Diurnal Winds in the Himalayan Kali Gandaki Valley. Part III: Remotely Piloted Aircraft Soundings

2042 MONTHLY WEATHER REVIEW VOLUME 130

JOSEPH EGGER,* SAPTA BAJRACHAYA,ϩ RICHARD HEINRICH,* PHILIP KOLB,* STEPHAN LAÈ MMLEIN,# MARIO MECH,* JOACHIM REUDER,* WOLFGANG SCHAÈ PER,@ PANCHA SHAKYA,ϩ JAN SCHWEEN,* AND HILBERT WENDT* *Meteorologisches Institut, UniversitaÈt MuÈnchen, Munich, Germany ϩDepartment of Hydrology and Meteorology, Ministry of Science and Technology, Kathmandu, Nepal #Fachbereich Maschinenbau, Fachhochschule Regensburg, Regensburg, Germany @Astrium, Friedrichshafen, Germany

(Manuscript received 26 July 2001, in ®nal form 18 February 2002)

ABSTRACT In 1998 a ®eld campaign has been conducted in the north±south-oriented Kali Gandaki valley in Nepal to explore the structure of its extreme valley wind system. Piloted ballon (pibal) observations were made to map the strong upvalley winds as well as the weak nocturnal ¯ows (Part I). The strati®cation of the valley atmosphere was not explored. In Part II of this multipart paper, numerical simulations are presented that successfully simulate most of the wind observations. Moreover, the model results suggest that the vigorous upvalley winds can be seen as supercritical ¯ow induced by contractions of the valley. Here, the results of a further campaign are reported where remotely piloted airplanes were used to obtain vertical pro®les of temperature and humidity up to heights of ϳ2000 m above the ground. Such pro®les are needed for an understanding of the ¯ow dynamics in the valley and for a validation of the model results. This technique is novel in some respects and turned out to be highly reliable even under extreme conditions. In addition four automatic stations were installed along the valley's axis. Winds were observed via pibal ascents. These data complement the wind data of 1998 so that the diurnal wind system of the Kali Gandaki valley is now documented reasonably well. It is found that the fully developed upvalley ¯ow is con®ned to a turbulent layer that tends to be neutrally strati®ed throughout the domain of observations. The strati®cation above this layer is stable. A capping inversion is encountered occasionally. This ®nding excludes explanations of the strong winds in terms of hydraulic theories that rely on the presence of strong inversions. Pairs of simultaneous ascents separated by 5±10 km along the valley axis reveal a remarkable variability induced by the topography and, perhaps, by an instability of the ¯ow. The analysis of the surface data as well as that of the soundings shows that the ¯ow above the neutral layer affects the surface pressure distribution and, therefore, the acceleration of the extreme upvalley winds.

1. Introduction south. A mountain pass leads to the Tibetan Plateau about 20 km to the northeast of Lo Manthang. The Kali Gandaki valley in Nepal stands out both Before 1998, scattered information was available in- because of its extreme geometry and the intensity of the dicating that the diurnal wind system of the valley ex- diurnal upvalley winds. The Kali Gandaki River orig- hibits rather strong upvalley winds (ϳ20 m sϪ1) between inates near the town Lo Manthang (see Fig. 1) and ¯ows Marpha and Kagbeni (see Egger et al. 2000, hereafter southward through the Mustang basin. It cuts through KG1, for details and relevant literature). This upvalley the Himalayan barrier between the villages of Marpha wind is called Lomar by the locals (``southerly wind''; and Ghasa forming there one of the deepest valleys on we change here the spelling ``Lhomar'' as used in KG1 Earth. Farther south, the river rushes down into a gorge and ZaÈngl et al. (2001, hereafter KG2) to ``Lomar'' to to reach the lower parts of Nepal at an altitude of ϳ1000 be consistent with the spelling of Lo Manthang where m above MSL. The Mustang basin extends from Marpha lo refers also to the south). Nocturnal downvalley winds to Lo Manthang. It is con®ned by the towering mountain appeared to be weak. chains to the east and west and by the Himalayas in the In fall 1998 the Meteorological Institute of the Uni- versity of Munich and the Department of Hydrology and Meteorology in Kathmandu conducted a joint ®eld Corresponding author address: Joseph Egger, Meteorologisches campaign in order to explore the structure of this wind Institut der UniversitaÈt MuÈnchen, Theresienstr. 37, 80333 MuÈnchen, Germany. system in detail (KG1). The following conclusions of E-mail: [email protected] KG1 are based on about 100 double-pilot ascents per-

᭧ 2002 American Meteorological Society

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FIG. 2. Isentropes (contour interval, 1 K; solid) and wind (vectors) in a section along the Kali Gandaki valley as obtained in the reference run REF of KG2 in the afternoon (t ϭ 15 h). Shading: light, wind speeds 10±15 m sϪ1; medium, 15±20 m sϪ1; dark, Ͼ20 m sϪ1. See also Fig. 7a of KG2. The bold letters mark the locations of Tukuche, Marpha, Jomsom, and Kagbeni. Height is above MSL. The narrowest point of the valley is located near Marpha.

simulations included the Tibetan Plateau as the dominant topographic feature of the region. Five nests were FIG. 1. Map of the Kali Gandaki valley: airplane ascents, dots; needed to resolve the core region reasonably well with permanent surface stations, crosses with circles; villages and sites a grid size of 800 m. There are 38 levels in the vertical mentioned in the text, crosses. Pibal ascents were made in Jomsom, Dhumpha, Chuksang, Tangye, and Lo Manthang. LK is Langpoghyun with a maximum resolution of 100 m near the ground. Kola. Height contours, solid (m); contour interval is 500 m. Hori- The initial state is in thermal wind balance with the zontal distances as indicated at the axes (km). The map is based on meridional temperature gradient. The level of no winds topographic data with a resolution of 30Љϫ30Љ. These data have is chosen such that the atmosphere is almost at rest in been interpolated toa1kmϫ 1 km grid. See also Fig. 1 of KG1. the Kali Gandaki valley. The model calculations were Dashed, Kali Gandaki and LK. quite successful in that nearly all the observed features of the wind ®eld were reproduced in a reference run. formed at eight locations covering the distance from Sensitivity experiments were carried out in order to elu- Lete to Lo Manthang. cidate the mechanisms driving the valley wind system including precipitation [see also Barros et al. (2000) for 1) The upvalley winds start near the surface before a recent precipitation analysis of the area]. noon. The layer of strong winds with typical veloc- It is a key result of KG2 that a rather stable layer ities of ϳ15 m sϪ1 grows over about1htoadepth with strong upvalley winds forms during the day in the of ϳ1000±1500 m. entrance and, in particular, in the core region (see Fig. 2) The breakdown of the upvalley wind regime after 2). This layer of 1000±1500-m thickness is found up- sunset begins close to the ground. The upvalley ¯ow stream of the widening of the valley near Marpha. The ceases before midnight. isentropes descend between Marpha and Jomsom. The 3) Nocturnal downvalley ¯ows are quite weak. ¯ow accelerates in the descending branch to attain a 4) Upvalley winds are less powerful both in the so- maximum speed of 23 m sϪ1 near Jomsom. The layer called exit region between Chuksang and Lo Man- of rapid ¯ows stays close to the ground farther to the thang and in the entrance region (Ghasa±Tukuche) north and ascends toward Tibet with slightly reduced than in the core region (Marpha±Kagbeni). ¯ow speeds. Note, however, that in¯ow of moderate Stimulated by these results, ZaÈngl et al. (2001) per- speed occurs above this layer up to a height of ϳ5000 formed numerical simulations of the Kali Gandaki wind m. The stability of the Lomar layer increases from Tuk- system using the Pennsylvania State University±Na- uche to Jomsom. The Brunt±VaÈisaÈlaÈ frequency is N ϳ tional Center for Atmospheric Research ®fth-generation 1.2 ϫ 10Ϫ2 sϪ1 in Marpha and 1.6 ϫ 10Ϫ2 in Jomsom Mesoscale Model (MM5). The total ¯ow domain of the where the isentrope ␪ ϭ 315 K and the ground are

