Diurnal Winds in the Himalayan Kali Gandaki Valley. Part I: Observations

Transcription

1 1106 MONTHLY WEATHER REVIEW Diurnal Winds in the Himalayan Kali Gandaki Valley. Part I: Observations JOSEPH EGGER Meteorologisches Institut, Universität München, Munich, Germany SAPTA BAJRACHAYA Department of Hydrology and Meteorology, Ministry of Science and Technology, Kathmandu, Nepal UTE EGGER, RICHARD HEINRICH, AND JOACHIM REUDER Meteorologisches Institut, Universität München, Munich, Germany PANCHA SHAYKA Department of Hydrology and Meteorology, Ministry of Science and Technology, Kathmandu, Nepal HILBERT WENDT AND VOLKMAR WIRTH Meteorologisches Institut, Universität München, Munich, Germany (Manuscript received 4 February 1999, in final form 28 May 1999) ABSTRACT The diurnal wind system of the Kali Gandaki Valley in Nepal was explored in September and October 1998 in a field campaign using pilot balloons as the main observational tool. This valley connects the Plateau of Tibet with the Indian plains. The river crosses the Himalayas forming the deepest valley on Earth. Intense upvalley winds blow up this valley during the day. Observations were made along the river at various spots selected between the exit point from the Himalayas and the source close to the Plateau of Tibet. The strongest upvalley winds were found between Marpha and Chuksang with typical speeds of m s 1.Theupvalleywindsets in first at the ground but an upvalley wind layer of m depth forms quickly after the onset. This deep inflow layer persists up to Lo Manthang, a town located a few kilometers south of the Plateau of Tibet. Deceleration in the late afternoon and evening also appears to commence near the ground. Weak drainage flow forms late in the night. The causes of these phenomena are discussed. 1. Introduction The valley of the Kali Gandaki River in Nepal is presumably the deepest valley on Earth. The river originates close to Tibet in the Mustang Himal and passes the town of Lo Manthang under the name Mustang Khola to flow then essentially southward through the Mustang district till the main barrier of the Himalayas is reached near Marpha (Fig. 1). The Himalayan ranges are crossed between Marpha and Ghasa. It is near Lete and Ghasa that Annapurna and Dhaulagiri tower above the river making the valley s depth more than 5000 m. The valley follows a steep descent from Ghasa to Tatopani. Corresponding author address: Dr. Joseph Egger, Meteorologisches Institut, Arbeitsgruppe für Theoretische Meteorologie, Universität München, Theresienstraße 37, Munich 80333, Germany. J.Egger@lrz.uni-muenchen.de The most conspicuous climatological feature of the region is the pronounced decrease of precipitation up the valley. Monthly mean precipitation is given in Table 1forsixstationsonthewayupfromTatopanitoLo Manthang for the months September and October, that is, for the two months of the field campaign to be described below. While the summer monsoon clearly affects the precipitation in Tatopani and Lete, there is little impact in Marpha just a few kilometers farther north. There is virtually no precipitation in Lo Manthang in the fall. The same features are apparent in the climate atlas of Nepal (ICIMOD 1996). The upper part of the valley is famous not only for its depth but also for its strong diurnal upvalley winds. These winds blow virtually every day and have been noted by travelers as a rather unpleasant feature of the area (Tucci 1977; Peissel 1992). In particular, the wind tends to raise a lot of dust in the afternoon. Figure 2 shows the monthly mean wind speed V as observed in 2000 American Meteorological Society

2 APRIL 2000 EGGER ET AL FIG. 1.MapoftheKaliGandakiValley:dots,observationsites;crosses,villagesandtowns mentioned in the text. Height contours solid (m). Horizontal distances in (km). The map is based on topographic data with a resolution of Thesedatahavebeenaveragedto a1km 1kmgrid. Kagbeni (see Fig. 1) in September and October Wind direction was not recorded so that it is not clear whether upvalley or downvalley winds prevailed during the night. However, there is no doubt with respect to TABLE 1. Mean monthly precipitation along the Kali Gandaki Valley in (mm) as observed during the years From Department of Hydrology and Meteorology (1997). Sep Oct Tatopani Lete Marpha Jomsom Ghami Lo Manthang the winds during the day. Strong upvalley winds set in between 0800 and 1000 LST, and reach an impressive maximum of 14 m s 1 at 1400 LST to decay later on. An inspection of the daily observations reveals rather little variability from day to day. The daily maximum speed varies only between 11.6 and 17.5 m s 1 within these two months. The strength of these winds is rather extreme. For example, typical valley winds in the Alps exhibit intensities of about 5 m s 1 (see Whiteman 1990 for a review). The asymmetry between day and night is also extreme. Typically upvalley winds in the Alps are about as intense as downvalley winds (e.g., Dreiseitl et al.

