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Along-Valley Structure of Daytime Thermally Driven Flows in the Wipp Valley. Rucker, Magdalena; Banta, Robert M.; Steyn, Douw G. 2008-03-31

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Along-Valley Structure of Daytime Thermally Driven Flows in the Wipp ValleyMAGDALENA RUCKER*Department of Earth and Ocean Sciences, The University of British Columbia, Vancouver, British Columbia, Canada, andNOAA/Earth System Research Laboratory, Boulder, ColoradoROBERT M. BANTANOAA/Earth System Research Laboratory, Boulder, ColoradoDOUW G. STEYNDepartment of Earth and Ocean Sciences, The University of British Columbia, Vancouver, British Columbia, Canada(Manuscript received 4 October 2005, in final form 16 May 2007)ABSTRACTHigh-resolution Doppler lidar observations obtained during the Mesoscale Alpine Program (MAP) 1999field campaign are used to investigate the along-valley structure of daytime valley flows in the Wipp Valley,Austria. The observations show that under varying ambient conditions the valley flow increases in speedthrough a narrow section of the valley. Furthermore, the along-valley volume flux diverges along the valleysegment under investigation, which suggests that the observed along-valley acceleration of the valley flowcannot be explained by the horizontal constriction of the valley sidewalls. It is hypothesized that thealong-valley acceleration of the flow is caused by an intravalley change in the horizontal pressure gradientinduced by differential heating rates of the valley atmosphere.1. IntroductionThermally driven valley wind systems are an impor-tant feature of the atmospheric environment in moun-tainous terrain. Prevalent under fair-weather condi-tions, they regularly influence flow characteristics inmountain valleys and hence play an important role inenvironmental issues related to transport and diffusionof atmospheric trace species, such as air quality, emer-gency response, and agricultural applications, as well asapplications sensitive to wind behavior, such as fireweather and aviation. Valley winds have been studiedfor many years, giving us a basic understanding of thetemporal evolution and thermodynamic mechanismdriving valley winds. In conceptual terms, valley windsform as a result of horizontal pressure gradients thatdevelop because of temperature differences between avalley and the adjacent plain. Wagner (1932) first sug-gested that the diurnal temperature variation betweenvalley and plain was due to the smaller volume of air inthe valley that needed to be heated or cooled comparedto that over the plain. This volume effect has been es-timated using the topographic amplification factor(TAF; Whiteman 1990), which considers a valley’scross-sectional area, or the area–height distribution(Steinacker 1984), which also takes into account heat-ing effects of tributaries and high-lying plateaus. Mc-Kee and O’Neal (1989) used TAF calculations to showthat pressure gradients, which develop within a valleybecause of changes in the valley geometry, can also beimportant in the formation of valley winds. A limitationof these methods to quantify the volume effect is thatthey are purely geometrical measures of the valley to-pography and do not take into account flow dynamics.Although the formation of valley wind systems is at-tributed to topography, little is known about the spatialstructure of valley flows along the valley axis and howthe intensity of the flow relates to the topography of the* Current affiliation: Sea Breeze Power Corp., Vancouver, Brit-ish Columbia, CanadaCorresponding author address: Dr. Douw Steyn, Department ofEarth and Ocean Sciences, The University of British Columbia,6339 Stores Road, Vancouver, BC V6T 1Z4, Canada.E-mail: dsteyn@eos.ubc.caVOLUME 47 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY MARCH 2008DOI: 10.1175/2007JAMC1319.1© 2008 American Meteorological Society 733JAM1319valley. In studying the along-valley flow structure, it isuseful to examine the mass or volume budget of thevalley atmosphere, which accounts for the effects ofchanging valley cross sections on the flow. Along-valleyvolume flux divergence has been observed for thenighttime case (Rao 1968; Whiteman and Dreiseitl1984; Post and Neff 1986), whereby the increase in thealong-valley volume flux is at least partially attributedto air draining into the valley from tributaries and fromkatabatic flows. For the daytime case, it is often as-sumed that the along-valley volume flux converges asair in a valley is gradually removed through tributaryand slope flows (e.g., Freytag 1988). Along-valley vol-ume flux convergence indeed has been observed for thedaytime case in midsized tributaries or end valleys(Buettner and Thyer 1965; Hennemuth 1987; Prévôtetal. 1998; Henne et al. 2004). On the other hand, along-valley volume flux divergence has been reported fordaytime valley flows in the Inn Valley (Freytag 1987)and in the Kali Gandaki Valley (Egger et al. 2000).In this contribution, we investigate the along-valleystructure of daytime flows in the Wipp Valley, Austria,using high-resolution Doppler lidar observations.These measurements, which also show along-valley vol-ume flux divergence, were obtained in fall 1999 as partof the Mesoscale Alpine Program (MAP) field cam-paign (Bougeault et al. 2001). Although the MAP–Wipp Valley field work focused on foehn and gap flowevents, a sequence of fine weather days during 11–17October 1999 provided an opportunity to study ther-mally driven flows while taking advantage of existinginstrumentation.The outline of the paper is as follows. Section 2briefly describes the study area, the instrumentationused in the field study as well as synoptic conditions. Insitu and Doppler lidar observations of flow in theWipptal are presented in section 3, and the along-valleyvolume flux is analyzed in section 4. In section 5, theresults of this study are discussed in context of similarfindings in other valleys. Section 6 summarizes the find-ings of this study.2. Description of field studya. Location, topography, and period ofobservationsThe Wipp Valley, a predominantly NNW–SSE-oriented valley located in North Tyrol, Austria (Fig. 1),is a major tributary valley of the Inn Valley. It runsfrom the city of Innsbruck (575 m MSL) south to theBrenner Pass (1373 m MSL). The valley is roughly 35km long and has an average slope angle along the valleyaxis of 1.3°. The Wipp Valley has several tributaries,the largest one being the Stubai Valley near the mouthof the Wipp Valley.The valley segment under investigation lies roughlywithin a 6-km radius of the Doppler lidar site (see Fig.1). At the northern terminus of the segment (near theentrance of the valley), the Wipp Valley is roughly Vshaped and 6 km in width at 800 m AGL. Just south ofthe mouth of the Stubai Valley, the Wipp Valley nar-rows to less than 4 km in width at 800 m AGL andremains this narrow for approximately 3 km beforewidening again near the southern end of the segment.Because of the complexity of the terrain (such as bendsin the valley, discontinuous ridgelines due to tributar-ies, and high-lying alpine plateaus that are part of thesurrounding mountains), it is difficult to objectively de-termine an effective ridge height. Near its entrance, theslopes on either side of the Wipp Valley provide a reliefof roughly 1200 m. Further south, the terrain definingthe eastern side of the Wipp Valley reaches a height ofapproximately 2400 m MSL, whereas the western ridge-line that separates the Stubai Valley from the WippValley gradually increases to a height of 1700 m MSL.The depth of the Wipp Valley in the narrow segment isapproximately 800 m. Surrounding mountain peaksreach heights between 2600 and 2800 m MSL.Measurements were obtained on 11, 13, 14, 16, and17 October 1999. In this study, we present observationsfrom 11, 16, and 17 October. Although thermally drivenvalley flows were observed on all five days, the 13 and14 October are excluded from the present study be-cause of external influences on the valley flow system.On 13 October, a downslope windstorm descendingeast of Stafflach (S; Fig. 1) disrupted the developmentof the thermally driven valley wind system, whereas on14 October the valley flow was strongly influenced bythe upper-level flow through down-mixing of momen-tum.b. InstrumentationA comprehensive description of the instrumentationin the Wipp Valley study area during the MAP SpecialObserving Period (SOP) can be found in Mayr et al.