Unauthenticated | Downloaded 09/28/21 10:45 PM UTC 2044 MONTHLY WEATHER REVIEW VOLUME 130 chosen as reference surfaces (␪ equals potential temperature). The ¯ow pattern in Fig. 2 is reminiscent of the results of laboratory and hydraulic model studies of ¯ows through lateral contractions. For example Arakawa [(1969); see also Baines (1995) for an updated outline of the theory] analyzed single-layer ¯ow through a valley of variable width where the Froude number F ϭ U(gЈH)Ϫ1/2 is the key parameter (U, ¯ow speed; gЈ, reduced gravity; H, depth of the ¯ow). There exists a class of solutions where F ϭ 1 at the narrowest point and where F Ͼ 1 downstream. This type of ¯ow is similar to that in Fig. 2 where event indications of a hydraulic jump are seen downstream of the maximum contraction FIG. 3. Monthly mean values of the hourly mean wind speed U (m near Marpha. Such models have been invoked by Pettre sϪ1) as observed in Kagbeni in Feb±Mar 1990 at a height of 9 m. (1982), Jackson and Steyn (1994a, b), Pan and Smith (1999), and others to explain strong wind storms in valleys and gaps, Structures as displayed in Fig. 2 can on the strati®cation of the upvalley ¯ow both in the also be found in two-layer models (Armi 1986; Baines narrow part of the valley and in the Mustang basin and 1995) and in continuously strati®ed ¯ows underneath a to verify in this way the model results of KG2 as dis- free surface representing an inversion (Armi and Wil- played in Fig. 2. liams 1993). The observations described in KG1 were made in the In principle, hydraulic theory cannot be applied to fall. By choosing February and March we wished to thermally driven ¯ows simply because sources and sinks learn more about the seasonal variability of the Kali of heat are not included. Here, however, the situation Gandaki valley wind system. The mean wind speeds in is somewhat different. As explained in more detail in February and March 1990 in Kagbeni are shown in Fig. KG2, the diurnal heating of the Mustang basin generates 3. These curves are fairly similar to those for September a pressure difference between, say, Jomsom and the and October (see Fig. 2 of KG1) except that the winds ``free'' atmosphere to the south of Ghasa. The strong are slightly less vigorous than in the fall. Note that Fig. winds can be seen as a response to this pressure gradient 2 represents equinoctial conditions in the simulations of in much the same way as gap winds respond to imposed KG2 that do not take into account the observed cli- large-scale pressure ®elds. Moreover, Fig. 2 suggests matological mean ¯ow. Therefore, a validation of Fig. that the Lomar is separated by a rather stable top layer 2 is possible in March as well as in September. The from the atmosphere above. All this indicates that an climatological ¯ow has a southerly component at upper application of hydraulic theory might help to better un- levels in the fall while westerlies prevail in the spring derstand both the observations and the model results. (e.g., Ramage 1971). Throughout the campaign the ¯ow In turn, information on the thermal structure of the val- at 500 hPa was generally westerly although many per- ley atmosphere is needed in order to better understand turbations moved over the area from the west. the vigorous Kali Gandaki valley winds. Such infor- This paper is organized as follows. The equipment is mation was not provided by the campaign of 1998. Stan- described in section 2. The surface observations are dard instrumentation for vertical soundings [see e.g., presented in section 3, the soundings in section 4. A Clements et al. (1989) for the design of a valley ¯ow discussion is presented in section 5, and concluding re- experiment] like radiosondes is impractical in these remarks are given in section 6. mote areas where the gas needed to ®ll the balloons has to be carried up by porters. Tethersondes cannot be used 2. Instrumentation for the same reason. Moreover, the winds are too violent. a. Airplanes However, battery-powered model airplanes with remote control (remotely piloted vehicles, RPV) are highly suit- The idea to use model airplanes as a carrier of in- able for this purpose. They are light, their energy de- struments in meteorology is not new. Corresponding mand is low, and they can be carried by porters to almost experiments were conducted successfully in the 1970s any starting position in the Mustang region. Their max- (Konrad et al. 1970; M. Reinhardt 1997, personal com- imum ascent height of ϳ2000 m is suf®cient to penetrate munication). More recent activities are described in the layer of strong winds. Such planes were used during Renno and Williams (1995), Chilson et al. (1999), and the ®eld campaign in 2001 to be described in this paper Stephens et al. (2000). The new airplanes were designed (18 February±27 March). Double theodolite pilot bal- by W. SchaÈper (WS) and built by WS and S. LaÈmmlein loon observations were carried out simultaneously. In in cooperation with Modellbau Ulrich and Blue Airlines. addition, an array of surface stations was installed. It They can be ¯own up to heights of at least 5000 m was the main goal of this effort to collect information above sea level in highly turbulent wind ®elds. The

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The planes carry sensors for an observing system designed and built by IngenieurbuÈro WuÈrtenberger con- sisting of a miniaturized datalogger and specially adapt- ed sensors for pressure, temperature, and humidity. The pressure sensor is based on the Motorola MPX 2100. Its signal is ampli®ed and compensated for changes of temperature. The resolution is ϳ1 hPa. Humidity is recorded via the HIH-3605-B Honeywell sensor with 0.6% resolution. The response time is ϳ5 s. Temperature is obtained from a LM50 C National semiconductor sensor. The resolution is ϳ0.2 K and the response time is Յ1 s. Ascents for an adiabatic lapse rate must be expected to be 0.3ЊC warmer than descents because of the related hysteresis effect. Three wind generators provided the electric power to recharge the batteries of the planes and all portable computers. One plane out of 11 was lost during the campaign, while another 1 was dam- aged but could be repaired. It is impossible to ¯y the planes during the night; therefore, we concentrated on the daytime ¯ow evolution during this campaign.