3 1108 MONTHLY WEATHER REVIEW FIG. 2.MonthlymeanvaluesofthehourlymeanwindspeedV (m s 1 )asobservedinkagbeniinsepandoct1990ataheightof30 ft. 1980; Whiteman 1990). In Kagbeni, the wind roars up the valley in the afternoon, while gentle breezes are typical of nighttime flow. Aside from these observations in Kagbeni little information is available on the diurnal winds along the Kali Gandaki River. Although winds are measured with high temporal resolution at the airport in Jomsom, these data are not recorded. Winds and other meteorological data are collected by the Mustang Development Service Association in Jomsom. However, these data have not yet been published. The Department of Hydrology and Meteorology of H. M. Government of Nepal collects wind data in Lo Manthang and Jomsom. Wind roses show a clear preference for southwesterlies in Jomsom and southerlies in Lo Manthang in September and October with weaker winds in Lo Manthang. Ohata and Higuchi (1978) tried to infer the intensity of the upvalley wind from the deformation of trees. This method is bound to fail in the upper Mustang area to the north of, say, Jomsom simply because there are few trees in this extreme environment. However, Ohata and Higuchi arrived at interesting results in the lower part of the valley, to be discussed further below. Neininger and Reinhardt (1986) report on attempts to probe the valley atmosphere with an instrumented motorglider. On four flights from Pokhara (situated about 50 km to the southeast of Tatopani) to Kagbeni, distinct upvalley winds were encountered only on 7 February 1985 when it was possible to fly into the valley in the afternoon. Additional surface observations were made in Jomsom for 4 7 February. Strong upvalley winds were observed with a pressure minimum in the afternoon. There was little pressure change in Pokhara during the day. In short, except for the near-surface wind data from Kagbeni as collected in 1990 and for the observations just mentioned, there are no further wind data available from the area. Nothing is known about the vertical structure of these winds and very little about changes of the winds along the valley. Given this lack of data on the wind field a joint campaign was undertaken by the Meteorological Institute of the University of Munich and the Department of Hydrology and Meteorology in Kathmandu in the fall of 1998 to explore the three-dimensional structure of the diurnal wind field in Mustang. The months of September and October were chosen to minimize the interference of the monsoon with the valley wind system. Winter conditions, however, may be found in upper Mustang late in October so that the campaign could not be extended beyond that month. Spring would be equally well suited for conducting such a campaign. It was decided to cover as large a portion of the valley as possible. It is only in this way that wind differences along the river can be detected. Therefore, observations were made all the way from Lete, close to the point of the river s descent into the lowlands, up to Lo Manthang, that is, close to the source of the river. Pibal ascents were made at eight locations (see Fig. 1 and Table 2). Major weather perturbations are unusual in this area at this time of the year. Thus there was good reason to expect that weather would be fine on a sufficiently large number of days so that observations at different locations would be comparable. The data to be presented below were collected by the authors of this paper. It was the primary goal of this field campaign to provide a gross description of the diurnal wind field in the upper Kali Gandaki Valley, with particular emphasis on the diurnal evolution of the vertical structure of the wind and on its variation along the valley. In this paper, we report the results of the campaign. Moreover, partial explanations of the strength of the winds and the asym- TABLE 2. Characteristics of theodolite baselines (see also Figs. 1 and 4): z is the height difference between theodolites T 1 and T 2 in m. Negative values indicate that T 1 is higher than T 2. Location Lete Tukuche Marpha Jomsom Kagbeni Chuksang Nyi La Lo Manthang T 1 theodolite position NLat ELong Altitude Baseline Length Direction () z

4 APRIL 2000 EGGER ET AL metry between day and night are provided. These explanations will be tested in the more theoretically oriented Part II of this paper. 2. Equipment In Mustang there are no roads for vehicles and virtually no electricity. All equipment had to be carried by porters or by mules. Electric power had to be generated. It is impossible to use heavy equipment under such circumstances. Moreover, demands on electrical power had to be low. Given these limitations with respect to weight and power supply, the double theodolite method (e.g., Reger 1935) seemed to be best suited for wind speed measurements. With this method, a helium-filled balloon is released and tracked by two theodolites (Warren Knight model 89 AFP). Of course, the helium had to be carried up as well. Each theodolite is connected to a 12-bit A/Dconverter that transforms the potentiometer readings of azimuth and elevation into a digital signal. The data are transmitted to a portable computer (serial RS 322 port). Specifically designed data acquisition software controls the calibration procedure before each ascent as well as the data transfer and storage. It also provides an online display of data from the ascent. This enables an in situ check of data quality and ascent characteristics. Mainly, Vaisala TA20-balloons were used during the day, but it was necessary to launch the larger TA30 balloons at night to compensate for the additional weight of a battery and a lamp. Storage batteries for the portable computers were recharged with a wind turbine. All instruments functioned properly through the end of the campaign despite the fact that the equipment was exposed to strong and dusty winds on many occasions. The average ascent velocity of the balloons was 2.5 ms 1.Duringanascentmeasurementsweremadeevery 10 s. The corresponding vertical resolution of the wind profile was 25 m. The accuracy of the azimuth and elevation angle readings was 0.1. The datasets from both theodolites were combined to calculate balloon positions every 10 s and to derive wind velocity estimates from these data. Error estimates of position and speed were based on equations provided by Hennemuth et al. (1980). Corresponding error bars are provided in most of the corresponding figures (e.g., Fig. 7). There is, of course, a tendency for error growth as the balloon s distance to the base line increases. The error can be reduced by averaging over consecutive observations. The error was kept below a threshold by selecting appropriate averaging intervals; Fig. 9, discussed later, provides a good example. Close to the ground, the errors are small and the points are densely packed. Higher up the distance between the data points is relatively large in order to keep the error below the threshold. In addition to the theodolite equipment, a hygrothermograph and three anemometers were used in the FIG. 3.The500-hPageopotentialheight(gpdam)at287N lat, 8315 Elong;ECMWFanalysisfortheobservationdays. experiments. Positions of all sites were determined via the Global Positioning System (GPS). Even this modest equipment had a weight of 350 kg. More than 10 porters were needed to carry this load. Soundings of the thermal structure of the atmosphere were not possible because the extra effort to put, say, a mobile radiosonde station into operation would have been substantial. Thus, we decided to concentrate on the winds during this exploratory campaign. Weather conditions were generally good during the campaign except for our stay in the entrance region where perturbations presumably linked to the monsoon affected the valley flow. Moreover, rain fell on 29 September, 2 October, and 18 October. It is notoriously difficult to relate such events to flow patterns of larger scale in such extreme terrain. Nevertheless, the 500-hPa heights taken from the analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) at a grid point close to Jomsom show that these precipitation events are linked to episodes with decreasing heights (Fig. 3). Inspection of the related weather maps yielded additional clues to be described below. However, gradients are quite weak in this subtropical domain. The expedition was conducted in a trek-type mode in which observations were made at individual locations as the expedition proceeded. Observations began on 19 September. We arrived in Lo Manthang on 10 October, started downward on 17 October, and the last observation was made on 24 October. 3. Observation sites The choice of observational sites during this campaign was partly dictated by the observing system. A theodolite baseline of sufficient length as necessary for reasonably accurate measurements was not always easy to find in this rugged terrain. Moreover, one has to have agoodviewupthevalleysothatsightoftheballoon

