mini-sodar observations of drainage flows in the rocky mountains

1 Department of Meteorology and Physics, University of Agricultural Sciences, Vienna, Austria2 NOAA/ERL/ETL, Boulder, Colorado, U.S.A.

Mini-Sodar Observations of Drainage Flows in the Rocky Mountains

F. A. WoÈlfelmaier1, C. W. King2, E. Mursch-Radlgruber1, and G. Rengarajan1

With 8 Figures

Received October 21, 1996Revised November 30, 1998

Summary

Vertical pro®les of drainage winds were monitored con-tinuously by a Doppler-Mini-Sodar during case studies intwo valleys, on both sides of the U. S. Continental Divide.A tethered balloon provided additional information on thevertical temperature and wind structure up to the Dividelevel. Ambient wind data were collected by a radar windpro®ler on the west side, and a tower on the crest of theDivide. The onset, evolution and breakup of the drainage¯ow were studied on two nights, when the ridge-top windswere westerly and skies were clear. To study the in¯uenceof the ambient ¯ow on drainage winds, changes in drainagewind speed, direction and depth, along with the volume ¯uxwere examined. It was found that, on the leeward side, thedrainage was strongly in¯uenced by the ambient winds(King, 1995b), which led to interruption and erosion of thelocally generated valley ¯ow. The drainage on the wind-ward side of the Divide was almost undisturbed. Acomparison of balloon and sodar wind pro®les showedvery good agreement during steady drainage conditions.

1. Introduction

Various ®eld experiments have been carried outto investigate the behavior of drainage winds.During the years 1982±1990, the AtmosphericStudies in Complex Terrain (ASCOT) programprovided extensive studies in western Coloradousing in situ and remote sensing instruments. Theresults demonstrated that these local, thermo-dynamically-driven wind systems are oftenin¯uenced by the ambient atmosphere abovethe ridge-tops. Meteorological factors such as

reduced surface radiational cooling due to cloudsand humidity, the strength and direction of upper-level winds, as well as unfavorable valley topog-raphy can all inhibit the development of drainage¯ow (Barr and Orgill, 1989; Whiteman, 1990;Gudiksen et al., 1992). In the Alps, drainage ¯owwas studied in large valleys such as the RhineValley and the Inn Valley during the mesoscaleexperiment in the area of Kufstein and Rosen-heim, MERKUR (Freytag and Hennemuth, 1983)and the mesoscale climate project, MESOKLIP(Fiedler and Prenosil, 1980). Smaller valleydrainage was investigated in the Swiss DischmaValley during the Dischma valley climate study,DISKUS (Freytag and Hennemuth, 1981). Mostof the ®eld campaigns lasted only 1±2 weeksduring clear and undisturbed ambient weatherconditions and were conducted over simpletopography. More recently, the in¯uence of theambient atmosphere was examined by Barr andOrgill (1989) in Brush Creek Valley, Colorado.They show a strong correlation between thestrength of the ambient ¯ow and the structure ofvalley drainage and stability. Using a Mini-Sodarin the Front Range of Colorado, Coulter andGudiksen (1995) examined the drainage ¯ow ofCoal Creek Canyon. They found a markeddependence of the strength and depth of thevalley ¯ow on the along valley component of theexternal, mesoscale winds.

Theor. Appl. Climatol. 64, 83±91 (1999)

The speci®c aim of this work is to study thestructure and dynamics of drainage winds duringtwo case studies, with respect to the ambient¯ow. The difference to former case studies is thatthe experiment was performed in two oppositelyoriented valleys, which are located on both sidesof the Continental Divide. The case studies werepart of a two-year measurement program, withinASCOT, to investigate the in¯uence of the ambientatmosphere on nocturnal drainage winds. Theywere carried out under ambient westerly windconditions. This gives a good opportunity toanalyze the effects of the ambient ¯ow on thedrainage winds on the windward and the leewardside of the Divide. From a wind-pro®ler andsurface station based one-year climatology ofambient winds and valley winds, we can tell howthe cases ®t into longer time ¯ow conditions.This climatology shows prevailing ambientwesterlies, a high frequency of drainage on thewest side and less occurrence of drainage on theeast side (King, 1997).