(2004). This study focuses on observations obtainedfrom the Doppler lidar system as well as a network ofsurface meteorological stations.The TEACO2 (so called because of its transverselyexcited atmospheric pressure discharge laser configura-tion, with carbon dioxide laser technology) Doppler li-dar of the National Oceanic and Atmospheric Admin-istration (NOAA) Environmental Technology Labora-tory was deployed in the Wipp Valley near the villageof Gedeir at an elevation of 1065 m MSL. The lidartransmits pulses of infrared light, a fraction of which is734 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47scattered back to the instrument by aerosol particles.The intensity of the backscattered energy is related tothe concentration, size, and composition of the aero-sols, and the Doppler-shifted frequency of the receivedsignal reveals the radial wind velocity component alongthe direction of the lidar beam. More detailed informa-tion on the lidar can be found in Post and Cupp (1990).This Doppler lidar has been used successfully in a num-ber of studies of flow over complex terrain, includingsea breezes (Banta et al. 1993), nocturnal canyon andvalley flows (Post and Neff 1986; Banta et al. 1995,1999), wind-flow patterns in the Grand Canyon (Bantaet al. 1999), and nocturnal low-level jets (Banta et al.2004). Doppler lidar observations of gap/foehn flowsduring MAP are discussed in Flamant et al. (2002),Durran et al. (2003), Gohm et al. (2004), and Weiss-mann et al. (2004).A pulse repetition frequency (PRF) of 10 Hz wasused for this study. To reduce noise and improve reli-ability of the velocity estimates, five pulses were aver-aged for each velocity estimate, resulting in an effectivePRF of 2 Hz. The minimum range varied between 1.2and 1.5 km, and the maximum range was generally 7–8km. The range gates were at 300-m intervals, and thevelocity accuracy was 0.6 m sH110021(Post and Cupp 1990).A number of different scans were conducted in rota-FIG. 1. Topographical map of the Wipp Valley area (contour interval 150 m), showingmeasurement sites reported in this study. The Doppler lidar site is shown with a solid square(DL; 1065 m MSL), while the surface weather stations are marked with solid triangles. InnValley: V H11005 Volders (601 m MSL), H H11005 Hall (612 m MSL), I H11005 Innsbruck City (609 m MSL);Wipp Valley: P H11005 Patsch (960 m MSL), M H11005 Matreiwald (985 m MSL), G H11005 Gedeir (1132 mMSL), T H11005 Tienzens (1168 m MSL), S H11005 Stafflach (1149 m MSL), Gr H11005 Gries (1326 m MSL),L H11005 Luegg (1228 m MSL), Pa H11005 Patscherkofel (2247 m MSL). The solid dot indicates thelocation of the Innsbruck Airport (IA; 579 m MSL) and the tethersonde site is marked witha solid diamond (TS; 1400 m MSL). The heavy line segments show the location of the Dopplerlidar vertical-slice scans. Partial range rings are shown at 2-km intervals from the lidar.MARCH 2008 RUCKER ET AL. 735tion during this study. Vertical-slice scans along thecenter of the valley (320° down valley of the lidar siteand 178° up valley of the lidar site) were used to resolvethe vertical structure of the valley flow along the valleyaxis, whereas conical scans at various elevation angleswere used to provide information on the horizontalflow structure. All scans were postprocessed to flag andremove terrain-contaminated range gates as well asrange gates with weak signal-to-noise ratios. Verticalcross sections presented for 16 and 17 October wereobtained by averaging two consecutive vertical-slicescans, taken over a 6-minute period, and horizontallyprojecting the radial velocity components to removethe effect of changing elevation angles. Given an aver-age valley floor inclination of 1.3°, the horizontal pro-jection differs from the terrain-following projection by0.03%. Vertical-slice scans were not available for 11October 1999. To facilitate comparison to the otherstudy days, pseudovertical cross sections were com-posed from conical scans, averaging radial velocities ateach range gate over a 40° sector centered on the 178°and 320° azimuths. (Note that for the first two pseudo-vertical cross sections shown in Fig. 4 below, the colorcoding does not reflect the true flow direction. Thisproblem is an artifact of the graphing package and iscaused by a combination of weak wind speeds andsparse data coverage. Arrows were therefore added tothe images to indicate the correct flow directions asdetermined from the original conical scans.) The signconvention for velocities shown in the vertical crosssections is positive (negative) for flow in the up-valley(down-valley) direction, whereas in the conical plots,positive (negative) velocities denote flow toward (awayfrom) the lidar.In interpreting Doppler lidar radial velocity compo-nents in the context of three-dimensional flow fields, apriori assumptions generally have to be made about theflow. For the vertical cross sections shown in this study,we impose a two-dimensional view; that is, we assumethat the flow within the valley sidewalls follows thedirection of the valley axis. Since the azimuth angles ofthe vertical-slice scans are similar to the along-valleydirection of the Wipp Valley (see Fig. 1), the radialvelocity components in those scans do closely approxi-mate the along-valley wind components. Several stud-ies in other valleys (e.g., Hennemuth and Schmidt 1985;Weigel and Rotach 2004) have shown that circulationsexist in the cross-valley direction and that the valleyflow is not necessarily a two-dimensional flow phenom-enon. These types of circulations, which are generallyweak, are not taken into account in this study as it is notpossible to identify cross-valley flow components usingsolely radial velocity measurements. However, the ex-clusion of cross-valley circulations does not affect theresults presented here. For winds above the valley side-walls, conical scans were analyzed to determine windspeed and direction of the flow aloft.During the MAP-SOP, 35 surface weather stationswere positioned throughout the study area. For thisstudy, a subset of 11 stations located near the valleybottoms of the Inn Valley and Wipp Valley (see Fig. 1)was used to examine the diurnal behavior of the near-surface winds in the valley. Data from all of these sta-tions were available as 10-min averages, with the ex-ception