5 1110 MONTHLY WEATHER REVIEW is not lost too early. In other words, observations can be made only if the riverbed is reasonably straight and wide. All in all, eight locations were selected (see Table 2andFig.1).Thesewillbedescribedbrieflyinthe following, beginning at the lowest site at Lete to the south and following the river up to Lo Manthang. a. Lete As has been mentioned, the steep descent of the river begins at Ghasa. A gorge of a few hundred meters width and with rather steep sidewalls of about 1000-m height leads downward from Ghasa to Tatopani. The gorge opens at Ghasa toward a basin situated between Dhaulagiri and Tukuche Peak to the west and the Annapurna range to the east. The baseline was established in the fields of the Lete village at a height of 2500 m with agoodviewuptheriver. b. Tukuche Tukuche is situated at the northeastern end of a rather straight section of the river. The valley bottom is flat, afewhundredmeterswide,andcoveredbygravel.the mountains rise steeply along the river with Nilgiri as the towering peak above Tukuche (top height 7061 m). Mountain slopes are covered by forest up to a height of about 4000 m. A rather short baseline was established to the south of Tukuche at a height of 2570 m. A dangerous crossing of the river with part of the equipment would have been necessary in order to establish a longer baseline. c. Marpha and Jomsom Upstream of Tukuche a gorge leads to Marpha, a rather large and impressive village situated to the west of the river. Marpha may be seen as the gateway to the Mustang area. While the valley is quite narrow in Marpha it widens upstream (Fig. 1) and changes its character. One enters the world of the semiarid valleys to the north of the Annapurna range. For example, the valley of Langpoghyun Khola (Fig. 4a) extends eastward from Kali Gandaki up to a height of almost 5000 moveradistanceof12km.thejhongkholariver joins the Kali Gandaki near Kagbeni, originating 20 km to the east. Such long tributaries are not found to the south of Marpha where the main valley is quite narrow. The peaks marking the border of the Mustang area (Fig. 1) typically reach heights somewhat above 6000 m and are about 20 km distant from the Kali Gandaki River on its eastern side. In general, the terrain is much steeper to the west and the peaks are relatively close to the river. One theodolite (T 1 )wasplacedonahilloverlooking the valley both at Marpha and at Jomsom. The other theodolite was close to the river at Marpha. The baseline extended over the river at Jomsom. Valley winds are impeded between Marpha and Jomsom by a ridge of 200-m height extending from the bridge over the Kali Gandaki toward the east (Fig. 4a). The slopes to the east of Marpha are covered by forest, while few trees are seen to the west. The northern slopes of the Annapurna range are covered by forest as well. There is little vegetation in the Mustang basin except near villages where irrigation systems are needed to grow crops on terraces. d. Kagbeni Shortly upstream of Jomsom one enters a winding gorge with steep walls. A few kilometers to the south of Kagbeni the valley widens again and becomes straight (Fig. 4b). There, a baseline was established across the river. Low-level upvalley winds to the south of the baseline are perturbed by a sharp bend of the river. The valley narrows again near Kagbeni. e. Chuksang The path to Lo Manthang is close to the river all the way up from Lete to Chuksang, but climbs steeply near Chele (Fig. 4c). The gorge to the north of Chele is spectacular and so narrow that a passage is too difficult for standard travel and transport. However, a good spot for establishing a baseline was found near Chuksang where Narsing Khola joins the Kali Gandaki. f. Nyi La pass After the climb from Chele the track to Lo Manthang continues at an altitude of about 3500 m with several passes to be overcome. Nyi La pass (3950 m) is the highest of these (Fig. 4d). Strictly speaking, Nyi La is hardly a pass. One simply has to cross a shoulder protruding from the western mountains. The river is a few kilometers to the east of the pass. To the south, the Annapurna Range is seen, while the Plateau of Tibet is clearly visible to the north. g. Lo Manthang The town of Lo Manthang is located on an eastward sloping plain in between two creeks flowing down from the Mustang Himal (Fig. 1). The baseline was established to the south of Lo Manthang (Fig. 5). Theodolite T 1 was placed on a ridge above our campsite. There is an east west-oriented ridge to the south of the baseline. The winds blowing up the Kali Gandaki basin in the afternoon have to cross this ridge. In what follows, the section from Lete to Marpha will be called the entrance region. The upvalley wind does not reach full strength in this section, where the valley is narrow and the ground is mostly covered by grassland and forest. Marpha marks the beginning of the core section that extends to Chuksang. The valley is wide and many tributaries flow into the Kali Gandaki from