For the case studies we analyze the dynamicsof the drainage ¯ow together with in¯uences ofupper-level winds on the valley ¯ow. We discussthe difference between these in¯uences on boththe windward and leeward side of the Divide. Atime series of the drainage volume ¯ux iscalculated, using the continuous sodar data.Variations in the ¯ux re¯ect changes in theambient atmosphere.

2. Experiments

The two case studies were performed in the JimCreek valley on the night of 19±20 October 1995and Mammoth Gulch valley on the night of 15±16 October 1995. To compare the drainage inboth valleys with one sodar system, similarambient conditions of the two cases wererequired. Both experiment nights showed clearskies and westerly winds. At Jim Creek, Divide-level ambient winds ranged from 11±18 msÿ1,opposing the drainage ¯ow. At Mammoth Gulch,ambient winds were lower and varied from 5±14 msÿ1, and the drainage direction was about 60degrees south of the ambient westerly winddirection. Upper level winds were monitoredwith a NOAA 915-MHz wind pro®ler (Ecklundet al., 1990) west of the Divide and a meteor-ological tower on the crest. A Mini-Sodarobserved continuous wind pro®les, which arecompared with simultaneous tethered balloonmeasurements.

2.1 Topography and Sites

The study was performed in two separate valleys;one on each side of the U. S. Continental Divide(Fig. 1), west of Denver, Colorado. The twovalleys, Jim Creek and Mammoth Gulch areoppositely oriented. Jim Creek on the west sidedrains towards the west-northwest, Mammoth

84 F. A. WoÈlfelmaier et al.

Fig. 1. Topography of the study area along the upper portions of Mammoth Gulch and Jim Creek, located on opposite sides ofthe Continental Divide west of Denver, Colorado, USA. The shaded regions represent the drainage airshed areas, upvalleyfrom the sodar and tethersonde sites, indicated with SOD. The meteorological tower on the Divide is marked with `̀ CDT''(adapted from King, 1995)

Gulch on the east side towards the northeast. JimCreek valley is 7.5 km long and ranges in depthfrom 200 m at the valley mouth to 500 m fartherupvalley. The sodar and tethersonde sites,indicated with `̀ SOD'' in Fig. 1, were locatedin the valley center, at an elevation of 2860 mASL. Mammoth Gulch valley is 7.0 km long andranges in depth from 250 m at the valley mouthto 500 m farther upvalley. The sodar andtethersonde sites (Fig. 1) were also located inthe valley center, at an altitude of 2950 m ASL.The airshed size, upvalley from the sodar sitewas 13.9 km2 at Jim Creek and 16.6 km2 atMammoth Gulch. Both valleys had similarvegetation coverage consisting of coniferousforest in the lower portions of the valley andtundra in the upper portions. The meteorologicalstation on the Continental Divide was located ata high point on a north/south orientated ridge(Fig. 1) at an elevation of 3700 m ASL. Withprevailing westerly winds this station is upstreamfrom the Mammoth Gulch sodar site and down-stream from the Jim Creek site. A NOAA 915-MHz wind pro®ler operated near Granby, 32 kmnorthwest of the Continental Divide site, in alarge mountain basin at an elevation of 2500 mASL. The basin opens to the west, enabling thepro®ler to obtain measurements of relativelyundisturbed westerly ¯ow (King, 1995a).

2.2 Instrumentation and Data

The BOKU Doppler-Mini-Sodar (Mursch-Radl-gruber and Wolfe, 1993) is a monostatic, four-beam, multi-frequency acoustic sounder (Fig. 2).

Its transmit frequencies were set at 4.0 and4.3 kHz during this case study. It providedvertical wind pro®les with 10 m resolution upto a maximum height of 200 m AGL. These dataare averaged over 15 minutes. A tethered-balloonsystem measured vertical pro®les of wind andtemperature every two hours. The maximumballoon height was 700 m above the valleybottom. At higher wind speeds on the Divide orunder gusty conditions the balloon coverage waslimited to lower levels. NOAA's 915-MHz windpro®ler in Granby obtained hourly-averagedvertical pro®les of wind speed and direction over100-m range gates to maximum heights rangingfrom 4.5 to 6.5 km ASL. On top of the Divide,wind speed and direction were measured 3 mabove the surface with a R.M. Young WindMonitor and averaged over 10 minutes.