of Gedeir and Gries, which were available as1-h averages. In addition to these surface stations,mountain stations on the peaks of Patscherkofel (2247m MSL) and Zugspitze (2960 m MSL; located approxi-mately 35 km WNW from Innsbruck) were used to in-terpret ambient conditions. Synoptic network balloonsoundings were launched from Innsbruck Airport at0000 UTC for all days, with two additional soundings at1200 and 1800 UTC on 17 October. Finally, tethersondesoundings obtained approximately 1.5 km from the li-dar site on the slopes of a small tributary valley wereavailable for 11 and 17 October.All observations are presented using coordinateduniversal time (UTC). Solar noon as well as 1200 CET(central European time) corresponds to 1100 UTC.Theoretical times of sunrise and sunset are 0530 and1630 UTC, respectively. These times do not account forthe elevated horizon due to mountains or local shadingeffects. Near Gedeir, the valley bottom was sunlit atroughly 0745 UTC and shaded by 1430 UTC.3. Synoptic and local conditionsFair-weather conditions generally prevailed over theWipp Valley area during the study period. An upper-level ridge was located over central Europe on 11 Oc-tober (Fig. 2a), with synoptic flow over the study regionpredominantly from the WNW. By 13 October, the up-per-level ridge had amplified, forming a classic blockingomega ridge. During its life cycle (until 18 October),the omega ridge weakened somewhat, but generally re-mained over central Europe. During the latter half ofthe study period (Fig. 2b), the western Alpine regionremained under weak gradients, whereas the easternpart was under the weak influence of a low situated tothe east of the blocking ridge, producing northwesterlyflows aloft.Local ambient wind and cloud conditions for 11, 16,and 17 October are summarized in Table 1. On 11 Oc-tober, the westerly winds observed at Innsbruck Air-port and on top of the Zugspitze were in agreementwith the general zonal synoptic flow of that day. Lidar736 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47observations, however, showed a weak (4 m sH110021) south-erly flow layer above 2200–2500 m MSL (also see Fig. 4,described below), which persisted throughout the day.The lower portion of this flow layer was also capturedby tethersonde soundings. An early morning conicalscan captured another flow reversal at approximately3100 m MSL (not shown), suggesting that in the morn-ing hours the southerly flow layer was roughly 600 mdeep. Although it is not possible to determine the exactnature of the southerly flow, we surmise that the flowwas caused by the north–south geostrophic pressuregradient, a process called pressure-driven channelingby Whiteman and Doran (1993). The separation of thevalley wind system and the westerly synoptic flow bythe southerly flow layer indicates that the synoptic flowhad little or no influence on the thermally forced valleyFIG. 2. Geopotential heights and winds at 700 mb over central Europe at 0000 UTC for (a)11 Oct and (b) 17 Oct. The solid square marks the location of Innsbruck.MARCH 2008 RUCKER ET AL. 737flow system on this day. For both 16 and 17 October,the WNW–N flows above the ridgetops of the Inn Val-ley and Wipp Valley corresponded with the generalsynoptic conditions. On 16 October, the synoptic influ-ence on the valley flow system was minimal because ofvery light ambient winds. The ambient flow on 17 Oc-tober was stronger, but as will be discussed in the fol-lowing section, the valley flow layer was likely de-coupled from the ambient flow through an inversionlayer. Although the ambient flow on 17 October wasstronger than on 11 and 16 October, the observed val-ley wind speeds in the Wipp Valley were only slightlylarger on 17 October than on the other two days. Thesimilar valley wind speeds suggest that, for the dayspresented in this paper, the enhancement (weakening)of the valley flow in the Wipp Valley due to ambientwinds in the same (opposite) direction was minimal.This is in contrast to results obtained by Weigel andRotach (2004) where considerably higher valley windspeeds were found in the Riviera Valley on days whenthe ambient flow was in the up-valley direction.4. Observationsa. Surface observationsThe primary purpose of the continuous surface ob-servations presented in this section is to affirm the ther-mal nature of the flows observed during the study pe-riod, thus providing an important context for the inter-pretation of the lidar observations. An underlyingassumption for this analysis is that the surface winds arepart of the valley wind system; that is, they are drivenby the same thermal forcing that affects the entire val-ley atmosphere. While this assumption is reasonable forobservations in the Inn Valley where the valley floor isalmost horizontal, it may not be entirely appropriatefor the Wipp Valley, which has a sloped valley bottom.In the latter case, the perceived up-valley winds at thesurface may be described more accurately as slopeflows along the valley axis [also called up-floor winds,e.g., Whiteman (1990)]. With the available observa-tions, however, it is not possible to determine un-equivocally to what extent the surface flows are due tothe sloping valley bottom.To determine if the observed flow was primarily ther-mally driven, the valley wind criteria of Dreiseitl et al.(1980), which essentially test for the twice-daily rever-sal of near-surface winds, were applied to all surfacestations. The results, listed in Table 2, show that thevalley wind criteria were satisfied on all three studydays for the majority of stations. Failure of the valleywind criteria at Stafflach (S), Hall (H), and Volders (V)on 16 and 17 October was primarily due to the lack oforganized nighttime down-valley flows, which meantthat no wind reversal occurred during the morninghours. However, a marked increase in up-valley windspeeds during the daytime followed by calm winds inthe evening was generally observed at these three sta-tions. It is assumed that valley fog, which was present inthe area on the mornings of these two days, hinderedthe development of the nocturnal portion of the valleywind circulation near the surface. This is also evident inthe sea level pressure readings (not shown) at Inns-bruck and Kufstein (located in the Inn Valley, approxi-mately 70 km down valley from Innsbruck) which showfor both 16 and 17 October a well-developed pressuregradient during daytime, but not at nighttime (Rucker2003).Figure 3 shows the daytime evolution of surface windspeeds at Tienzens, located 5 km up the valley from theTABLE 2. Dreiseitl’s valley wind criteria applied to surfaceweather stations shown in Fig. 1. Failure of the criteria is indicatedby “N,” and success by “Y.” Dashes indicate missing data.DayWipp Valley Inn ValleyLGrSTGM P I HV11 Oct Y Y Y Y Y Y ——YY16 Oct Y Y N Y Y Y ——YN17 Oct Y Y N Y Y Y Y Y N NTABLE 1. Summary of local ambient conditions during the study period. Wind conditions at Innsbruck Airport are based on 0000UTC (11 and 16 Oct) and 1200 UTC (17 Oct) radiosonde soundings, while wind conditions at Zugspitze, Patscherkofel, and Gedeirsummarize daytime conditions. Conditions at Gedeir are based on vertical wind profiles derived from Doppler lidar conical scans. On16 Oct, wind direction at Zugspitze was not available.DayInnsbruck Airport(2800–3000 m MSL)Zugspitze(2962 m MSL)Patscherkofel(2247 m MSL)Gedeir(2400–2800 m MSL) Cloud/fog