6 APRIL 2000 EGGER ET AL FIG. 4.Mapsofthe(a)MarphaandJomsom,(b)Kagbeni,(c)Chuksang,and(d)NyiLaareaswithbaselines.Thedotin(a)marksthe same position as the dot in Fig. 18. the east. Upvalley winds reach maximum strength in the core region. Chuksang is a transition point where the exit section begins. The river is winding through rather narrow gorges so that intense upvalley flows at the ground cannot be maintained. As will be seen, there is, however, a deep layer of flow well above the gorge. The valley basin is even wider in the exit section than in the core section. 4. Observations a. Entrance region The observations made in the entrance region, that is, at Lete and at Tukuche, do not contain a clear message. Inflow during the day was observed in Lete on 18 and 19 September, but flow speeds where relatively small. Wind observations near the Tukuche baseline were made for September but the flow was gen-

7 1112 MONTHLY WEATHER REVIEW FIG. 5.SketchoftheLoManthangareawithbaseline.Nomap was available of sufficient resolution to represent the small-scale features that are important in this case. This drawing is based on a photo taken from the hills north of Lo Manthang. The view is toward the south. The walls of Lo Manthang form a rectangle of 300 m 160 m in the north south by east west direction. Theodolite T 1 is located 120 m above the base camp. erally perturbed by southeasterly flow aloft. For example, the upvalley winds during daylight on 20 September continued throughout the night (Fig. 6). During this night, there was rain and the cloud base was rather low so that nighttime ascents had to be canceled. Lhasa reported weak southerlies at 500 hpa on 20 September ahead of a weak trough over western Tibet. This trough disappeared on 22 September (see also Fig. 3). Observations of cloud motion suggested that the inflow layer has a depth of at least 1000 m with typical velocites of 5 10 m s 1.Althoughthereisnodoubtthattheupvalley wind system extends at least down to Lete there were too many perturbations to allow for statements with respect to the strength and depth of the inflow. One would have to stay longer in this region to make sure that unperturbed days are part of the sample. However, Ohata and Higuchi (1978) found strongly deformed trees between Tukuche and Marpha, while such deformations are absent in the gorge downstream of Ghasa and are moderate at Ghasa and Lete. This indicates that winds become stronger between Lete and Marpha. b. Core region As was mentioned, Marpha is at the transition from the entrance region to the core region. We discuss the results from Marpha in this section because the flow characteristics are rather similar to those found in Jomsom and Kagbeni. The first ascent at Marpha on 23 September (not shown) revealed vigorous upvalley winds throughout a layer of 500-m depth and southwesterly flow aloft. Wind speeds in this layer hardly strengthened during the day and there was still upvalley wind at midnight at least up to 1000 m. This suggests that the valley circulation was perturbed on this day as well. Weak drainage flows evolved during the morning of 24 September (Fig. 7). This downvalley flow had a speed of 4 ms 1 and a depth of almost 1000 m. The balloon moved about 2 km down the valley until entering a layer of rather weak southwesterly flow. The FIG.6.Near-surfacewindspeedV (m s 1 )asobservedinthecamps near Tukuche, Marpha, and Jomsom at the days indicated. The wind direction changed in Jomsom from downvalley to upvalley between 0930 and Half-hourly mean values. balloon was tracked to a large height to make sure that the situation was not perturbed on this day. The Marpha ascents of 24 September at 0911, 1000, and at 1124 LST document the growth of the upvalley wind layer with time (Fig. 8). At 0911 LST, the katabatic layer is completely replaced by upvalley flow with V 5ms 1 at the surface. The southwesterlies aloft were slightly stronger than the upvalley flow. At 1000 LST, wind velocities have increased substantially over a depth of about 1000 m. The surface wind observations at the Marpha campsite show also a rapid rise of wind velocities at that time (Fig. 6). Wind speeds peaked at a height of about 1000 m at 1124 LST. It is, however, difficult to interpret such details since the upvalley flow was rather turbulent at that time. The balloons underwent rapid accelerations and decelerations during ascents because of this turbulence. Wind velocities at the camp site reached maximum values near 1400 LST and decayed slowly thereafter (Fig. 6). Parallel observations of surface winds near the suspension bridge between Marpha and Jomsom ranged between 8 and 18 m s 1 on this day. It is not clear, however, whether this increase of wind speed with upvalley distance reflects an overall acceleration. The valley is quite narrow at the bridge. Ascents during the following night (0204, 0234 LST; not shown) showed extremly weak winds in the lowest 500 m, which were replaced by downvalley winds by the next ascent at 0617 LST (Fig. 9). At that time there was virtually no flow aloft. Tibet was covered by a huge high pressure system on this day. Weak northeasterlies prevailed at 500 hpa above Lhasa. Thus, this day (25 September 1998) can be seen as a calm day without noticeable large-scale perturbations. The balloon moved down the valley during the ascent shown in Fig. 9 for about 15 min and then climbed up one of the slopes. Finally the balloon crossed the baseline. Arapidbuildupoftheupvalleywindsystemwas observed on this day. The depth h u of the upvalley wind layer was 500 m at 0911 LST, 900 m at 0948 LST, 1500 mat1029,andmorethanthatat1128lstandpossibly