3. Sodar-Observations

3.1 Jim Creek

Figure 3 shows the temporal evolution and thevertical structure of the drainage on the night of19±20 October 1995, from 1800-0900 LST(Local Standard Time), observed by the Mini-Sodar in Jim Creek. The isolines of the windspeed magnitude are plotted against time andheight. Positive numbers indicate down-valleywinds, negative numbers up-valley winds both inmsÿ1. The arrows are the horizontal wind vectorfor each height and time.

At 1800 LST the drainage starts to developalong the valley ¯oor. This is seen by the tilteddrainage isolines (1, 2 msÿ1) between 1800 and1900 LST. In the upper layers of the valley, theup-valley wind is still active. Within one hour thedrainage rises to a height of more than 180 m and®lls the entire valley. At about 1930 LST a jetstructure starts to develop in the wind pro®le. Adisturbance occurs at 2300 LST, as lower windspeeds extend down to the jet region (see 3 msÿ1-isoline in Fig. 3), only to regain strength tovalues of 4.5 msÿ1 near the jet. Until 0300 LSTthe jet is expanding and increasing its maximumspeed, shown by the 4 msÿ1 isoline in Fig. 3.From this time on the ¯ow remains steady until itstarts to erode from above at 0600 LST. At 0700LST, the drainage also begins to erode from below,indicated by the shape of the 3 msÿ1-isoline in

Fig. 2. The BOKU Mini-Sodar at Jim Creek with a viewdown-valley

Mini-Sodar Observations of Drainage Flows in the Rocky Mountains 85

Fig. 3, as the convective boundary layer (CBL)develops.

3.2 Mammoth Gulch

Figure 4 shows the horizontal wind vector on thenight of 15±16 October 1995, from 1700-0600LST, in Mammoth Gulch. Isolines represent thewind speed. The height coverage of the sodar isonly about 100 m in this case, because of thewell-mixed air in the valley upper atmosphere.This air contains few temperature ¯uctuations,which are essential for the backscattered signalof the monostatic sodar.

At 1730 LST, the lowest 100 m of the valleyatmosphere decouple from the ambient wind anddrainage starts to build up along the valley ¯oor.

Wind speeds in Fig. 4 show a decrease at 1730LST and the onset of a drainage at 1800 LST.The direction of the drainage ¯ow is south tosouthwest. A weak jet structure develops with amaximum wind speed of 2.5 msÿ1. Balloonpro®les show that the drainage is limited to amaximum height of 60±100 m (Fig. 5b). At 2000LST, a disturbance from above enters the lowervalley atmosphere (see 3 and 4 msÿ1 isoline inFig. 4) and interrupts the drainage for about 2hours. A weak drainage structure then developslasting for another 2.5 hours. This drainage hasnearly the same characteristics as before withlight winds limited to a height of 80 m and a jetheight of about 30 m. At around 0000 LST, thedrainage is disrupted from above and is com-pletely eroded by 0100 LST.

Fig. 3. Isopleths of wind speed magnitudeon the night of 19±20 October 1995 at JimCreek. Positive numbers indicate down-valley winds and negative numbers up-valley winds in msÿ1. Arrows show thehorizontal wind vector

Fig. 4. Temporal and vertical structure ofthe wind ®eld on the night of 15±16October 1995 at Mammoth Gulch. Valuesare in msÿ1 and arrows show the horizontalwind vector


4. In¯uence of Ambient Winds

4.1 Jim Creek

Compared to climatological data from Jim Creek,the onset of the drainage on the night of 19±20October was about 2 hours delayed. This isbecause of the stronger ambient winds on thisday. The mean westerly wind speed is about5 msÿ1 lower (King, 1995a). The drainage duringthat night can be considered undisturbed. Thetemperature inversion in the valley is 100±200 mdeep, around 5 �C (Fig. 5a) and results in a strongdrainage ¯ow. There are variations in the strengthand depth of the drainage ¯ow but these aresmall, and a well-established ¯ow remains activeduring the entire night. Between 2200 and 0200LST the Divide winds accelerated from 13 to18 msÿ1 (Fig. 6). Suitable indication-parametersfor the in¯uence of ambient ¯ows on thedrainage wind are the volume ¯ux and thedrainage depth (Barr and Orgill, 1989). Underundisturbed conditions the drainage ®lls thewhole valley, resulting in maximum values ofthe volume ¯ux and the drainage depth. If thesevalues are smaller, an ambient disturbance isindicated. Both, the volume ¯ux and the baseheight of the transition layer have smaller valuesat around 2300-0000 LST. This may be causedby the acceleration of the ambient ¯ow. Theresults of the volume ¯ux calculations aredescribed in section 4.3.