cover11 Oct 4 m sH110021WNW 8 m sH110021W Light and variable 4 m sH110021S Clear in morning; afternoonconvective clouds16 Oct 2 m sH110021N3msH1100211–2msH110021NW 2–4msH110021NW–N Morning valley fog; afternoonconvective clouds17 Oct 6 m sH110021WNW 3–6msH110021W–NW 3 m sH110021NW 8msH110021WNW Morning valley fog; afternoonconvective clouds738 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47Doppler lidar site, and Matreiwald, located 2 km downthe valley from the lidar site for all three study days.Although care has to be taken when comparing surfacemeasurements due to possible influences of local topog-raphy, the surface winds at Tienzens are consistentlystronger than those at Matreiwald. As will be shown inthe following section, the increase in wind speed withdistance in the up-valley direction is also evident in theDoppler lidar observations.Based on the surface observations as well as synopticeffects discussed in the previous section, we concludethat the valley flows in the Wipp Valley were primarilythermally driven on the study days. In the followingsections, Doppler lidar observations are used to inves-tigate the spatial characteristics of the valley flow.b. Doppler lidar measurements along the WippValley1) LIGHT, SOUTHERLY EXTERNAL FLOW(11 OCTOBER 1999)Figure 4 shows the temporal evolution of the along-valley flow structure on 11 October. During the previ-ous night, the 0000 UTC sounding at Innsbruck Airportnear the entrance of the Wipp Valley (not shown)showed a layer of southerly flow between 1400 and2400 m MSL, overlying the nocturnal, down-valleyflow, which in the Inn Valley was westerly. The top ofthis outflow layer from the Wipp Valley thus nearlycoincided with the height of the ridge line (approxi-mately 2400 m MSL) surrounding the Wipp Valley. Asthe up-valley wind layer (shown in yellow–magenta col-ors) developed through the morning, it was clearly dis-tinguishable from the southerly flow aloft (blue–purplecolors) which remained relatively weak throughout theday.The onset of the up-valley flow at the surface oc-curred at roughly 0900 UTC. Lidar measurements atthe same time captured a complex flow structurewhereby a layer of southerly flow was sandwiched be-tween layers of flow directed up valley. The elevatedlayer of southerly flow below 2200 m in the first twolidar images was most likely a remnant of the nocturnal,down-valley flow in the Wipp Valley. Similar flow fea-tures during the morning transition period have beenobserved in other valleys (e.g., Brehm and Freytag1982; Whiteman 1982) whereby the elevated down-valley flow is associated with remnants of the nocturnalinversion.By 1015 UTC, the up-valley flow layer had intensi-fied and extended well above the lower ridgeline(dashed line) to the west of the Wipp Valley. The layerof southerly flow was almost completely eroded, al-though remnants of it were still visible north of the lidarsite. By 1100 UTC, the up-valley flow was fully devel-oped and extended almost 1400 m above the valleyfloor, or roughly to the height of the immediate terrainto the east of the Wipp Valley (solid terrain outline).Although the spatial coverage between up-valley anddown-valley pointing lidar scans differed, the observa-tions strongly point to a gradual increase in wind speedwith distance in the up-valley direction. At 1230 UTC,for example, maximum wind speeds of 6–7msH110021wereobserved at about 250 m AGL north of the lidar site,whereas wind speeds of 8 m sH110021were observed 500 mabove the valley floor south of the lidar site. Assuminga Prandtl-type profile for which the flow maximum oc-curs relatively close to the surface (e.g., Atkinson 1981),it is possible that wind speeds closer to the valley bot-tom exceeded 10 m sH110021south of the lidar site. At 1300UTC, flow north of the lidar site had weakened some-what but remained at the same intensity south of theFIG. 3. Ten-minute average wind speeds on 11, 16, and 17 Oct 1999 for surface stations at Tienzens (T) south of the lidar site(black circles) and Matreiwald (M) north of the lidar site (gray circles). Periods with up-valley flow are shown by filled circles.MARCH 2008 RUCKER ET AL. 739lidar site. By 1500 UTC, the valley wind had weakenedfurther but still attained speeds of 4–5msH110021at 500 mAGL south of the lidar site, whereas the flow at thesame height north of the lidar site was no more than1msH110021. The valley wind near the surface ceased at 1500and 1530 UTC for Tienzens and Matreiwald, respec-tively.2) LIGHT, NORTHERLY EXTERNAL FLOW(16 OCTOBER 1999)Figure 5 shows vertical cross sections of the along-valley winds for 16 October, when the ambient windwas weak and from a northwesterly direction. Up-valley flow at the surface was first observed at 0700UTC for Matreiwald and 0900 UTC at Tienzens. By thetime of the first vertical-slice scans at 1100 UTC, theup-valley flow layer was already well established, with aclear increase in wind speed occurring with up-valleydistance. Similar to 11 October, a layer of southerlyflow was visible north of the lidar site between 1600 and2000 m MSL. A conical scan taken 10 min later showsthat this layer existed across the entire valley cross sec-tion, but was slightly deeper toward the center of thevalley than near the sidewalls. Although not visible inFIG. 4. Vertical cross sections along the valley centerline showing horizontally projected radial veloc-ities for 11 Oct 1999. Data north of the lidar site are based on partial conical scans covering elevationangles 0° to 30° (in increments of 5°), while data south of the lidar site are based on conical scans withelevation angles ranging from 10° to 25° (in increments of 5°). Several faulty scans (particularly for theimages at 0854, 1254, and 1456 UTC) were excluded from the analysis. Positive (negative) values indicateup-valley (down valley) flow. Arrows are superimposed to clarify wind direction. The thin lines mark theheight of individual layers with opposing flow whereby the error bars indicate the vertical resolution(proportional to the cosine of the scan elevation angle). Near-surface along-valley wind speeds atMatreiwald and Tienzens are presented as dots at H110022000 and 5000 m from the lidar site, respectively. Thesolid black area marks the valley floor. For reference, the eastern- and western-lying ridge lines areshown with solid and dashed lines, respectively. Up-valley direction is to the right.740 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47Fig 4 live 4/Cthe vertical cross section, a small remnant of down-valley flow aloft could also be seen in conical scanssouth of the lidar site (the down-valley flow appearedtoward the eastern sidewall and was therefore not vis-ible in the vertical scans along the center of the valley).The existence of down-valley flow in the Wipp Valleyalso is supported by the 0000 UTC sounding at theInnsbruck Airport, which showed outflow from theWipp Valley (i.e., down-valley flow) to a height of2300 m MSL. By 1130 UTC, the down-valley flow in theupper valley segment had disappeared, while weakdown-valley flow still persisted aloft in the lower valleysegment. By 1200 UTC, the remnant of down-valleyflow north of the lidar site had disappeared as well, andthe up-valley flow now appeared fully developed. Aslight increase in wind speed with distance in the up-valley direction could be seen within the vertical scanspointing to the north of the lidar, whereas a very no-ticeable increase in wind speed occurred between thedown-valley and up-valley pointing segments of thescans. A clear vertical structure in the valley flow layerwas not discernible north of the lidar site. South of thelidar site, vertical-slice scans until 1230 UTC showed adistinctly layered flow structure. After 1300 UTC, thevertical wind shear decreased somewhat as strongerwind speeds near the valley bottom were mixed tohigher elevation levels. An analysis of vertical aerosolbackscatter profiles along the Wipp Valley (RuckerFIG. 