8 APRIL 2000 EGGER ET AL FIG. 7.PibalascentatMarphaat0616LST24Sep1998:(a)windspeedV (m s 1 )and(b)directionasafunctionofheightz (m). Upvalley direction: 220. Onlythelower2000moftheascentareshownoutofatotalof10000m m at 1413 LST (Fig. 10). Wind speeds in the lowest 1000 m were quite high in this case, leading to a rapid increase of the balloon s distance from the baseline and to a correspondingly rapid error growth. The surface pressure dropped by 5 hpa between morning and afternoon on 24 and 25 September. Our observations in Marpha strongly suggest that the total mass flow in the valley wind system in the lowest 1500 m above ground increases between, say, Lete and Marpha. If so, air must descend into this layer from above. Supporting evidence for this downward flow comes from cloud observations. It is typical to see FIG. 8.PibalascentsatMarphaat0911,1000,and1124LST24 Sep 1998: Wind speed V (m s 1 )asafunctionofheightz (m). Ascents are interpolated manually for the sake of clarity. clouds to the south but to have clear sky from, say, Tukuche toward the north. Mean sunshine duration exhibits a distinct minimum to the south of Jomsom in October (ICIMOD 1996). Observations in Jomsom commenced on the morning of 27 September. Weak northwesterly winds were recorded at 0924 LST (not shown) throughout the lowest 3000 m. The following ascents until noon were rather similar to those of 25 September in Marpha except that the flow speeds were somewhat higher. The upvalley flow sets in close to the ground. The inflow layer grows afterward to a depth h u of almost 2000 m. The wind speed profiles throughout the day are presented in Fig. 11. The layer s depth was less than 500 m at 0956 and about 500 m at Note the strong acceleration of the flow near the ground. Forty minutes later h u grew to about 800 m with an overall increase of speed of about 3 m s 1.Intheafternoon,theflowvelocitieswere high throughout a layer of 1300-m thickness. Flow speeds are reduced first near the ground in the evening (not shown). The surface observations during the day reveal a more dramatic rise of wind velocities between 1000 and 1100 LST than had been observed at Marpha (Fig. 6). However, the anemometer was placed in a maize field in Marpha while there was little surface roughness near the observation site in Jomsom. Thus the increased flow strength in Fig. 6 presumably reflects the choice of site. After sunset, the flow speed decreased quickly. However, there was no katabatic flow in the morning. The next day brought little change when compared to 27 September except that flow velocities were above 20 m s 1 in the afternoon. The pressure fall in Jomsom was 5 hpabetweenmorningandafternoon. On 1 October an attempt was made to explore the

9 1114 MONTHLY WEATHER REVIEW FIG. 9.PibalascentatMarphaat0617LST25Sep1998:(a) wind speed V (m s 1 ), (b) direction as a function of height, and (c) trajectory. Upvalley direction: 220.Theballoonwastracked up to 7000 m above the ground. Only the lowest 2000 m are presented. variability of the valley winds along the valley. Nearsurface winds were recorded near Tukuche, Jomsom, and Kagbeni (Fig. 12) simultaneously. Winds in Tukuche and Jomsom turned out to be relatively weak on that day. Strong winds were recorded in Kagbeni. In particular, the increase of wind speed in Kagbeni between 1000 and 1130 LST is quite dramatic. It must be pointed out that the spot chosen for observations near Kagbeni was at a relatively narrow gap underneath the spot where theodolite T 1 was placed a few days later (see Fig. 4). Wind speeds tend to be high at this spot. Nevertheless, there was a lot of dust raised during the day upstream of this point. It is virtually certain that winds were much stronger in this part of the valley than in Jomsom, let alone in Tukuche. This raises the question how this acceleration comes about, given the widening of the valley near Marpha and the small slope of the valley floor. Mass continuity requires descending motion between Jomsom and Kagbeni. Theodolite observations in Kagbeni began on the morning of 3 October. However, clouds were so low and the situation appeared to be so strongly perturbed that we decided to save helium. The Mustang area was located ahead of an extended trough on that day. Upper-

10 APRIL 2000 EGGER ET AL FIG. 10.Windspeedsinthelowest1500maboveMarphaon25 Sep 1998 as obtained from four consecutive ascents. Wind directions (not shown) indicate the depth of the inflow layer of 600 m, 1200 m, 1500 m, and 1500 m. level winds on 4 October were from the WSW with speeds of 5 10 m s 1.The500-hPaflowaboveLhasa had the same direction. The low-level flow accelerated and a deep upvalley flow layer was established by 1151 LST (Fig. 13). The southwesterly layer on top is clearly visible in Fig. 13. Later in the day wind speeds were larger than 15 m s 1 at least up to 1300 m except for the lowest 200 m (not shown). Surface wind observations at theodolite T 1 on 4 October showed rather similar characteristics as those obtained 120 m below on 1 October (see Fig. 12). In particular, there were also two separate phases of rapid increase. Weak downvalley winds were found in the morning of 5 October. The ensuing buildup of the upvalley flow layer did not differ much from that of the previous day. Aratherclearpictureemergedforthecoreregion. Late in the night, a weak drainage flow tends to occur. The upvalley acceleration sets in first near the ground. After that, the inflow layer grows rapidly reaching typical depths of h u m in the afternoon. Deceleration appears to set in first near the ground. FIG. 11.WindspeedV as a function of height on 27 Sep in Jomsom at various times as indicated. For the sake of clarity, the 0956 winds are presented for the lowest 400 m only. Manual interpolation. c. Exit region Observations in Chuksang were made on 23 and 24 October after the return from Lo Manthang. As was mentioned, Chuksang is close to the northeastern end of that section of the valley where the river meanders through a relatively wide flat bed covered with gravel. The gorge near Chele blocks all upvalley flow close to the river. So Chuksang is a transition point between the core region and the exit region. Wind speeds were found to be somewhat weaker in Chuksang than in Kagbeni and Jomsom. For example, FIG. 12.WindspeedV as recorded on 1 Oct in Tukuche, Jomsom, and Kagbeni. Also given are the observations at theodolite 1 at Kagbeni on 3 Oct. Ten-minute mean.