4.2 Mammoth Gulch

The drainage at Mammoth Gulch is weak withmaximum speeds of 2.5 msÿ1. Balloon data (Fig.5b) show a 7±10 �C inversion in the lowest 80 m.After 2000 LST, a stronger disturbance fromabove enters the lower layers of the valleyatmosphere. The in¯uence of the ambient windin Fig. 4 is clearly seen: The wind pro®le showsno jet, speeds are increasing with height, and thedirection is shifting more westerly. At 2020 LST,the upper limit of the inversion descends down to50 m, but still is 9 �C. At 2130 LST, the winddirection in the former jet region (30±40 m) alsoshifts more westerly. At 2200 LST, the drainageredevelops, with similar characteristics to thosebefore the interruption. The increased ambient¯ow (Fig. 6) leads to erosion and disruption ofthe drainage from above, between 0000 and 0030

LST. The 0200 LST balloon pro®le showed notemperature inversion and gusty winds, re¯ectingthe topographically modi®ed ambient ¯ow (Fig.5b).

4.3 Volume Flux

The volume ¯ux for the two valleys wascalculated, assuming horizontal homogeneity,from the along valley component of the sodarwind pro®le in the valley center (ui in (1)) and thearea cross section of the valley in vertical steps of10 m, (ai in (1)). The index i represents thedifferent levels. The result was then multiplied by0.7, a factor that accounts for the lower windspeeds near the slopes of the valley and was foundindependently in different studies (Clements et al.,1989; King, 1989; Vergeiner et al., 1987). In thecalculation we used the following formula:

volume flux � 0:7XN

i�1

ai � ui �1�

The temporal variation of the ¯uxes in bothvalleys are shown in Fig. 7. The volume ¯ux forthe along-valley wind component at Jim Creekwas calculated for winds with directions rangingfrom 40±160 degrees and heights from 0±200 m.The average volume ¯ux, during well-developeddrainage conditions was 165,000 m3sÿ1. Thevolume ¯ux varied between 10±20% of the meanvalue during the night. The results in Fig. 7 showa 2-hour period with reduced ¯ux values between2230 and 0030 LST. This may be the result ofthe acceleration of the ambient winds aloft, atthe level of the Continental Divide. The winddirection range for the volume ¯ux calculation inMammoth Gulch was 140±250 degrees. Before1730 and after 0100 LST, stronger winds fromabove were channeled into the valley. Thesepro®les were not included in the calculation. Onthe night of 15±16 October, the drainage wasrelatively shallow, with a depth varying between60 and 100 m. Levels above this height were notconsidered in the calculation. The mean drainage¯ux in this case was 140,000 m3sÿ1. This is lowcompared to the ¯ux in Jim Creek, consideringthat the airshed of Mammoth Gulch is larger. Thestrong disturbance of the drainage between 2000and 2200 LST is also visible in the volume ¯uxdata. During this time the ¯ux varies over a wide


range. The maximum value at 2030 LST isdisturbed drainage, which is forced by theambient winds, but still is in the direction ofthe drainage ¯ow.