5. As in Fig. 4, but for 16 Oct 1999. Cross sections are based on vertical-slice scans.MARCH 2008 RUCKER ET AL. 741Fig 5 live 4/C2003) showed a well-defined jump in aerosol backscat-ter with height between 1800 and 2000 m MSL prior to1225 UTC, but not in profiles taken at a later time. Thecorrespondence of the observed “up mixing” of thewind field and dissipation of the layered flow structurewith the disappearance of the step change in the verti-cal aerosol backscatter profiles suggests that a cappingtemperature inversion was eroded in the afternoonwhich allowed the valley flow to become coupled withthe weak flow aloft, similar to the afternoon couplingfound by Banta and Cotton (1981) and Banta (1984) inthe Colorado Rocky Mountains.Around 1500 UTC, the up-valley wind layer as de-fined by the minimum in wind speed (Whiteman andDreiseitl 1984) extended roughly 1600 m above the val-ley floor. It is of interest to note that although the valleyflow layer was still well established at 1500 UTC, thesurface winds in the up-valley direction had greatly di-minished. The up-valley flow at the surface ceased at1700 and 1500 UTC for Matreiwald and Tienzens, re-spectively.Figure 6 shows the vertical wind structure for con-stant range gates north and south of the lidar site. Itshould be noted that since the data were taken fromconstant range gates, the profiles are not truly vertical,but follow an arch. The maximal horizontal differencefrom the true vertical is 350 m. In addition to a verypronounced increase in wind speed that occurred in theup-valley profile, a change in the shape of the verticalwind profile is also evident. Although the lidar scans inthe down-valley direction did not cover the lowest 300m of the flow, the wind profiles down valley from thelidar site nevertheless appear consistently flat or weaklycurved. Up valley from the lidar, the flow maximumwas more pronounced and occurred closer to the sur-face. A similar change in the vertical wind profile wasalso observed along a 18-km stretch of the Inn Valleyduring the Mesoscale Experiment in the Region Kuf-stein-Rosenheim (MERKUR; Reiter et al. 1984). TheDoppler lidar measurements presented in this study,however, show that such change in the wind profile cantake place over a distance of several kilometers. Thereason for change in the shape of the vertical windprofile is not known. As a final point of interest, thewind speed north of the lidar site appeared almost sta-tionary during the period that the Doppler lidar wasoperational, whereas south of the lidar site a more no-ticeable increase in wind speed occurred.3) MODERATE, NORTHERLY EXTERNAL FLOW(17 OCTOBER 1999)The vertical cross sections obtained on 17 October(Fig. 7) show a well-developed up-valley flow regimethat is clearly separated from the upper-level flow by awind speed minimum. The upper-level flow on this daywas moderate and from the northwest. The wind speedminimum coincided with a strong inversion layer atroughly 1800 m MSL, which was evident in the 1200UTC sounding at Innsbruck Airport and also in theafternoon tethersonde soundings near Gedeir. It there-fore appears that the valley flow was decoupled fromthe upper-level flow by this inversion layer. The depthof valley flow, as defined by the wind minimum, wasroughly 800 m. As was the case for the other study days,the intensity of the valley flow increased across theminimum range of the lidar. In contrast to 16 October,however, the valley flow on 17 October continued toincrease south of the lidar site, with the maximum windspeed occurring roughly 4 km downstream from thelidar. The increase in wind speed along the valley seg-ment was not just limited to the valley proper, but ex-tended to a height of 2600 m MSL. Above 2600 m MSL,velocity–azimuth display (VAD) analysis showed thewind speed to be constant and from the northwest. Theapparent decrease in the along-valley (radial) windcomponent in the vertical cross sections at this heightand above are due to the different azimuth angles withrespect to the flow above ridgetops with which the up-and down-valley scans were obtained.Greater lidar range on this day allowed the verticalslices in Fig. 7 to be extended into the entrance regionof the Wipp Valley (H110027toH1100210 km mark). In this re-gion, the a priori assumption of along-valley flow is notapplicable, and hence care has to be taken when inter-preting the Doppler lidar data.Under thermally driven conditions, it is generally as-sumed that air is drawn from the main valley into thetributary. Northerly flow, or flow toward the mouth ofthe Wipp Valley, was indeed observed during daytimeat the Innsbruck surface station (see Fig. 1). The weaknegative radial velocities obtained with the Doppler li-dar, however, indicate that the direction of the flowbetween 1200 and 2000 m MSL was 50° or greater; thatis, the flow at those elevations was oriented more alongthe ENE–WSW direction of the Inn Valley rather thanthe NNW–SSE direction of the Wipp Valley. InnsbruckAirport soundings at 1200 and 1800 UTC confirmed thepresence of up-valley flows in the Inn Valley to heightsof 1800 to 2000 m MSL. From the available observa-tions, it is not possible to ascertain if the higher-levelflow continued along the Inn Valley, or entered theWipp Valley along the western half of the valley.Strong channeling of valley winds has been observed inthe Riviera Valley (Weigel and Rotach 2004), but thishas been primarily attributed to the curvature of theterrain near the mouth of the Riviera Valley.742 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47The vertical wind profiles (Fig. 8) are similar to thoseobserved on 16 October. Again, the wind profile northof the lidar appeared to have only slight curvature,whereas the wind profile south of the lidar showed apronounced wind speed maximum approximately 200m above valley bottom. The ratio of the height aboveground of the wind speed maximum and the depth ofthe up-valley flow layer (800 m) for the wind profileFIG. 