11 1116 MONTHLY WEATHER REVIEW FIG. 13.Pibalascentat11:514Oct1998atKagbeni:(a)windspeedand(b)directionasafunctionofheightz (m). Upvalley direction: 220. FIG. 14.Pibalascentsof23OctinChuksang.Windspeed(ms 1 ) at various times as indicated. Upvalley direction: 220. Manual interpolation. the speed of the surface winds in the camp in Chuksang did not exceed 5 m s 1 on 23 October. Additional observations on the slopes at a height of 400 m above the river gave maximum winds of 9 m s 1.Thisisdefinitely less than that observed at Kagbeni. However, there appears to be rather little wind in Chele just a few kilometers up the river. Observations were made on the buckwheat terraces immediately to the north of Chele at the rim of the gorge about 100 m above the river. Maximum wind speeds were 2.7 m s 1 on 23 October. AspecialeffortwasmadeinChuksangtodocument the deceleration of the wind in the afternoon. The ascent at 1735 LST on 23 October showed the familiar profile of a deep anabatic layer with southwesterlies aloft (Fig. 14). At 1951 LST the low-level winds speeds were reduced substantially. The upper branch of the inflow was still quite strong at that time. Rather weak anabatic flow was recorded up to a height of 1500 m at 2200 LST and even lower speeds were encountered at 2300 LST. This series demonstrates clearly that the deceleration sets in at the ground. The flow acceleration the next day followed the familiar pattern but maximum flow speeds did not exceed 15 m s 1 in the lowest 1500 m. Although the low-level winds at Chuksang cannot penetrate the gorges north of Chele, there are sections farther north where the river is relatively wide. Occasional observations from higher terrain showed that dust is raised there. However, no data were collected close to the main river in the exit region. An excursion on 18

12 APRIL 2000 EGGER ET AL FIG. 15.Ascentsat1013and1529LST20OctatNyiLapass:(a)and(c)windspeedV (m s 1 )and(b)and(d)winddirectionasa function of height z (m). October to Yara at the eastern side of Kali Gandaki turned out to be a failure. It began to rain before noon. There was little wind during this day. Figure 15 shows the wind observations taken on 20 October at Nyi La pass. The situation at this pass differs profoundly from that in the valley. The air ascends from the south and descends toward the north. One balloon ascent failed because the balloon disappeared into the valley leading toward Nyi La from the north. The ascent at 1013 LST revealed a layer of southerly flow up to 2300 m above the pass. Aloft, winds were westerly (not shown). The turning of the wind occurs roughly at the height of mountaintops bordering the valley to the west. The acceleration during the day was substantial in the lowest 1000 m above the pass (Fig. 15). This demonstrates clearly that the upvalley winds extend horizontally at least from the riverbed to the mountains to the west. The top of the inflow layer was at 5000 m above sea level on that day. The observations at Lo Manthang support the findings at Nyi La pass. Early on 12 October little wind was found below a height of 1000 m. Upvalley winds linked to the local topography were quite weak. Aloft, there were westerlies of 10 m s 1.Windsremainedweak until noon but there was at least some southerly inflow at 1122 LST (Fig. 16a,b). Note the pronounced turning of the wind at 1000 m above the ground. This inflow became rather vigorous in the afternoon (Fig. 16c,d) but

13 1118 MONTHLY WEATHER REVIEW FIG. 16.Ascentsof12OctinLoManthangatvarioustimesasindicated.WindspeedV (m s 1 )andwinddirectionas afunctionofheightz (m).