5. Comparison of Wind Data fromthe Sodar and Tethered Balloon

The backscatter for monostatic sodar systemsdepends most strongly on the vertical tempera-

ture gradient and on the vertical shear of thehorizontal wind (Neff, 1988). During the experi-ment night in Jim Creek, a statically stablepro®le existed throughout most of the valleyatmosphere. This stability, together with a well-established drainage and jet structure, providedoptimal conditions for acoustic backscatter. As aresult, the sodar range extended up above 180 m.Because of the steady drainage ¯ow, the 15-minaveraged sodar pro®les and the balloon data

Fig. 5. a) Vertical pro®les of the horizontal wind vector and temperature from tethersonde soundings on the night of 19±20October 1995 at Jim Creek. Vector length corresponds to the wind speed (see wind speed legend). The down-valley direction isgiven for reference. Between the morning and evening transition a well-developed drainage ¯ow is seen in the lowest 200 m. b)Same parameters as in Fig. 5a on the night of 15±16 October 1995 at Mammoth Gulch. Despite the strong inversion in the lowest100 m only very weak drainage ¯ow can be seen in the wind pro®les. In the 0200 LST pro®le the inversion near the ground isalready eroded by the accelerating ambient winds


showed very good agreement. Figure 8 shows anexample of wind speed and direction pro®lesfrom the sodar and balloon for both valleys. InMammoth Gulch, the temperature inversion hada maximum height of 100 m. Within this height

the inversion was very strong, at 9 �C. Because ofa weak jet structure, and the low drainage windspeeds, the wind shear was small. Above the100 m level, the ambient winds dominated, theair was well mixed and did not contain strong

Fig. 7. Time series of the drainage volume ¯uxes in JimCreek (solid) and Mammoth Gulch (dotted)Fig. 6. Time series of wind speed on 19±20 October 1995

(solid) and 15±16 October 1995 (dotted) on the crest of theContinental Divide

Fig. 8. Comparison of wind speed and direction from sodar and tethered balloon pro®les at Jim Creek, 20 October 1995, 0400LST, (left) and Mammoth Gulch, 15 October 1995, 20 LST, (right)


temperature ¯uctuations necessary for acousticbackscatter. The height coverage of the sodar inthis case was limited to 100±120 m. The sodarand the balloon data are in agreement, but due tothe variability of the drainage the deviations arelarger than the in the Jim Creek case (WoÈlfel-maier et al., 1996).

6. Conclusions

The results of the two case studies show theability of the Mini-Sodar to monitor temporalvariations and vertical structure of drainage ¯ow,and also to resolve the in¯uence of ambientwinds. The height coverage of the acousticsounder was dependent on atmospheric condi-tions. In good drainage situations with strongtemperature inversions and a distinct jet, found inJim Creek, the height coverage was more than180 m. In Mammoth Gulch, the drainage wasdisturbed, shallow and weak, and the sodarcoverage was limited to approximately 100 m.A summary of the basic drainage and ambientwind characteristics during the two cases is givenin Table 1.

The agreement of sodar and tethered balloonwind pro®les depends on the variability of thedrainage ¯ow. In undisturbed conditions bothwind pro®les compared very well. Duringdisturbed and variable drainage, greater differ-ences were found, between the point datameasured by the balloon and the time- andvolume-averaged sodar data.

Drainage on the windward side of the Dividewas almost undisturbed, although ridge-top windspeeds were greater than during the second case.On the leeward side, the drainage in the valleywas strongly in¯uenced by the ambient ¯ow,despite lighter ambient winds and strong thermalforcing, which existed in both cases. In¯uences

of the ambient atmosphere were also re¯ected inthe variation of the drainage volume ¯ux. Themain phenomena causing these disturbances andthe role of valley topography and orientation aresubject of further investigations.

Acknowledgements

The ®rst author would like to thank the Austrian Ministry ofScience and the Institute of Meteorology at the Universityof Agricultural Sciences for ®nancial support, that made thestay at ETL possible. Many thanks go to Dr. Clark King forsupport and data access, to Dr. William Neff for theinvitation, to Brian Templeman and Catherine Russell forhelp at the Boulder Atmospheric Observatory (BAO) towerand to many other individuals from ETL staff for theirassistance.

References

Barr, S., Orgill, M. M., 1989: In¯uence of external meteor-ology on nocturnal valley drainage winds. J. Appl.Meteor., 28, 497±517.

Clements, W. E., Archuletta, J. A., Hoard, D. E., 1989: Meanstructure of the nocturnal drainage ¯ow in a deep valley.J. Appl. Meteor., 28, 457±462.

Coulter, R. L., Gudiksen, P., 1995: The dependence ofcanyon winds on surface cooling and external forcingin Colorado's Front Range. J. Appl. Meteor., 34, 1419±1429.

