear analog of Tgoil water ten- sion. The unit i s c a l i b r a t e d by placing the sensing head i n a water bath and l i f t i n g the mercury column to varied heights with corresponding nota- tions of deflections of the Rustrak recorder. The major d i f f i c u l t y with the unit i s that at sub-freezing tem- peratures the water i n the system freezes disturbing the trace and i n fac t may destroy the s l i d e wire by f r o s t action on the resistance wire windings. This happened i n the i n s t a l l a t i o n during the l a t t e r part of February 1968. This deficiency could be removed i n the future by burying the e n t i r e system i n the s o i l or placing the above surface portion of the unit i n an enclosure heated to just above the ice point. Another problem was the gradual d i f f u s i o n of a i r out of the s o i l water which moved into the system as a r e s u l t of several wetting and drying cycles. This problem was eliminated by adding an a i r trap to the system. This modification should permit the use of the measurement system i n remote s i t e s where laboratory de-aired water i s not a v a i l a b l e . 135 Humidity and Dew Point Measurements: A i r dew points were calculated from the wet bulb depression of aspirated wet and dry bulb mercury thermometers.

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