14 APRIL 2000 EGGER ET AL FIG. 17.CrosssectionsoftheKaliGandakiValleyatMarphaandJomsom(dashed). disappeared completely at low levels after sunset (Fig. 16e,f). The next day, a buildup of the southerlies was observed again. However, velocities were less than 10 ms 1 in the lowest 1000 m. The observations at Chele revealed a complete blocking of the low-level flow at the entrance to the Chele gorge. However, vigorous diurnal upvalley flow was found at Nyi La and in Lo Manthang within a deep layer extending vertically up to at least 5000 m above mean sea level and horizontally to the mountains to the west. It is obvious from the trajectories that this diurnal southerly flow reaches Tibet. It is also obvious that this flow is an extension of the upvalley winds in the core region. There are no observations available in the eastern part of the exit region. Thus it is impossible to estimate the width of the inflow layer. 5. Discussion Two of the most surprising features of the Kali Gandaki diurnal wind system were known before our campaign, namely the asymmetry between day and night and the extreme intensity of the winds (see Fig. 2). We have found in addition that there is a strong acceleration between, say, Tukuche and Kagbeni. This a truly surprising feature. It is a generally accepted explanation of diurnal valley winds that their intensity is closely linked to the so-called volume effect [also called the topographic amplification factor; Whiteman (1990)] as proposed by Wagner (1932) and as demonstrated quantitatively by Steinacker (1984). The topographic amplification factor can be evaluated locally per unit length of the valley. One has to determine the cross-sectional area A(H) ofthevalleyuptoaheighth assumed to be the depth of the diurnal circulation systems. Let L be the width of the cross section at height H. The area A must be compared to the corresponding area HL out in the plain. Thus HL/A is the local topographic amplification factor. The larger this factor, the smaller is the mass of air to be heated in the valley and the larger the temperature excess of the valley atmosphere with respect to the atmosphere outside the mountain massif. Finally, the larger the temperature excess the stronger the wind into the valley (see Whiteman 1990 for a concise review of this concept). For example, McKee and O Neal (1989) demonstrated that drainage flow can be closely related to local values of the topographic amplification factor. This explanation is not readily compatible with the shape of the valley between Tukuche and Kagbeni. Cross sections of the valley are shown in Fig. 17 for Marpha and Jomsom for H 2500 m. The valley is essentially v-shaped in Marpha and u-shaped in Jomsom so that the flow enters between Marpha and Jomsom a section with reduced topographic amplification factor. Despite this, the upvalley winds appear to accelerate on their way to Kagbeni instead of slowing down. It is not obvious, however, whether the evaluation of topographic amplification factors on a local basis is appropriate for the Kali Gandaki Valley. Roughly speaking, the Lete basin opens toward the free atmosphere to the south at the gap near Ghasa and is connected to the large Mustang basin by a narrow and winding valley tube. Vergeiner (1987) has proposed a conceptual model for such situations. He considered a basin linked to the foreland by a narrow valley. According to Vergeiner it is the topographic amplification factor of the basin that is important and not the width and shape of the connecting valley tubes. The temperature in the basin is higher during the day than that over the plain at the same altitude. This leads to a pressure difference and to corresponding upvalley winds. However, it must be kept in mind that the Kali Gandaki Valley opens to the free atmosphere and not to a plain in Ghasa. It is this access to the free atmosphere where the topography of the Kali Gandaki Valley differs from that envisaged by Vergeiner (1987). The free atmosphere to the south of Ghasa is hardly heated up during the day. Because of this a strong upvalley wind would have to be expected into the Mustang basin even if this basin were like a box with vertical walls. The temperature in this box would be considerably higher than in the free atmosphere to the south of Ghasa. Therefore the pressure would be lower and winds begin to blow in the valley tube. However, even a cursory inspection of the terrain shows that there is also substantial topographic amplification in the Mustang basin. In particular, there are

15 1120 MONTHLY WEATHER REVIEW FIG. 18.TreedeformationstothesouthofLangpoghyunKhola.Thedirectionofthetrunk s declination is given by the arrow. The maximum deviation of branch directions is indicated by the two straight lines. Tree 10 is also strongly deformed but there were little branches growing opposite to the directions of trunk declination. Creeks are denoted by dashes. The locations of the trees were determined using GPS. The dot marks the same position as the dot in Fig. 4a. several valleys leading from the Kali Gandaki toward the east (see section 3c). These valleys are km long and at least the valleys near the Annapurna Range have a strong diurnal circulation of their own. These diurnal winds are so strong that a remarkably clear deformation of trees results. These deformations have been documented for that valley which joins the main valley south of Lhungpoghyun Valley (Fig. 18). Ohata and Higuchi (1978) did not perform their deformation analysis in this valley. The valley bottom ascends toward the east so that upvalley winds are westerly. Pine trees of about 10-m height were selected to measure these deformations. Most trees show an eastward inclination of their trunk (arrows in Fig. 18). There are many trees without branches pointing toward the west. Instead, the branches form an angle within which all the twigs are found. This angle has been determined simply by using acompass.ofcourse,onetendstoselectgoodexamples. However, these good cases would not have been found if these diurnal winds were not extremly strong. Trees 5, 14, and 13 belong to the wind system of Langpoghyun Khola while nearly all the other trees are deformed by the winds blowing up the slopes of the neighboring valley to the south. The ridge to the northwest of trees 13 and 14 is covered by forest, which shows little wind impact. The evaluation of tree deformation was performed on a cloudy day without winds (29 September 1998). However, two of us (JE and UE) were measuring wind speeds in this valley on 24 September 1994, a fine day. Then upvalley winds up to 23 m s 1 velocity were observed. We conclude that this tributary has diurnal upvalley winds at the surface that are similar in strength to those found in the main valley. Although nothing is known about the depth of the upslope flow, there must be compensating return flow aloft with descent above the main valley. Adiabatic warming and a corresponding decrease of pressure are induced in the main valley by the tributaries (Egger 1990). This warming is part of the volume effect of the larger basin. It contributes to a northward upstream acceleration of the winds at the exit of the valley tube near Marpha. All this suggests that the strength of the upvalley flow is at least partly due to the specific topography. Moreover, as has been mentioned, the Mustang basin is dry so that fluxes of sensible heat are larger there than, say, in the Lete basin where the ground is covered by vegetations. This leads to additional acceleration. On the other hand, the albedo of the arid slopes of the Mustang basin is larger than that of the forests farther south. The relatively high altitude of the basin is an additional factor. Given the solar input, the increase of temperature during the day is larger the less dense the air. Corresponding accelerations are also larger and so are the wind speeds. This argument applies, of course, to all valleys in the area and may explain the strong deformations of the trees displayed in Fig. 18. Smith and Shi (1992) have shown that the bulk longwave cooling at high elevation sites may be smaller if the surface is mostly covered by rocks as in upper Mustang than by leaf vegetation as in Lete. Again, this might contribute to enhanced upvalley winds during the day and weaker katabatic winds at night. Cloud formation was observed quite often during the night throughout the experiment. This hinders the development of katabatic winds. Our stay in the area was too short, however, to see how the intensity of katabatic winds in the morning is related to nighttime cloudiness. The specific topography of the Kali Gandaki Valley