b. Permanent stations Four permanent stations were installed. They con- tained instrumentation for temperature, wet-bulb temperature, pressure, and wind speed and direction. These instruments were mounted on a 3-m mast. Power was provided by a solar panel. Global radiation was recorded at one station (Jomsom). Observations are made with a time step of Dt ϭ 120 s. The surface stations were positioned in Kagbeni on the roof of a house, in Jomsom close to the airstrip, in FIG. 4. (a) Prototype Kali. (b) The pilot is wearing the special binoculars needed to follow the plane at heights of more than 1000 m Marpha in an open ®eld near the river, and in Tukuche above the ground. again on the roof of a house (Fig. 1). It would have been preferable to have all stations located close to the axis of the valley. The stations in Marpha and Jomsom prototype Kali is shown in Fig. 4a. The plane has a satis®ed this requirement quite well, and that in Kagbeni length of 1.29 m and a wing span of 2.10 m. The total came close as the house chosen (Hotel Niligiri) is ex- mass in 3 kg. Flight velocities are in the range of 10± posed to the full force of the wind. On the other hand, 40msϪ1. The optimum climb rate for highest altitude the village of Tukuche is sheltered to some extent and is ϳ5msϪ1. The propeller is driven by an electromotor so were our instruments. Wind speeds at this location (Hacker HBR 50S26). Power is provided by 14±16 re- underestimate the wind strength found in the riverbed chargeable NiMH cells. To ease control of the planes (see KG1). The stations were in operation as follows: in turbulent ¯ow, the planes are equipped with gyro kagbeni, 25 February±23 March; Jomsom, 22 February± systems for stabilization around the roll axis. Special 26 March; Marpha, 23 February±21 March; Tukuche, binoculars have been developed by Firma Zeiss so that 23 February±21 March. the plane can be followed visually up to heights of ϳ2000 m above the ground (Fig. 4b). To increase color c. Pilot balloons contrast, blue blocker sunglasses, also made by Zeiss, were used. The pilot controls the plane by a Robbe radio Two theodolites were used to track a helium-®lled gear control. Nets are used for landing in dif®cult terrain balloon. The system is identical to that described in and in strong winds. A sounding is normally completed KG1. Observations were made in Jomsom, Dhumpha, within 15 min. The ¯ight path during ascent is chosen Chuksang, Tangye, and Lo Manthang (Fig. 1). to maximize the ascent height; that is, the pilots try to exploit slope winds and thermals. Descent is performed 3. Surface observations so as to equal time spans for ascent and descent. The planes were steered by several of the authors (P. Kolb, The surface observations are presented ®rst because S. LaÈmmlein, and WS). they contain detailed information on the diurnal cycle

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Ϫ1 FIG. 5. Time series of wind speed ␷ (m s ) (bold) and direction (dots), temperature T (ЊC) and wet-bulb temperature Tw (ЊC) (dashed), speci®c humidity q (g kgϪ1), and surface pressure p (hPa) in Marpha for 19 and 20 Mar. of ¯ow conditions in the valley. This information helps ence between temperature and wet-bulb temperature in the interpretation of the soundings. The days of 19 was quite large during the day. Relative humidities were and 20 March are selected to demonstrate the main char- ϳ35%. acteristics of the time series in Kagbeni and Marpha The diurnal pressure oscillations are represented (Figs. 5 and 6). Marpha is located in the narrow part of clearly in the surface pressure time series. According the valley, which opens toward the Mustang basin just to Dai and Wang (1999) both the diurnal and the semi- north of Marpha (see Fig. 1). According to Fig. 2, Mar- diurnal tidal oscillation reach maximum values of 0.6± pha is situated upstream of the region of ¯ow acceler- 0.8 hPa in Nepal, the semidiurnal one being slightly ation while Kagbeni is downstream. The wind speeds larger. By and large, our observations (see Table 1) are recorded in Marpha were moderate, with maximum in agreement with this ®nding. However, as can seen speeds of ϳ7msϪ1. There were weak northeasterlies from Table 1, the amplitude of the diurnal (semidiurnal) on 19 March until the Lomar set in and continued until pressure oscillation increased from 0.6 (0.8) hPa in Tuk- midnight. The next day was essentially calm until 0900 uche to 1.2 (1.1) hPa in Kagbeni. This must be a local LST when the Lomar set in again. The global radiation effect that is presumably related to the strong upvalley data of Jomsom (not shown) indicate that the afternoon winds. In Figs. 5 and 6 there is a pressure maximum of 19 March was partly cloudy. Visibility was unusually about 2 h after sunrise and another one before midnight. low on the morning of 20 March. Clouds covered the The minimum in the afternoon is more pronounced than sky at ϳ1300 LST and rain fell for a few hours. Thus, that early in the day. 20 March is a day with perturbations in the afternoon. The Lomar set in at Kagbeni later on both days and Temperature and the wet-bulb temperature were rising was more powerful with maximum speeds of 18 m sϪ1. relatively late on both days because of the shadow cast The weak winds in the morning of 20 March were main- by the Annapurna massif. Moisture decreased in the ly from the south. There was, however, a distinct onset morning of 19 March as is typical at this station. It of Lomar on the day as well. Temperatures in Kagbeni increased after the onset of the Lomar, which appeared were quite similar to those in Marpha. The diurnal and to bring moister air up the valley. However, the differ- semidiurnal oscillations of temperature were about the

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FIG. 6. The same as Fig. 5 but for Kagbeni. same at all stations (Table 1). The evaluation of the 2 gives the mean delay between all pairs of available related phases shows that temperature maxima occur at stations. Here, we de®ne the onset time as that moment about the same time (1310±1325 LST). where the upvalley wind reaches half the maximum The data for Jomsom (not shown) were similar to strength of that day. This de®nition avoids ambiguities those in Kagbeni except that the Lomar commences due to the existence of weak upvalley winds before the earlier but with reduced intensity. Maximum wind onset of the strong winds. It is clear from Table 2 that speeds were ϳ12 m sϪ1 on both days. Those in Tukuche the upvalley wind regime moves from Marpha to Jom- were ϳ8msϪ1. The increase of the intensity of the som and farther up to Kagbeni. Typical speeds of prop- valley winds from Tukuche to Kagbeni is also re¯ected agation are 5 m sϪ1. The variability of the delays from in the diurnal and semidiurnal variations of the wind day to day is large. For example, the maximum delay speed in Table 1. between Kagbeni and Marpha is 116 min while lomar Both the delay of the onset of the upvalley wind was recorded a few minutes earlier in Kagbeni on 7 regime in Kagbeni with respect to Marpha and the re- March. Data from Tukuche are not included because of duction of the diurnal pressure variation in Marpha with the general weakness of the winds there, which makes respect to Kagbeni were found almost every day. Table it sometimes dif®cult to determine an onset time. The winds are driven by pressure gradients. In principle, pressure gradients can be evaluated by reducing TABLE 1. Amplitudes of the diurnal (®rst entry) and semidiurnal (second entry) oscillations of pressure, temperature, and wind velocity at the four permanent stations. TABLE 2. Mean delay (min) of the onset of the valley wind regime at the station in the left column with respect to the station in the top Pressure Temperature Wind row. In parentheses, number of days with negative delays, total num- Station (hPa) (ЊC) (m sϪ1) ber of days, and propagation velocity in m sϪ1. Tukuche 0.6/0.8 3.8/1.6 2.2/0.9 Kagbeni Jomson Marpha Marpha 0.7/1.0 4.2/1.9 2.6/0.8 Jomsom 1.0/1.0 4.4/1.7 3.8/1.5 Kagbeni 0 24 (3/22/6) 50 (1/21/5) Kagbeni 1.2/1.1 4.1/1.8 6.0/1.9 Jomsom Ð 0 22 (1/23/4)

