Diurnal Winds in the Himalayan Kali Gandaki Valley. Part I: Observations
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1106 MONTHLY WEATHER REVIEW VOLUME 128 Diurnal Winds in the Himalayan Kali Gandaki Valley. Part I: Observations JOSEPH EGGER Meteorologisches Institut, UniversitaÈt MuÈnchen, 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, UniversitaÈt MuÈnchen, Munich, Germany PANCHA SHAYKA Department of Hydrology and Meteorology, Ministry of Science and Technology, Kathmandu, Nepal HILBERT WENDT AND VOLKMAR WIRTH Meteorologisches Institut, UniversitaÈt MuÈnchen, Munich, Germany (Manuscript received 4 February 1999, in ®nal 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 ®eld 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 15±20 m s21. The upvalley wind sets in ®rst at the ground but an upvalley wind layer of 1000±2000-m depth forms quickly after the onset. This deep in¯ow 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 ¯ow forms late in the night. The causes of these phenomena are discussed. 1. Introduction The most conspicuous climatological feature of the region is the pronounced decrease of precipitation up The valley of the Kali Gandaki River in Nepal is pre- the valley. Monthly mean precipitation is given in Table sumably the deepest valley on Earth. The river originates 1 for six stations on the way up from Tatopani to Lo close to Tibet in the Mustang Himal and passes the town Manthang for the months September and October, that of Lo Manthang under the name Mustang Khola to ¯ow is, for the two months of the ®eld campaign to be de- then essentially southward through the Mustang district scribed below. While the summer monsoon clearly af- till the main barrier of the Himalayas is reached near fects the precipitation in Tatopani and Lete, there is little Marpha (Fig. 1). The Himalayan ranges are crossed be- impact in Marpha just a few kilometers farther north. tween Marpha and Ghasa. It is near Lete and Ghasa that There is virtually no precipitation in Lo Manthang in Annapurna and Dhaulagiri tower above the river making the fall. The same features are apparent in the climate the valley's depth more than 5000 m. The valley follows atlas of Nepal (ICIMOD 1996). a steep descent from Ghasa to Tatopani. 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 Corresponding author address: Dr. Joseph Egger, Meteorologisch- es Institut, Arbeitsgruppe fuÈr Theoretische Meteorologie, UniversitaÈt area (Tucci 1977; Peissel 1992). In particular, the wind MuÈnchen, Theresienstraûe 37, Munich 80333, Germany. tends to raise a lot of dust in the afternoon. Figure 2 E-mail: [email protected] shows the monthly mean wind speed V as observed in q 2000 American Meteorological Society Unauthenticated | Downloaded 09/30/21 12:10 PM UTC APRIL 2000 EGGER ET AL. 1107 FIG. 1. Map of the Kali Gandaki Valley: dots, observation sites; crosses, villages and towns mentioned in the text. Height contours solid (m). Horizontal distances in (km). The map is based on topographic data with a resolution of 3003300. These data have been averaged to a1km3 1 km grid. Kagbeni (see Fig. 1) in September and October 1990. the winds during the day. Strong upvalley winds set in Wind direction was not recorded so that it is not clear between 0800 and 1000 LST, and reach an impressive whether upvalley or downvalley winds prevailed during maximum of ;14ms21 at ;1400 LST to decay later the night. However, there is no doubt with respect to on. An inspection of the daily observations reveals rath- er little variability from day to day. The daily maximum speed varies only between 11.6 and 17.5 m s21 within TABLE 1. Mean monthly precipitation along the Kali Gandaki Val- these two months. ley in (mm) as observed during the years 1974±96. From Department of Hydrology and Meteorology (1997). The strength of these winds is rather extreme. For example, typical valley winds in the Alps exhibit in- Lo Man- tensities of about 5 m s21 (see Whiteman 1990 for a Tatopani Lete Marpha Jomsom Ghami thang review). The asymmetry between day and night is also Sep 189 137 52 41 13 10 extreme. Typically upvalley winds in the Alps are about Oct 64 59 40 32 29 10 as intense as downvalley winds (e.g., Dreiseitl et al. Unauthenticated | Downloaded 09/30/21 12:10 PM UTC 1108 MONTHLY WEATHER REVIEW VOLUME 128 topani) to Kagbeni, distinct upvalley winds were en- countered only on 7 February 1985 when it was possible to ¯y 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 ®eld a joint cam- paign was undertaken by the Meteorological Institute FIG. 2. Monthly mean values of the hourly mean wind speed V (m of the University of Munich and the Department of Hy- s21) as observed in Kagbeni in Sep and Oct 1990 at a height of 30 drology and Meteorology in Kathmandu in the fall of ft. 1998 to explore the three-dimensional structure of the diurnal wind ®eld in Mustang. The months of September and October were chosen to minimize the interference 1980; Whiteman 1990). In Kagbeni, the wind roars up the valley in the afternoon, while gentle breezes are of the monsoon with the valley wind system. Winter typical of nighttime ¯ow. conditions, however, may be found in upper Mustang Aside from these observations in Kagbeni little in- late in October so that the campaign could not be ex- formation is available on the diurnal winds along the tended beyond that month. Spring would be equally well Kali Gandaki River. Although winds are measured with suited for conducting such a campaign. high temporal resolution at the airport in Jomsom, these It was decided to cover as large a portion of the valley data are not recorded. Winds and other meteorological as possible. It is only in this way that wind differences data are collected by the Mustang Development Service along the river can be detected. Therefore, observations Association in Jomsom. However, these data have not were made all the way from Lete, close to the point of yet been published. The Department of Hydrology and the river's descent into the lowlands, up to Lo Manthang, Meteorology of H. M. Government of Nepal collects that is, close to the source of the river. Pibal ascents wind data in Lo Manthang and Jomsom. Wind roses were made at eight locations (see Fig. 1 and Table 2). show a clear preference for southwesterlies in Jomsom Major weather perturbations are unusual in this area at and southerlies in Lo Manthang in September and Oc- this time of the year. Thus there was good reason to tober with weaker winds in Lo Manthang. Ohata and expect that weather would be ®ne on a suf®ciently large Higuchi (1978) tried to infer the intensity of the upvalley number of days so that observations at different loca- wind from the deformation of trees. This method is tions would be comparable. The data to be presented bound to fail in the upper Mustang area to the north of, below were collected by the authors of this paper. say, Jomsom simply because there are few trees in this It was the primary goal of this ®eld campaign to extreme environment. However, Ohata and Higuchi ar- provide a gross description of the diurnal wind ®eld in rived at interesting results in the lower part of the valley, the upper Kali Gandaki Valley, with particular emphasis to be discussed further below. Neininger and Reinhardt on the diurnal evolution of the vertical structure of the (1986) report on attempts to probe the valley atmosphere wind and on its variation along the valley. In this paper, with an instrumented motorglider. On four ¯ights from we report the results of the campaign. Moreover, partial Pokhara (situated about 50 km to the southeast of Ta- explanations of the strength of the winds and the asym- TABLE 2. Characteristics of theodolite baselines (see also Figs. 1 and 4): Dz is the height difference between theodolites T1 and T2 in m. Negative values indicate that T1 is higher than T2. T1 theodolite position Baseline Location N Lat E Long Altitude Length Direction (8) Dz Lete 288379500 838369380 2450 965.0 61.0 2106.0 Tukuche 288429170 838369100 2700 380.0 185.0 22.5 Marpha 288459210 838419210 2700 879.2 125.0 2246.0 Jomsom 288469440 838439130 2800 793.5 155.0 250.5 Kagbeni 288489480 838469210 2900 1078.8 319.0 67.2 Chuksang 288559030 838499090 3050 949.8 149.0 207.0 Nyi La 298029440 838519150 3900 1216.1 108.0 16.8 Lo Manthang 298109500 838579140 3800 980.0 98.1 2120.0 Unauthenticated | Downloaded 09/30/21 12:10 PM UTC APRIL 2000 EGGER ET AL.