6. Vertical profiles of the horizontal along-valley wind component for range gates 8–12 (2.25–3.45km from lidar) north (solid circles) and south (solid squares) of the lidar site for 16 Oct 1999. Thin lineswith dots show individual profiles, whereas heavy lines with solid circles–squares indicate averagedprofiles.MARCH 2008 RUCKER ET AL. 743south of the lidar is 0.25. This ratio closely correspondsto that of a Prandtl-shaped profile (Atkinson 1981). It isnot implied, however, that the mechanism proposed byPrandtl (1942) for simple slope flows is applicable toup-valley flows.c. Doppler lidar measurements across the WippValleyAlthough the primary orientation of the Wipp Valleyis NNW–SSE, the valley exhibits several bends along itslength. Research in other valleys with curvature, suchas the Loisach Valley and the Riviera Valley, hasshown that thermally driven valley flows can exhibitcharacteristics of curved channel flow (Reiter et al.1983; Weigel and Rotach 2004). In this section, we ex-amine the cross-valley flow structure from conical scansto see if similar banking of the valley flow occurred inthe Wipp Valley.Figure 9 shows the radial velocity for a series of low-level conical scans obtained north of the lidar site on 11October. These scans, which can be considered as in-stantaneous snapshots of the flow field, show fluctua-tions in the cross-valley flow structure which may beassociated with flow under convective conditions. Per-sistent asymmetries in the cross-valley structure, how-ever, are not evident. The positive radial velocity com-ponents seen in Fig. 9 near the entrance region areconsistent with up-valley flow branching off from theInn Valley. The exact orientation of the flow in theentrance region, however, cannot be determined fromthe Doppler lidar scans.Radial velocities for conical scans on 11, 16, and 17October are shown in Fig. 10 for a slightly higher el-evation angle. Although the valley orientation south ofthe lidar site changes from 165° to 180°, no persistentbanking of the valley flow toward the eastern sidewall isevident. Weak radial velocities south of the lidar site onthe west side as well as negative radial velocities northof the lidar site on the east side occurred near the en-trances of tributaries, and hence are interpreted as flowbranching off into the tributaries. Measurements wereFIG. 7. As in Fig. 5, but for 17 Oct 1999.FIG. 8. As in Fig. 6, but for 17 Oct 1999.744 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47Fig 7 live 4/Cnot available to confirm the presence of up-valley flowsin those tributaries, but up-valley flows were clearlyobserved in the small tributary to the east of the lidarsite, and hence it is assumed that up-valley flows werepresent in other tributaries as well. On 17 October,influence of the tributary extended farther into theWipp Valley than on 11 and 16 October. It is postulatedthat the temperature inversion that was present on thisday at 1800 m MSL may have limited the vertical move-ment of air, hence increasing the horizontal range fromwhich air is drawn into the tributary. Alternatively, thetemperature inversion may have intensified the valleyflow circulation by confining the heat input to certainaltitudes in the valley (Steinacker 1984).The apparent lack of banking of the daytime up-valley flow in the Wipp Valley in comparison to obser-vations made in the Loisach Valley or the Riviera Val-ley may be due to the smaller changes in direction. Inthe Wipp Valley, changes in the valley orientation areless than 20° while in the Loisach Valley and RivieraValley, the bends are closer to right angles.5. Along-valley volume flux calculationsIn the previous section, it was shown that under vari-ous ambient conditions the flow in the Wipp Valleyincreased in wind speed with up-valley distance. Themain along-valley acceleration occurred in an areawhere the valley narrows considerably. In this section,we present calculations of the along-valley volume fluxFIG. 9. Partial conical scans at 0° elevation for 11 Oct at (a) 1015 UTC, (b) 1102 UTC, (c)1217 UTC, and (d) 1254 UTC. The images show the radial velocity component, with positivevalues indicating flow toward the lidar and negative values denoting flow away from the lidar.Superimposed is the topographic contour line at the same height of the scan (1065 m MSL).The range rings mark the horizontal distance in 2000-m intervals from the lidar site, locatedat the lower right corner of each figure.MARCH 2008 RUCKER ET AL. 745Fig 9 live 4/Calong the Wipp Valley to examine how the increase inwind speed relates to the horizontal constriction of thevalley sidewalls.a. AnalysisIn the following analysis, we assume the flow to besteady and incompressible. The incompressibility as-sumption allows the air density to be treated as a con-stant and mass fluxes to be expressed as volume fluxes.The steady-state volume flux budget for a valley seg-ment is given byVxoutH11002 VxinH11001 VtributaryH11001 VslopeH11001 VwH11005 0, H208491H20850where Vxoutis the along-valley volume flux leaving thevalley segment, Vxinis the along-valley volume flux en-tering the valley segment, Vtributaryis the volume fluxleaving through tributaries, Vslopeis the volume fluxleaving via slope flows, and Vwis the volume flux leav-ing the segment through the top (fluxes are positive forthe direction of flow indicated in the definition of thefluxes). From Eq. (1), we can infer that for the daytimesituation, mass leaving the valley segment throughtributary and/or slope flows is balanced either throughflow convergence along the valley axis (VxoutH11002 VxinH11021 0)or through subsidence (VwH11021 0) at the top of the valleysegment. However, if there is volume flux divergencealong the valley axis (VxoutH11002 VxinH11022 0), then subsidencemust occur to balance both the along-valley flow diver-gence and air leaving through slope and tributary flows.This analysis concerns itself primarily with the along-valley volume flux, and the vertical motion implied bythe along-valley volume flux divergence–convergence.The magnitudes of the tributary and slope flows are notknown. However, these flows act in the same sense asalong-valley flow divergence in that they remove air-mass from the valley, and hence inclusion of these flowsin the volume flux budget would not change the sign ofthe vertical motion, only the magnitude.The along-valley volume flux was calculated for eachrange gate using the formulaVxH20849xH20850H11005H208850hH20849xH20850FuH20849x, zH20850WH20849x, zH20850 dz, H208492H20850where Vx(x) denotes the total along-valley volume flux(m3sH110021) as a function of along-valley distance x (m),W(x, z) (m) is the width of the valley as a function ofdistance and height above ground level z (m), u(x, z)(m sH110021) is the lidar-measured, along-valley wind com-ponent as a function of distance and height, and h(x)(m) is the depth of the valley flow. The weighting factorF represents the ratio of the mean cross-sectional,along-valley wind speed and the wind speed measuredat the center of the valley. This weighting factor, whichis very similar to the flux ratio used by King (1989),FIG. 10. Conical scans at 10° elevation for (a) 11 Oct at 1050 UTC, (b) 16 Oct at 1453 UTC, and (c) 17 Oct at 1333 UTC. The imagesshow the radial velocity component, with positive values indicating flow toward the lidar and negative values denoting flow away fromthe lidar. Superimposed are topographic contour lines in intervals of 400 m, beginning at 800 m MSL. The range rings mark thehorizontal distance in 2000-m intervals from the lidar site (shown with a solid square). The corresponding heights of the range rings are1420, 1770, and 2120 m MSL. The straight lines mark the 178° and 320° azimuths.746 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47Fig 10 live 4/Ctakes into account that the along-valley flow compo-nent varies across the valley. For this study, a value of0.87 was estimated for the weighting factor F using ra-dial velocities from conical scans. Effects of tributaryflows on cross-valley profiles were excluded. Verticalprofiles of the horizontally projected radial velocitieswere smoothed using a Gaussian filter with a sigma of50 m. Since the lidar observations did not extend to thevalley