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TABLE 3. Mean ratio of the pressure decrease between the maximum in the morning and the minimum in the afternoon at the station in the left column and that in the upper row. The second entry in the ®rst row is the mean pressure decrease (hPa) at the station indicated on top. Kagbeni Jomsom Marpha Tukuche Kagbeni 1.0/4.5 1.22/3.9 1.44/3.4 1.67/3.1 Jomsom Ð 1.0 1.14 1.35 Marpha Ð Ð 1.0 1.11 all pressure to the height of Jomsom. However, the ba- rometers used at the permanent stations differed in the range of ϳ1 hPa. This deviation is too large for a pres- FIG. 7. Correlation of the temperature and the wind velocity in sure reduction to be useful. Instead we simply argue Marpha (bold) and Kagbeni (dashed) for positive lags; i.e., wind perturbations are shifted by ␶ (h) with respect to temperature. Diurnal that the nocturnal winds are extremely weak. Therefore, and semidiurnal cycles are removed. there is no appreciable pressure gradient force in the Mustang basin at night. The pressure decrease during the day is different at the various stations and the station Differential heating from the ground between, say, Tuk- with the largest decrease is the one with the lowest uche and Kagbeni is presumably not important in gen- pressure. The mean value of the pressure decrease from erating the strong upvalley winds. the morning maximum to the minimum in the afternoon The covariance functions for all time series have been is given in Table 3 for each station (®rst row). The evaluated as well. Of course, the diurnal and semidiurnal decrease is smallest in Tukuche and largest in Kagbeni. signals are dominant. However, long timescales prevail This yields a mean pressure difference between Kagbeni even after these periods are removed. The ®rst zero and Jomsom of 0.6 hPa in the afternoon. The mean crossing of autocorrelations of single station data is pressure difference between Marpha and Jomsom is 0.5 found in the case for lags of 4±6 h. Delays are the most hPa. The reference simulation in KG2, where the af- interesting features to be extracted from these functions. ternoon pressure minimum is located slightly north of For example, Fig. 7 shows the correlation of temperature Jomsom, is broadly consistent with this ®nding. and wind in Marpha for positive lags. The wind veloc- As has been mentioned, the daily ranges of temper- ities peak at a lag of about 25 min. Thus, the reaction ature are about the same in Marpha, Jomsom, and Kag- of the winds in the narrow part of the valley to changes beni. This implies that the enhanced pressure changes of the temperature is quite fast. On the other hand, the in Kagbeni are due to dynamical processes on top or winds have cooling impact on the temperature (not above the boundary layer. Pressure difference between shown). The correlation of temperature and wind in various stations can be explained by variations of the Jomsom reaches a maximum after ϳ60 min, that is, depth of the upvalley wind layer if there is a pronounced considerably later than in Marpha (Fig. 7). This differ- inversion on top of this layer. As will be shown in the ence may be due to the fact that Marpha is located in next section, such an inversion exists only occasionally. a valley where the ¯ow is more constrained. One may, however, argue that the observed widening and intensi®cation of the ¯ow between Marpha and Kag- 4. Vertical structure beni is possible only if air descends from above into the Lomar layer. The related warming and pressure de- As has been mentioned, the airplane soundings were crease appears to be largest in Kagbeni. performed to explore the strati®cation of the valley at- The northward delay of the onset of strong winds as mosphere during the day and to validate the model re- quanti®ed in Table 2 is seen in the diurnal and semi- sults of KG2 as presented in Fig. 2. The model predicts diurnal wind oscillations as well. The wind maximum a deep in¯ow layer for Marpha and shallow and rather in Kagbeni as descried by these oscillations lags that in stably strati®ed ¯ow in Jomsom. Marpha by 30 min and that in Tukuche by 35 min. The By far the largest number of ascents were made at related pressure extrema are delayed in an opposite airport in Jomsom (15±17, 19±21, 23±25 March), that sense. The pressure maximum in the morning occurs in is, in the core region of the Kali Gandaki wind system. Kagbeni at 0817 LST, in Jomsom at 0830, in Marpha A selection of these results will be presented ®rst. Par- at 0850, and in Tukuche as late as 0947. The pressure allel ascents upstream of Jomsom were made in Dhum- minimum in the afternoon is recorded in Kagbeni at pha (21 March) and Marpha (16 and 17 March). The 1525 LST, in Jomsom 1530, in Marpha at 1535, and at only soundings in the entrance region took place near 1635 in Tukuche. This shows again that the evolution Tukuche (23±25 March). Further parallel ascents in Ek- of pressure gradients between the various stations is lobati (see Fig. 1; 19 and 20 March) provide information hardly linked to the temperature of the Lomar layer. on the strati®cation downstream of Jomsom. In addition,

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minated before noon. Model plane ascents were not per- mitted during that time. Visual control is impossible before sunrise, leaving little time for early aircraft observations. A ridge was moving toward the Mustag area on that day but gradients at 500 hPa were weak. Light northeasterly winds prevailed throughout the ®rst 2 km (not shown). The morning atmosphere was stably strat- i®ed at least up to the maximum ascent height of 1450 m above the ground (N ϳ 1.3 ϫ 10Ϫ2 sϪ1). The air was slightly cooler during descent. The loop during descent close to the ground re¯ects, of course, a ¯ight maneuvre before landing. The temperature pro®le in Fig. 8 de- viates substantially from those found quite often early in the morning in valleys (Brehm and Freytag 1982; McKee and O'Neal 1989; Whiteman 1990) where an inversion extends from the ground to a height of a few hundred meters. This difference is presumable due to the absence of nocturnal downvalley ¯ow in Jomsom. The moisture decrease linearly with height. Ascent and descent values are almost identical. The absence of a clearly visible hysteresis effect in the moisture pro®le suggests that the difference of the potential temperature between ascent and descent are real because the response time of the temperature sensors is shorter than that of the humidity sensors.

2) UPVALLEY FLOW A fairly complete set of ascents has been obtained on 19 March and will be presented in detail below. A warm ridge was centered over Tibet on that day. The upvalley wind regime was just beginning to es- tablish itself at 1100 LST (Fig. 9a). Rather weak northeasterlies are found up to a height of 2000 m above a shallow layer of upvalley ¯ow. Convection appears to erode the stable layer established during the night so that an almost neutral layer of ϳ300 m depth is seen FIG. 8. Potential temperature ␪ (K) as a function of height during above a shallow superadiabatic layer. Higher up, the ascent (solid) and descent (broken) at 0620 LST 25 Mar in Jomsom. strati®cation is stable (N ϳ 10Ϫ2 sϪ1). Such pro®les are Also given is the speci®c humidity q (g kgϪ1) during ascent and common in valleys late in the morning (e.g., Whiteman descent (triangles). The vertical coordinate z is de®ned such that z ϭ 0 at Jomsom airport at 2751 m above MSL. 1990). Ascent and descent temperatures differ by 1±2 K close to the ground presumably because of additional heating before the start. The moisture is well mixed up the ¯ow in the exit region was investigated in Tangye to a height of ϳ700 m and is constant above z ϭ 1400 on 27 and 28 February, in Lo Manthang (3, 4, and 6 m. The ␪ pro®le in Eklobati is similar in shape to that March), and in Chuksang (10 and 11 March). It was in Jomsom but the air was warmer by about1Kand intended to obtain a reasonably complete dataset of tem- also drier. One hour later (1200 LST; Fig. 9b), the upval- perature and moisture pro®les throughout the Mustang ley wind was quite strong in the lowest 250 m of the basin during the day by undertaking this rather strenuous valley atmosphere. A layer of weaker upvalley winds part of the campaign. extended at least up to a height of 1300 m. The velocity spike at z ϭ 1300 m is presumably real because tracking problems tend to occur only during the ®rst minutes of a. Core region an ascent when the balloon is close to the observers. This pro®le of velocities is reminiscent of those reported 1) EARLY MORNING in KG1 (Figs. 10 and 11 of KG1) for the early stages Only one ascent was made early in the morning in of the Lomar. In general, ascent temperatures are higher Jomsom (Fig. 8; 25 March). Flight operations at the than those found in descent. The systematic temperature airport usually began before 0700 LST and were ter- differences of more than2Kasrecorded up to heights

