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Study of needle ice events at Vancouver, Canada, 1961-1968 Outcalt, Samuel Irvine

Abstract

Prediction of needle ice events requires an understanding of energy and water transfer between the atmosphere and the soil. A project in southwestern British Columbia was conducted during 1961-1968 for the purposes of (1) constructing a general model of needle ice growth, (2) characterizing the processes which combine to produce ice needle events, and (3) explaining the variations in ice needle morphology. The problem was approached by (1) comparing environmental conditions on event and non-event nights, (2) identifying the heat sinks which produce surface cooling and fusion, (3) determining the range of the variability in soil water and heat flow properties produced by changing soil water content, (4) statistically analyzing the event, non-event record, and (5) determining the time dependence of the components of the energy-water transfer system during needle ice growth. Statistical and physical analysis demonstrated the overriding control of the thermal and evaporative heat sinks in needle ice growth and indicated that the equivalent radiant temperature of the night sky must drop below -15°C before a needle ice event is probable. Further, it was demonstrated that after ice crystals form at the surface the magnitude of the surface heat sink (equilibrium surface temperature) and the soil water content control the depth of the normally frozen soil cap above the needles and the spatial homogeneity of needle growth. The statistical study of the event record produced a simple dew point-cloud cover empirical model for event prediction and a listing of favorable-unfavorable conditions for needle ice growth. A general model for needle ice growth was developed indicating the relationship between surface equilibrium temperature and soil water tension on the growth and the spatial temporal homogeneity of needle growth. Contributions to general micrometeorology were made by demonstrating the utility of combined measurement of surface temperature soil water tension and soil heave in the analysis of soil structural evolution during diurnal freeze-thaw cycles. Specifically, the anomalous positive "bump" which frequently occurs in nocturnal surface temperature curves was shown to be coincident with a thermally driven flux of warm subsoil water toward the surface and a sudden increase in soil water tension was demonstrated to occur at the time when heave (ice segregation) began. Finally specific problems which appear both tractable and rewarding were formulated for future investigators.

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