bottom, the wind profiles were extrapolated tothe surface using a generalized Prandtl formulation(Rucker 2003). The volume flux based on the extrapo-lated portion of the flow is 1%–7% of the total volumeflux south of the lidar site, and 6%–16% of the totalvolume flux north of the lidar site. Cross-sectionalwidths at each range gate were determined from a 100-m-resolution digital terrain model of Tyrol. In areaswhere the valley width was ill defined because of tribu-taries, cross-sectional profiles were linearly interpo-lated to profiles on either side of the tributary. All vol-ume flux calculations were based on a depth of 800 m,since for this depth most of the flow was confined bysidewalls on either side of the valley. Hence, volumeflux estimations for 17 October include the entire valleyflow layer, whereas estimations for 11 and 16 Octoberare based on only a portion of the valley flow.To estimate uncertainties in the volume fluxes due tomeasurement errors or analysis assumptions, the calcu-lations were also performed using values of 0.82 and0.92 for the weighting factor F. In addition, straight lineextrapolations were applied to wind profiles north ofthe lidar site to account for possible underestimationsof the along-valley volume flux by using the Prandtlformulation. Another problem in the volume flux esti-mations arose for profiles south of the lidar site wherethe valley widens. From conical scans (e.g., Fig. 10) itappears that the up-valley flow may not entirely fill thewidened valley segment. Hence, narrower cross-sec-tional profiles were estimated using radial velocity pat-terns from conical scans as guidance. Finally, volumefluxes north of the lidar site were also calculated as-suming a 200-m deeper flow layer (Rucker 2003).b. ResultsFigure 11 shows the along-valley volume flux, vol-ume flux density (which can be interpreted as the av-erage wind speed of the whole valley flow layer), andcross-sectional area along a 7-km-long segment of theWipp Valley for 17 October. North of the lidar site, thealong-valley volume flux is roughly constant, and hencethe slight increasing trend in the volume flux densitymay be attributed to the decrease in cross-sectionalarea north of the lidar site. A considerable increase inthe along-valley volume flux as well as volume flux den-sity, however, occurs over a distance of roughly 3 km,centered on the lidar site. Over this distance, the cross-sectional area of the Wipp Valley is almost constantwith only a slight increase toward the southern end.The fact that the along-valley volume flux divergesalong this valley segment even though the cross-sec-tional area remains almost constant, indicates that theincrease in wind speed cannot be explained by the hori-zontal constriction of the valley sidewalls. Farther southof the lidar site, the volume flux density continues toincrease even though the cross-sectional area remainsroughly constant, hence leading to an increase in thealong-valley volume flux downstream of the constric-tion.The mean vertical motion required to balance thealong-valley volume flux divergence on 17 October wascalculated by dividing the difference in volume flux be-tween the outflow and inflow boundaries by the totalarea at the top of the valley segment. For a 2700-m-longvalley segment centered on the lidar site, an averagesubsidence of 0.45 m sH110021is required to balance thealong-valley volume flux divergence at 1353 and 1425UTC. Taking into account uncertainties in the volumeflux calculations, the range in subsidence is 0.17–0.51msH110021and 0.24–0.49 m sH110021for 1353 and 1425 UTC, re-spectively. It should be noted that in calculating thesubsidence velocities, only flow in the along-valley di-rection was taken into account. In reality, slope andtributary flows also affect the volume budget in a valleysegment. For the daytime case, both slope and tributaryflows remove air from a valley segment. Hence, thesubsidence velocities represent minimum values as theydo not take into account additional flow divergence dueto slope and tributary flows.The subsidence rate estimated for this work is con-siderably larger than the subsidence rate determined byFreytag (1987) for the Inn Valley, where along-valleyvolume flux divergence was also observed. For a 37-km-long section along the Inn Valley, Freytag (1987)calculated a subsidence velocity of 0.05 m sH110021requiredto balance the along-valley volume flux divergence.The large difference in the two estimations may bepartly due to the different nature and scale of the mea-surements in the two valleys. For the Wipp Valley, thelidar observations capture very localized changes in theflow, whereas the calculations for the Inn Valley arebased on soundings placed roughly 18 km apart, andhence may be considered as subsidence rates averagedover a much larger region.A detailed volume flux analysis for the entire valleyflow layer is not presented for 11 and 16 October, sinceon these two days the valley flow extended above theMARCH 2008 RUCKER ET AL. 747surrounding ridgelines and thus was not laterally con-fined by valley sidewalls. However, estimations for an800-m-deep flow layer (Table 3) show similar increasesin the along-valley volume flux and volume flux densitybetween two ranges gates located approximately 2 kmnorth and south of the lidar site for 11 and 16 Octo-ber as found for 17 October. It may therefore beconcluded that the along-valley volume flux divergenceis not a function of ambient flow conditions. The sub-sidence rate based on the along-valley volume fluxdivergence for all three days varies between 0.38 and0.62 m sH110021.FIG. 11. (a) Along-valley volume flux and (b) volume flux density as a function of along-valley distance for 17 Oct 1999 at 1353 UTC (heavy line) and 1425 UTC (thin line). Uncer-tainty estimates are represented by the shaded areas. (c) Cross-sectional area as a function ofalong-valley distance is shown. The dotted line represents the cross-sectional areas for modi-fied cross sections. Up-valley direction is to the right.748 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47As discussed above, the volume flux analysis for 17October shows a continued increase in the along-valleyvolume flux density downstream of the valley constric-tion. Although the valley flow on 16 October clearlyintensifies through the valley constriction, an accelera-tion of the valley flow downstream of the constriction isnot evident (see Fig. 5). The reason for the differentflow behavior is not yet understood. The presence of atemperature inversion on 17 October suggests that theacceleration downstream of the constriction may be ex-plained using hydraulic theory. To determine if the flowup valley from the lidar site reached supercritical con-dition, we calculated the Froude number followingZängl et al. (2001) using the formula U(hgH9004H9258/H9258)H110021/2,where U is the mean wind speed across the flow layer,h is the depth of the flow, H9258 is the average potentialtemperature of the flow layer, and H9004H9258 is the jump inpotential temperature across the inversion. Using val-ues of U H11005 6msH110021and h H11005 800 m based on lidarobservations, and H9004H9258 H11005 5.5 K and H9258 H11015 285 K based onthe 1200 UTC sounding at Innsbruck Airport, theFroude number is roughly 0.5. Using more conservativevalues of U H11005 7msH110021, h H11005 600 m and H9004H9258 H11005 3K,theFroude