Unauthenticated | Downloaded 09/28/21 10:45 PM UTC 2050 MONTHLY WEATHER REVIEW VOLUME 130 of 1200 m are so large when compared to our estimate by about 3 K on top of the neutral layer so that the ¯ow of hysteresis effects that we speculate that these struc- is supercritical with F ϳ 1.5 in Jomsom. The related tures are linked to deep eddies. Rapid and strong var- pressure difference between Jomsom and Eklobati is 5± iations of the vertical motion of the RPV have also been 7 Pa. reported by the pilots. The mountains along the valley One hour later, the air above 1000 m was cooler and axis are quite rugged and have heights of 1000±2000 drier in Jomsom and the inversion disappeared (Fig. 9d). m with respect to the valley bottom. It is conceivable There is no obvious reason for this process. It is aston- that vortices are generated by the interaction of the ishing that the inversion still existed in Eklobati where upvalley ¯ows with these obstacles. Moisture appears a slight cooling aloft was accompanied by a moistening. to be affected by the eddies as well. Humidity is higher The q curves in Eklobati differ greatly between ascent during descent in contrast to what one would expect for and descent. This difference is not due to an observa- a hysteresis effect. All in all, the onset of the Lomar tional error. The plane carried two moisture sensors that led to a decrease of the temperature in the lowest 250 gave almost exactly the same result. m, as one would expect for thermally driven ¯ows. The Lomar layer reached a depth of at least 1500 m However, the temperature rose in the layer above the at 1500 LST in Jomsom (not shown). Ascent and descent strong winds. The pronounced increase of moisture differ again substantially at that time so that one cannot within1huptoaheight of 700 m must be due to assign a clear structure to this turbulent boundary layer. advection. Towering cumulonimbus clouds evolved in the after- Given a ¯ow speed of 10 m sϪ1 it takes just 10 min noon in the south above the mountains bordering the to advect air from Jomsom to Eklobati. Indeed, tem- kali Gandaki valley. This feature is typical of radiation peratures in Eklobati decreased in the lowest 500 m so days with clear skies in the morning. Like on many that Eklobati was no longer warmer than Jomsom. There other days, the clouds moved northward and showers is some similarity is both pro®les, but many details do were recorded in Jomsom. Thus, this last ¯ight of the not coincide. Advective moistening is seen in Eklobati day documents a late stage of the Lomar where the as well. valley ¯ow was affected by convective cells. The par- A well-mixed layer of 1000-m depth was capped by allel ascent in Eklobati was not successful. a pronounced inversion at 1300 LST in Jomsom (Fig. So far we have found that capping inversions may 9c). A stable layer was found higher up. The strong occur but do not appear necessary for the Lomar to winds were con®ned to the neutral layer. The moisture develop. The Lomar layer is neutrally strati®ed in con- was well mixed in the neutral layer and the moisture trast to the model results. Both warming and cooling content was strongly reduced above the inversion. As are seen to occur above the Lomar layer. This suggests compared to Fig. 9b, the potential temperature is now that conditions above the Lomar layer proper have an lower below the inversion and about the same above. impact on the evolution of the upvalley ¯ows in agree- The close proximity of ascent and decent in Fig. 9c is ment with the numerical simulations. surprising given the large ¯ow speeds in the Lomar On 21 March piloted-balloon (pibal) observations and layer. Moreover, the noisiness of the wind speed pro®le related airplane ascents were made in Dhumpha at the indicates that the ¯ow was highly turbulent. The ␪ pro- mouth of the Langpoghyun valley (see also Fig. 1). This ®le in Eklobati at 1300 LST is quite similar to that in valley ascends along the slopes of the Annapurna mas- Jomsom but there is warming above the lomar layer. sif. One expects maximum descent near Dhumpha ac- Warming from above is common in valleys during the cording to Fig. 2. The sounding at 1100 LST reveals day (e.g., Brehm and Freytag 1982) and is thought to an inversion at z ϳ 1200 m that descends until noon be due to the downward branch of the cross-valley cir- by 200 m due to an increase of the temperature aloft. culation with slope winds forming the upward branch. The parallel soundings in Jomsom do not reveal inver- Given the large width of the Mustang basin it is doubtful sions but the temperature aloft is increasing as well. The if this explanation is suf®cient in this case. It is likely ¯ow is neutrally strati®ed in the Lomar layer. that upvalley ¯ow descending above the Lomar layer In 1998, tree deformations were mapped in the Lang- contributes to the warming (see also Fig. 2). poghyun valley (see Fig. 18 of KG1). It was found that The neutral layer in Eklobati was not as deep as in strong upvalley ¯ows must prevail in this valley. These Jomsom. Therefore, we have here an example of a winds form a branch of the Kali Gandaki wind system downward sloping inversion where hydraulic modelling but are oriented almost normal to the standard ¯ow di- may be appropriate. The potential temperature ``jumps'' rection of the lomar. On 21 March, such winds were

→

FIG. 9. Potential temperature ␪ (K) and speci®c humidity q (g kgϪ1) as a function of height during ascent and descent on 19 Mar in Jomsom and Eklobati. Also given is the wind speed (bold) and the wind direction (dots) as obtained at the base in Jomsom. As can be seen from Fig. 1 upvalley winds occur for 180Њ Յ dir Յ 240Њ: (a) 1100, (b) 1200, (c) 1300, and (d) 1400 LST. Wind velocities too noisy to be shown. Jomsom (Eklobati) is at 2751 m (2810 m) above MSL. Jomsom airport: z ϭ 0m.