Ecklund, W. L., Carter, D. A., Balsley, B. B., Currier, P. E.,Green, J. L., 1990: Field tests of a lower tropospheric windpro®ler. Radio Sci., 25(5), 899±906.

Fiedler, F., Prenosil, T., 1980: Das MESOKLIP-Experiment,mesoskaliges Klimaprogramm im Oberrheintal, Wiss.Berichte, Meteor. Inst. Univ. Karlsruhe, 107 pp.

Freytag, C., Hennemuth, B., 1981: DISKUS, Gebirgswin-dexperiment im Dischmatal-Datensammlung. Teil 1: Son-dierungen. Wiss. Mitt 43, Meteorologisches Institut,UniversitaÈt MuÈnchen, 250 pp.

Freytag, C., Hennemuth, B., 1983: MERKUR: MesoskaligesExperiment im Raum Kufstein-Rosenheim. Wiss. Mitt.48, Meteorologisches Institut, UniversitaÈt MuÈnchen,132 pp.

Table 1. Drainage Flow and Ambient Wind Characteristics during the Case Studies

Jim Creek Mammoth Gulch

Ambient ¯ow, Wind speed 11±18 msÿ1 5±14 msÿ1

Divide tower Wind direction WNW WNWDrainage height 170±220 m 60±100 mMaximum speed 4.5 msÿ1 2.5 msÿ1

Valley atmosphere Duration 11 hours 4 hoursInversion height 150±200 m 80±130 mInversion strength 4±5 K 7±9 K


Gudiksen, P. H., Leone, J. M., King, C. W., Ruf®eux, D.,Neff, W. D., 1992: Measurements and modeling theeffects of ambient meteorology on nocturnal drainage¯ow. J. Appl. Meteor., 31, 1023±1032.

King, C. W., 1989: Representativeness of single verticalwind pro®les for determining volume ¯ux in valleys.J. Appl. Meteor., 28, 463±466.

King, C. W., 1995a: Modi®cation of wind velocity over theContinental Divide. Preprints, Seventh Conf. on MountainMeteorology, Breckenridge, Amer. Meteor. Soc., 156±159.

King, C. W., 1995b: Thermally forced circulations in oppo-sitely oriented airsheds along the Continental Divide inColorado. Preprints, Seventh Conf. on Mountain Meteor-ology, Breckenridge, Amer. Meteor. Soc., 332±337.

King, C. W., 1997: A climatology of thermally forcedcirculations in oppositely oriented airsheds along theContinental Divide in Colorado. NOAA Technical Mem-orandum ERL ETL-283, 152 pp.

Mursch-Radlgruber, E., Wolfe, D. E., 1993: Mobile high-frequency Mini-Sodar and its potential for boundary-layerstudies. J. Appl. Phys., B, 57, 57±63.

Neff, W. D., 1988: Observation of complex terrain ¯owsusing acoustic sounders: Echo interpretation. Bound.Layer Meteor., 42, 207±228.

Vergeiner, I., Dreiseitl, E., Whiteman, C. D., 1987:Dynamics of katabatic winds in Colorado's Brush CreekValley. J. Atmos. Sci., 44, 148±157.

Whiteman, C. D., 1990: Observations of thermally devel-oped wind systems in mountainous terrain. In: Blumen W.,(ed.) Meteorological Monographs: Atmospheric Processesover Complex Terrain. Amer. Meteor. Soc., 5±42.

WoÈlfemaier, F. A., Mursch-Radlgruber, E., King, C. W.,Rengarajan, G., 1996: Mini-Sodar observations of drai-nage ¯ows and the in¯uence of ambient winds in theRocky Mountains. Proc., Eighth Int. Symp. on AcousticRemote Sensing, Moscow, 7.25±7.31.

Authors' addresses: F. A. WoÈlfelmaier, E. Mursch-Radl-gruber, G. Rengarajan, Department of Meteorology andPhysics, University of Agricultural Sciences, TuÈrkenschanz-strasse 18, A-1180 Wien, Austria; C. W. King, NOAA/ERL/ETL, 325 Broadway, Boulder, Colorado 80303, USA.


mini-sodar observations of drainage flows in the rocky mountains

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