16 APRIL 2000 EGGER ET AL appears to be partly responsible for the observed asymmetry between day and night. The upvalley flow passes Marpha almost at full strength and forms essentially a jet after its exit from the valley tube there. Such jets maintain their intensity mainly through their inertia, loosing momentum via friction. Additional acceleration through tributaries as discussed above helps to strengthen the jet. This appears to be the only example of an upvalley jet known up to now. On the other hand, downvalley jets are rather common. They tend to form at valley exits during the night. For example, Pamperin and Stilke (1985) report on the cold air jet caused by the nocturnal outflow from the Inn valley into the Alpine forelands. This jet is observed over the plain at distances up to 20 km from the mouth. There is no symmetry between day and night in this case. The diurnal inflow to the Inn Valley is accelerated gently and does not form ajetintheplain.justasonewouldnotexpectthata downvalley jet forms, say, in Jomsom. These ideas will be tested in Part II of this paper. There are, however, also open questions. For example, drainage flow from the Mustang basin would enter the Lete basin and one would expect to find a nocturnal jet there. There is no evidence of such flows. It is conceivable, however, that the nocturnal cooling in the Lete basin is as strong as that farther north. In that case, there would be little reason for strong outflows. A similar situation is found near Ghasa. Again one would expect that the cold air would rush through the gap near Ghasa to flow down the steep canyon there. Again, there is no evidence of such flows. Quite to the contrary, trees are deformed by upvalley winds in Ghasa (Ohata and Higuchi 1978) albeit weakly. The absence of this downvalley flow may be explained by the fact that clouds tend to form in the Lete basin in the afternoon. This would reduce the nocturnal cooling. However, further observations are required to elucidate this point. The wind system of the Kali Gandaki Valley is embedded in the diurnal circulation of the Plateau of Tibet. Even in winter, there is low-level inflow (Murakami 1981) toward the plateau in the afternoon and outflow late in the night. It has been suggested, for example, by Reiter and Tang (1984) that the daily circulations of the North American plateaus affect local valley wind systems and that this might apply as well to the Plateau of Tibet. Calculations with a simple model by Egger (1987) support this suggestion. It is, therefore, conceivable that the valley winds explored here reflect at least partially the impact of the Plateau of Tibet. For example, pressure is relatively low over the plateau in summer. This imposes a pressure gradient in the upper part of the Kali Gandaki that might in turn drive upvalley winds and weaken downvalley flow. However, September and, in particular, October are months of transition from the summer situation to the winter, when the air on the plateau is cold and the surface pressure is relatively high. Therefore, a search for influences of the circulation of the Plateau of Tibet should be conducted in summer or winter but not in fall. What we found, however, is the existence of a deep layer of upvalley flow in the upper Mustang region. This flow extends at least to the Plateau of Tibet. There are, however, no observations available to determine whether the atmosphere above the plateau is affected by this airstream. 6. Conclusions In this concluding section, we summarize our findings: 1) The upvalley flow in the Kali Gandaki is accelerated between Lete and Marpha. There is some evidence for descending motion in this entry section. 2) The upvalley flow is fully developed in the core region. Acceleration in the morning and deceleration late in the day commence near the ground. The depth of the inflow layer may be more than 2000 m. The upvalley jet extends to the mountains to the west. 3) Although low-level flow is partially blocked in the exit region, the deep inflow layer can be identified up to the Plateau of Tibet. There are indications that this exit branch reaches full strength later in the day than in the core region. 4) Weak drainage flow of 1000 m depth was found on several occasions early in the morning. We hypothesize that the upvalley flow is induced in the valley tube linking Marpha and Lete by the heating of the Mustang basin. The upvalley flow throughout the core region is essentially a jet that is based on inertia. This explains part of the asymmetry between day and night. These speculations will be exposed to tests in a forthcoming second part of this paper where numerical experiments on the Kali Gandaki Valley winds will be described. Acknowledgments. We are grateful to M. Reinhardt for his enthusiastic support of the campaign, to J. Schween for writing most of the data evaluation programs, and, above all, to the supporting team for perfect help. The comments by all the referees and by G. Zängl were most helpful. REFERENCES Department of Hydrology and Meteorology, 1997: Precipitation records of Nepal, Ministry of Science and Technology, Nepal, 402 pp. [Available from His Majesty s Government of Nepal, Ministry of Science and Technology, Department of Hydrology and Meteorology, Babar Mahal, Kathmandu, Nepal, 2044.] Dreiseitl, E., H. Feichter, H. Pichler, R. Steinacker, and I. Vergeiner, 1980: Windregimes an der Gabelung zweier Alpentäler (Wind regimes at the bifurcation of two Alpine valleys). Arch. Meteor. Geophys. 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