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FIG.9.(Continued)

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total pro®le in Tukuche at 1330 LST. This is less than in Fig. 2. The warming at upper levels in Jomsom with respect to Tukuche is found also in the numerical simulations.

c. Exit region

Soundings in the exit region were carried out in Chuk- sang, Tangye, and Lo Manthang. Wind intensities in Chuksang were lower than in Jomsom in agreement with what had been found in 1998 (KG1). The Lomar layer was neutrally strati®ed without a capping inversion. The village of Tangye is located to the east of the Kali Gan- daki River. An escarpment overlooking the main valley was selected for the observations. Until that time, no ¯ow data had been collected in the eastern part of the upper Mustang basin. Observations were made on 27 and 28 February under a prevailing southwesterly upper- level ¯ow ahead of a trough at 500 hPa. Upvalley winds FIG. 10. Track (dots) of a balloon released at 1026 LST 21 Mar were of moderate strength on 27 February. The next near Dhumpha. Bold shows height lines (m); T1T2 shows the baseline. The numbers along the track give the height of the balloon above day extremely strong upvalley winds were observed in the starting position; y (x) axis pointing toward the north (east). the presence of strong convective activity in the south. Figure 12 shows a situation where at least the lower part of the Lomar layer is well mixed with maximum observed. The track of a balloon released at 1026 LST velocities of ϳ20 m sϪ1. This establishes that vivid at the axis of this side valley is depicted in Fig. 10. The upvalley winds occur in the eastern part of the Mustang balloon followed this valley at a velocity of ϳ10 m sϪ1. basin. The balloons could not be tracked for long enough It should be said, however, that the ¯ow above the bal- nor did the planes reach suf®ciently large altitudes as loon in Fig. 10 was southerly. At the moment, the cause needed for an estimate of the depth of the lomar layer. of these rapid northeastward accelerations is unknown. The dryness of the ¯ow indicates that the air, say, in Ascents in Marpha were made in unfavorable con- Kagbeni, is not simply advected up to Tangye. Dry air ditions. The upper-level ¯ow was southwesterly on both from higher levels must be mixed into the Lomar ¯ow. days and appears to have induced strong downward mo- It was hoped that strong katabatic winds or cold air tion above Marpha. It was frustratingly dif®cult for the out¯ows from the Tibetan Plateau would be centered in pilots to gain height. The ascents are, therefore, not Lo Manthang. Conditions late in winter tend to be fa- presented. vorable for such ¯ows. However, the observations were disappointing in that respect. The morning observations b. Entrance region of 3, 4, and 6 March revealed that the strati®cation was quite stable close to the ground (N 0.015 sϪ1). On 3 The only soundings in the entrance region of the Lo- ϳ mar were made near Tukuche. Examples are given in March upvalley winds were quite vigorous and the strat- Fig. 11. The pre-lomar ␪ pro®le at 1130 LST in Tukuche i®cation was neutral at least up to heights of ϳ1000 m. is quite similar to that in Jomsom but temperatures are No inversion was found. An interesting situation oc- slightly lower in the lowest 750 m than in Jomsom. curred on 6 March with a deep layer of stably strati®ed Note that the strati®cation is neutral even before the northerlies on a rather cold morning. The 500-hPa map onset of the Lomar. At 1330 LST the potential temper- shows that a small ridge moved over the area. The stable ature in Tukuche is essentially constant in the lowest layer disappeared quite rapidly (ascent at 0945 LST; not 500 m. The strati®cation is weakly stable above. The shown) but is was not before late in the afternoon that air is clearly warmer above the Lomar layer in Jomsom southerlies were observed. The Lomar layer of a depth than in Tukuche. Moisture is well mixed throughout the of ϳ1250 m was well mixed with maximum ¯ow speeds ascent in Tukuche but decreases slightly above the Lo- of8msϪ1. No inversion was detected. The northerly mar layer in Jomsom. All this indicates that there is adverse ¯ow conditions were almost too dominant for descent between Tukuche and Jomsom. It is dif®cult to the Lomar to reach Lo Manthang on that day. compare Figs. 2 and 10 because there are no wind ob- All in all, it became clear that the basic characteristics servations available in Tukuche so that the depth of the of the Lomar layer, namely neutral strati®cation and the Lomar layer cannot be estimated. The Brunt±VaÈisaÈlaÈ absence of an inversion, are found also in the upper part frequency is N ϳ 5 ϫ 10Ϫ3 sϪ1 when evaluated for the of the Mustang basin.

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nϩ1 n 5. Discussion [(pmkϪ p ) Ϫ (p mkϪ p )]/Dt

We have to conclude on the basis of the soundings nnn ϭ ␥ukmkϩ ␦(p Ϫ p ) (5.3) that the strong winds in the Kali Gandaki valley cannot be explained on the basis of hydraulic ¯ow theory, and captures this situation. Strong upvalley winds would we have, therefore, to look for other ways to understand lead to an increase of the pressure difference pm Ϫ pk, the observations. As a ®rst step let us ®t equations to provided ␥ Ͼ 0. A damping term is added on the right. the surface data in order to obtain information on the Note, that (5.3) is counterintuitive. One could argue that dynamics of the Lomar. For example, we may select the strong winds advect mass to Kagbeni. This would re- stations Kagbeni and Marpha to adopt the equation of duce the pressure difference and, then, ␥ should be neg- motion along the valley axis to the available surface ative. Of course, (5.1) and (5.3) are simply the equations data of wind and pressure. With of a regressive model of ®rst order relating the variables (unϩ1 Ϫ unnnn)/Dt ϭ ␣(p Ϫ p ) ϩ ␤u (5.1) uk and (pm Ϫ pk) to their changes in time. kk mkk The coef®cients ␣ Ϫ ␦ in (5.1) and (5.3) are deter- (t ϭ nDt; Dt ϭ 120 s; subscripts k, m for Kagbeni and mined by a least square ®t using all observations. The Marpha; p, pressure; u, along-valley wind component; resulting coef®cients are given in Table 4 for three pairs u Ͼ 0 for upvalley ¯ow), we have a simple equation to of stations. With ␣ ϭ 0.32 ϫ 10Ϫ4 (m sϪ1 PaϪ1)we be tested that asserts that changes of the along-valley Ϫ1 Ϫ1 obtain about half the correct value ␳ o Dxkm ϳ 0.6 ϫ wind speed uk in Kagbeni are caused by the pressure Ϫ4 10 for the pair Kagbeni±Marpha where Dxkm is the gradient force estimated by using the pressure data from distance of the villages. This means that the true ac- Kagbeni and Marpha. There is also a damping term. It celerations due to the pressure gradients are partly can- is understood in (5.1) that all terms are deviations from celed by an effect that is not represented properly in the time mean. Nickus and Vergeiner (1984) applied (5.1). Advection of momentum from above is presum- this equation to observations in the Inn valley. Of ably a good candidate. The damping term ␤ is negative course, (5.1) is a rather simple approximation to the full and implies a damping time | ␤ | Ϫ1 ϳ 40 min. Nickus equation of motion: and Vergeiner (1984) obtained a damping time of 30 .p min for the Inn valleyץ u 1ץ uץץ u 1ץ 2 ϭϪ u Ϫ ␷ Ϫ w Ϫ The coef®cient ␥ is positive with ␥ ϳ 6 ϫ 10Ϫ5 (Pa xץ z ␳oץ yץ xץ t 2ץ mϪ1). The pressure difference between Marpha and Kag- ϩ f ␷ Ϫ du, (5.2) beni increases by about 2 Pa within1hiftheanomalous Ϫ1 where y is the cross-valley coordinate; ␷, the cross- wind speed is 10 m s . This is a relatively weak effect. valley ¯ow; w, the vertical velocity; and f, the Coriolis The damping is quite strong with a damping time of Ϫ1 parameter. When using (5.1), we assume that cross-val- | ␦ | ϳ 20 min. The system (5.1) and (5.3) is, of ley advection, vertical advection, and rotation effects course, stable in the sense that any initial perturbation are negligible. This omission is presumably justi®ed would be damped out in an integration given the co- with respect to rotation and cross-valley winds. It is less ef®cients in Table 4. Without damping, however, the obvious if vertical advection is unimportant. Unfortu- system is unstable with an e-folding time of (␣␥)Ϫ1/2 ϭ nately, no data are available to check this assumption. 6.4 h. The related growth is slow but not negligible. nn2 2 It is surprising that ␣ is smaller for the Kagbeni± In principle, a term ϳ[(uukm ) Ϫ ()] could have been added on the right of (5.1) in order to include along- Jomsom pair than for Kagbeni±Marpha despite the fact valley advection. It would have been inconsistent, how- that Jomsom is closer to Kagbeni than Marpha. The ever, to include horizontal advection when vertical ad- cancellation of the accelerations due to the pressure gra- vection is excluded. dient by unrepresented effects must even be larger in We wish to introduce another more empirical equation Jomsom than in Kagbeni. The damping time is longer in combination with (5.1). As outlined above, there is and ␥ has just half the value of that of the Kagbeni± some evidence that descent and the related warming Marpha pair. On the other hand, the coef®cients attain aloft cause the pressure to fall deeper in Kagbeni in the the largest values for the Jomsom±Marpha pair. The e- afternoon than in Jomsom or Marpha (see Table 2). This folding time (␣␥)Ϫ1/2 is just 3.6 h in this case. This descent is presumably linked to the acceleration and indicates that the Jomsom±Marpha section is the most widening of the Lomar jet toward the north. If so, the important one for the evolution of the upvalley ¯ow. wind velocity in Kagbeni provides a gross measure of The interaction of the Lomar with the layers above is this effect. We assume that strongest there. However, this interaction extends clearly