number increases to 0.9, but still falls short ofunity. This suggests that the continued acceleration ofthe flow downstream of the constriction is likely notdue to hydraulic processes.6. DiscussionAlthough divergence of the along-valley mass/volume flux is logical for nighttime valley flows, it iscounterintuitive for the daytime case since flow is gen-erally removed from the valley atmosphere throughslope and tributary flows. Nevertheless, as in this study,along-valley volume flux divergence for daytime valleyflows has been inferred for two other valleys, the InnValley (Freytag 1987) and the Kali Gandaki Valley(Egger et al. 2000). It can be argued that, in all threevalleys, an increase in wind speed occurs in valley seg-ments where there is a decrease in the cross-sectionalarea. Increases in the along-valley mass/volume flux inthose valleys, however, suggest that the lateral constric-tions in the valley sidewalls cannot entirely explain theincreases in wind speed.A number of explanations have been suggested forthe observed increase in wind speed in the Kali Gan-daki Valley and Inn Valley. For the Kali Gandaki Val-ley, model simulations by Zängl et al. (2001) firstpointed to hydraulic theory as a way to explain theincrease in wind speed. Subsequent field measurements(Egger et al. 2002), however, have shown that a cappingtemperature inversion—a necessary requirement forhydraulic theory—is not present at all times, and hencea satisfactory explanation for the flow mechanism in theKali Gandaki Valley is still outstanding (Egger et al.2002). For the Inn Valley, Freytag (1987, 1988) postu-lated that the increase in mass flux was due to subsi-dence over the main valley, which quasi-locally com-pensated for flow into tributary valleys. In an earlierstudy, Vergeiner (1983) observed an increase in thedaytime surface wind speed and pressure gradientalong the same narrow segment in the Inn Valley [seeFig. 14 of Vergeiner and Dreiseitl (1987)], but usedconservation of the along-valley mass flux to explainthe increase in wind speed. More recently, a numericalstudy by Zängl (2004) suggested that flow enters theInn Valley through tributaries that link the Inn Valleywith the Alpine foreland and thus contributes to theincrease in mass flux in the Inn Valley.Work by Vergeiner and Dreiseitl (1987) points to afourth plausible explanation for the increase in windspeed in the Inn Valley, which has not yet receivedattention in the literature. Table 2 of Vergeiner andDreiseitl (1987) lists the diurnal ranges (between 0600and 1500 UTC) of vertically averaged temperature un-der fair-weather conditions for a number of stationsalong the Inn Valley and on the adjacent plain. Themonotonic, along-valley increase in the diurnal heatingrate leads to an along-valley temperature gradient,which in turn produces an along-valley pressure gradi-ent. The along-valley increase in the diurnal heatingrate is attributed to the general decrease in valley vol-ume along the length of the Inn Valley (Vergeiner andDreiseitl 1987). The same table, however, also showsTABLE 3. Along-valley volume flux, volume flux density, andsubsidence rate due to the along-valley volume flux divergence foran 800-m-deep flow layer at approximately 2 km north (RG7 DV)and 2 km south (RG7 UV) of the lidar site for 11, 16, and 17 Oct1999.DateTime(UTC)Volume flux(106m3sH110021)Flux density(m sH110021)Subsidence(m sH110021)RG7DVRG7UVRG7DVRG7UV11 Oct 1217 4.2 13.7 2.9 6.1 0.6216 Oct 1059 1.2 7.4 0.8 3.3 0.401133 3.3 8.3 2.3 3.8 0.321204 2.6 10.1 1.8 4.6 0.491225 3.4 9.3 2.4 4.5 0.381302 3.3 11.1 2.3 5.0 0.511405 3.4 12.0 2.4 5.4 0.561450 3.4 11.5 2.4 5.2 0.531516 4.3 11.4 3.1 5.1 0.4617 Oct 1353 4.1 11.0 2.9 5.0 0.451425 2.5 9.4 1.8 4.3 0.45MARCH 2008 RUCKER ET AL. 749that the rate of change of the diurnal heating rate is notconstant along the valley, but varies between the valleysegments. In particular, the largest rate of change of thediurnal heating rate occurs in the same narrow valleysegment (Rattenberg–Schwaz) in which the increase inwind speed has been observed in the other studies(Vergeiner 1983; Freytag 1987, 1988). It thus appearsplausible that the acceleration of the flow along the InnValley is due to a local increase in the along-valleypressure gradient which is caused by a higher heatingrate along the narrow portion of the valley. In additionto the volume effect, the heating rate of the valley at-mosphere is also likely influenced by flow dynamics.With the lack of thermodynamic data in the WippValley, it is not possible to determine if the along-valleyacceleration of the valley flow is due to differences inthe heating rate of the valley atmosphere. It is possible,however, that processes similar to those proposed forthe Inn Valley also occur in the Wipp Valley. The ef-fects of topography and flow dynamics on the heatingrates of the atmosphere in the Wipp Valley will beexplored in a future modeling study.7. SummaryIn this study, highly spatially resolved Doppler lidarobservations of thermally driven valley flows are pre-sented for three days. The observations show that, oneach of the days—despite somewhat different ambientflow conditions—the along-valley wind speed increasedwith distance in the up-valley direction through a nar-row section of the valley. In addition, the observationssuggest that the vertical wind structure changed alongthe valley from a relatively flat profile to a Prandtl-typeprofile. Volume flux analyses show that the along-valley volume flux diverged in the valley segment underinvestigation. This implies that the along-valley in-crease in wind speed cannot be explained by the lateralconstrictions of valley sidewalls. On days when themorning transition period was captured by the Dopplerlidar, the transition from down-valley to up-valley flowaloft occurred more quickly south of the lidar site thannorth of the lidar site.Since the along-valley volume flux divergence ap-pears to be a persistent flow feature in this valley seg-ment, it is hypothesized that the change in the geometryof the valley induces a localized change in the heatingrate of the valley atmosphere and hence also a localizedchange in the horizontal pressure gradient, which inturn causes the flow to accelerate. The effects of topog-raphy as well as dynamic processes on the spatial valleyflow structure will be examined in more detail with thehelp of numerical modeling in a future contribution.Acknowledgments. The authors thank Georg Mayrfor his organizational efforts of the MAP field cam-paign in the Brenner Pass area; Stephan Mobbs forproviding surface measurements; and Lisa Darby, JanetIntrieri, Richard Marchbanks, and Jim Howell for theirassistance in obtaining the lidar data. The authors alsothank the anonymous reviewers as well as Lisa Darbyand Dr. Jian-Wen Bao for helpful comments on themanuscript. This research was supported throughNSERC and CFCAS grants to D. Steyn and throughNOAA/ETL partial traveling funds to M. Rucker; R.Banta was supported through the Environmental Re-search Laboratories, Office of Atmospheric Research,Special MAP Project Funding.REFERENCESAtkinson, B. W., 1981: Meso-scale Atmospheric Circulations. Aca-demic Press, 495 pp.Banta, R. M., 1984: Daytime boundary-layer evolution overmountainous terrain. Part I: Observations of the dry circula-tions. Mon. Wea. Rev., 112, 340–356.——, and W. R. 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