←

FIG. 11. Potential temperature ␪ (K) and speci®c humidity q (g kgϪ1) during ascent (bold) and descent (broken) in Jomsom and Tukuche on 24 Mar. Also given are wind speed (bold) and direction (crosses) as observed at the Jomsom baseline: (a) 1130 and (b) 1330 LST. Jomsom (Tukuche start) is at 2751 m (2670 m) above MSL. Jomsom airport: z ϭ 0m.

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FIG. 12. Potential temperature and speci®c humidity during ascent (bold) and descent (broken) at 1200 LST 28 Feb in Tangye (3600 m above MSL). Also given are wind speed and direction; z ϭ 0 at the observing site. to Kagbeni. This conclusion is supported by the frequent and almost at rest. Sometimes there is an inversion, more observations of warming above the Lomar layer. often there is none. We have here, therefore, a variation We wish to address here one further issue that came of the classic Kelvin±Helmholtz instability problem up quite often during the presentation of the soundings. (e.g., Drazin and Reid 1981). As outlined in the appen- There is good evidence that the strong Lomar ¯ows are dix, the stability in the upper layer is only weakly damp- turbulent. Pro®les during ascent and descent differ ing. The discontinuity of the upvalley wind at the upper sometimes so strongly that it is quite unlikely that this boundary of the Lomar layer supports instability at all difference is due to the choice of the ¯ight track (see wavelengths L ϭ 2␲/k (k is the along-valley wave- Figs. 9 and 11). As has been mentioned, this turbulence number) provided an inversion is absent. However, may be generated by the orographic obstacles to the growth rates increase ϳk so that short waves are pre- ¯ow. There is, however, also the possibility that the ferred. It is conceivable that such instabilities play a con®guration of Lomar is inherently unstable. We may role in generating the deviations in Figs. 9 and 11. perceive the ¯ow in an idealized manner as a two-layer Finally, let us comment on the relationship of the system where the strong winds are con®ned to a mostly numerical simulations of KG2 to the observations. neutral layer. The air is stably strati®ed above this layer Many aspects of the wind pro®les obtained in our cam-

TABLE 4. Coef®cients ␣, ␤, ␥, and ␦ as determined from the station data by a least square ®t of (5.1) and (5.3) for the station pair given in the ®rst row.

␣ ␤ ␥ ␦ (10Ϫ4 msϪ2 PaϪ1) (10Ϫ4 sϪ1) (10Ϫ4 PamϪ1) (10Ϫ4 sϪ1) Kagbeni±Marpha 0.30 Ϫ4.4 0.62 Ϫ7.8 Kagbeni±Jomsom 0.23 Ϫ2.5 0.31 Ϫ7.9 Jomsom±Marpha 0.72 Ϫ6.5 0.82 Ϫ14.7

Unauthenticated | Downloaded 09/28/21 10:45 PM UTC AUGUST 2002 EGGER ET AL. 2057 paign are compatible with those presented in KG2. The on the strati®cation of these upper layers, the wind ob- onset of the upvalley winds close to the surface and the servations via pibals are too inaccurate at large heights following buildup of the upvalley wind layer as found to tell us much about the ¯ow conditions up there. in KG2 were observed this time, as in 1998 (KG1). Therefore, important aspects of the dynamics of the Kali Moreover, the model results agree with the observations Gandaki wind regimes have to await further clari®ca- of strong upvalley winds in Tangye (see Fig. 6 of KG1) tion. and Lo Manthang. It is, however, our impression that It is an important side result of the recent ®eld cam- the turbulence parameterization in MM5 underestimates paign in the Kali Gandaki valley that soundings up to the generation of turbulence in strong ¯ows as in Fig. heights of ϳ2000 m above the ground can be made by 2. It is presumably for that reason that the model predicts use of RPV even under extreme conditions. The vertical a stably strati®ed Lomar layer in contrast to the obser- resolution of the resulting pro®les is quite good as is vations. Pronounced gravity wave features have been the quality of the observations. This sounding system found in KG2 that where generated by the ridges near is quite mobile and vertical pro®les can be obtained Marpha (see Fig. 8 of KG2). It was suggested in KG2 almost anywhere. It is a drawback of this method that that these waves are important for the generation of the highly skilled pilots are needed at least under the ex- strong upvalley winds. This speculation can be ruled treme conditions of the Kali Gandaki valley. out on the basis of our observations. Internal gravity waves cannot be excited in neutrally strati®ed ¯ow. Acknowledgments. The campaign could not have been conducted without the ®nancial support by Deutsche Forschungsgemeinschaft. A great number of people sup- 6. Concluding remarks ported the project in many ways. We wish to express The following summarizing statements can be made our gratitude to Robbe Modellsport for help with respect on the basis of our observations. The period of weak to the remote control, to Hacker Antriebstechnik for help winds extends normally from late in the evening till with respect to the plane engines, to IngenieurbuÈroWuÈr- 0900±1100 LST. Although weak downvalley ¯ow pre- tenberger for support with respect to the sensors, to vails during that time, weak upvalley ¯ow occurs quite Zeiss for its generosity with respect to the development often. The rather limited number of soundings in the of the optical control systems, to Simprop Electronic morning suggests that the atmosphere is stably strati®ed for advice on the implementation of the motor, to H. at sunrise but that there is no inversion. After sunrise a MuÈller of Modellbau Ulrich and Blue Airlines for help convective layer of a few hundred meters depth evolves in building the plane, to K. Budion for advice with with a shallow superadiabatic layer at the ground. respect to batteries, and to Optik Schadow. We are grate- The upvalley winds set in close to the ground. A well- ful to G. ZaÈngl for comments and to L. Gantner for mixed upvalley wind layer is established within about providing information on the synoptic situation. The an hour with a depth of 1000±1500 m. Sometimes an comments by the referees helped substantially to im- inversion is found on top of this layer, but more often prove the presentations of the results. the neutral layer is simply topped by a stable layer. The fully developed upvalley ¯ow is presumably unstable. APPENDIX These ¯ows are found at all locations from Tukuche up to Lo Manthang. Instability of Two-Layer Flow The transition to the upvalley regime occurs ®rst in Marpha and Tukuche and moves with speeds of ϳ5m We consider a two-layer atmosphere. The lower layer sϪ1 upward to Kagbeni and farther on to Lo Manthang. of depth H represents the lomar layer with constant As outlined in KG1 and KG2, the upvalley winds are mean wind U1 and vanishing Brunt±VaÈisaÈlaÈ frequency 2 generated primarily by the heating of the Mustang basin N 1 ϭ 0. A second layer with vanishing wind U 2 ϭ 0 2 before noon. Low pressure is established there with re- and stable strati®cation N 2 Ͼ 0 is assumed to extend to spect to the atmosphere to the south of Lete at the same in®nity above this layer. Both layers are separated by a height as the basin. The ¯ow driven by this pressure material surface. There is no density jump. We assume gradient has to pass the narrow part of the valley where perturbations of the form exp[ik(x Ϫ ct)] and solve the the Kali Gandaki River cuts through the Himalayas. The well-known wave equation dynamically most interesting part of the ¯ow is that near 22wÃ Nץ and to the northeast of Marpha where the highest wind iiϩ wÃ ϭ 0 (A.1) z222i (U Ϫ c) Ϫ kץ speeds are found. The suggestion of KG2, that some []i kind of supercriticality is involved in generating these [i ϭ 1, 2; wÃ i(z) Fourier coef®cient of vertical velocity], high velocities, is not supported by the soundings. How- under the boundary conditions wÃ ϭ 0atz ϭ 0, pÃ ϭ ever, the soundings support and even extend the sug- 1 1 pÃ 2 at z ϭ H, and wÃ 2 ϭ 0 at in®nity. Moreover, the gestion of KG2 that descent above the Lomar is an kinematic condition important part of the dynamics of the Kali Gandaki wind system. Although the soundings provided information wÃ 112(c Ϫ U ) ϭ wÃ c (A.2)

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designs of the 1984 ASCOT ®eld study. J. Appl. Meteor., 28, relates the vertical velocities at z ϭ H. With wÃ 1 ϳ sinh(kz) and wÃ ϳ exp(Ϫnz), one obtains the polynomial 405±413. 2 Dai, A., and J. Wang, 1999: Diurnal and semidiurnal tides in global 24 2 2 3 surface pressure ®elds. J. Atmos. Sci., 56, 3874±3891. kc Ϫ 4kU1 cosh (Hk)c Drazin, Ph., and W. Reid, 1981: Hydrodynamic Stability. Cambridge

22 2 2 2 2 Monographs on Mechanics and Applied Mathematics, Cam- ϩ [6kU12cosh (kH) ϩ N sinh (kH)]c bridge University Press, 525 pp. Egger, J., S. Bajrachaya, U. Egger, R. Heinrich, J. Reuder, P. Shakya, 23 2 24 2 Ϫ 4kU11cosh (kH)c ϩ kU cosh (kH) ϭ 0. (A.3) H. Wendt, and V. Wirth, 2000: Diurnal winds in the Himalayan Kali Gandaki valley. Part I: Observations. Mon. Wea. Rev., 128, There are two real roots. One of them violates the upper 1106±1122. boundary condition and must be excluded. The other Jackson, P., and D. Steyn, 1994a: Gap winds in a fjord. Part I: Ob- one is c ϳ U /2 for L Յ 4 H. The complex pair is c ϳ servations and numerical simulation. Mon. Wea. Rev., 122, 1 2645±2665. U1(1 Ϯ i)/2 except for L Ͼ 4 H, where | c | is somewhat ÐÐ, and ÐÐ, 1994b: Gap winds in a fjord. Part II. Hydraulic analog smaller. In essence we recover the result of the Kelvin± simulations. Mon. Wea. Rev., 122, 2666±2676. Helmholtz problem. Konrad, T., M. Hill, J. Rowland, and J. Meyer, 1970: A small radio- controlled aircraft as a platform for meteorological sensors. APL Tech. Dig., 10, 11±19. REFERENCES McKee, Th., and R. O'Neil, 1989: The role of valley geometry and energy budget in the formation of valley wind. J. Appl. Meteor., 28, 445±456. Arakawa, S., 1969: Climatological and dynamical studies on the local Nickus, U., and I. Vergeiner, 1984: The thermal structure of the Inn strong winds, mainly in Hokkaido. Japan Geophys. Mag., 34, valley atmosphere. Arch. Meteor. Geophys. Bioklimatol., A33, 349±425. 199±215. Armi, L., 1986: The hydraulics of two ¯owing layers of different Pan, F., and R. Smith, 1999: Gap winds and wakes: SAR observations densities. J. Fluid Mech., 163, 27±58. and numerical simulations. J. Atmos. Sci., 56, 905±923. ÐÐ, and R. Williams, 1993: The hydraulics of a strati®ed ¯uid Pettre, P., 1982: On the problem of violent valley winds. J. Atmos. ¯owing through a contraction. J. Fluid Mech., 251, 355±375. Sci., 39, 542±554. Baines, P., 1995: Topographic Effects in Strati®ed Flows. Cambridge Ramage, C., 1971: Monsoon Meteorology. International Geophysics Monographs on Mechanics, Cambridge University Press, 482 Series, Vol. 15, Academic Press, 296 pp. pp. Renno, N., and E. Williams, 1995: Quasi-Lagrangian measurements Barros, A., M. Joshi, J. Putkonen, and D. Burbank, 2000: A study in convective boundary layer plumes and their implications for of the 1999 monsoon rainfall in a mountainous region in central the calculation of CAPE. Mon. Wea. Rev., 123, 2733±2742. Nepal using TRMM products and rain gauge observations. Geo- Stephens, G., and Coauthors, 2000: The Department of Energy's At- phys. Res. Lett., 27, 3683±3686. mospheric Radiation Measurement (ARM) unmanned aerospace Brehm, M., and C. Freytag, 1982: Erosion of the night-time thermal vehicle (UAV) program. Bull. Amer. Meteor. Soc., 81, 2915± circulation in an Alpine valley. Arch. Meteor. Geophys. Bio- 2937. klimatol., B31, 331±352. Whiteman, C., 1990: Observations of thermally developed wind sys- Chilson, Ph., P. Johansson, M. Johnsson, R. Moses, J. Stanojev, Th. tems in mountainous terrain. Atmospheric Processes over Com- Hedquist, A. Niva, and R. Scheifele, 1999: Ripan: A remotely plex Terrain, Meteor. Monogr., No. 45, Amer. Meteor. Soc., 5± controlled aircraft project for tropospheric and stratosphere re- 42. search. Proc. 14th ESA Symp. on European Rocket and Balloon ZaÈngl, G., J. Egger, and V. Wirth, 2001: Diurnal winds in the Hi- Programs, Potsdam, Germany, ESA SP-437, 111±116. malayan Kali Gandaki valley. Part II: Modeling. Mon. Wea. Rev., Clements, W., J. Archuleta, and P. Gudiksen, 1989: Experimental 129, 1062±1078.

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