Climate of Khumbu Himal*

66 Climate of Khumbu Himal Climate of Khumbu Himal* Jiro Inoue** Abstract The climate of the highland in Khumbu Himal is described in analysing the observational data taken at Lhajung (4420 m) in 1973 and 1974. There are marked rainy and dry seasons. 70-80% of annual precipitation (54 cm in 1973 and 35 cm in 1974) is concentrated in 4 months in the summer monsoon. The monsoon activity was somewhat weaker in 1974 than in 1973. Solar insolation is almost twice that of observed at sea level at the same latitude except in the monsoon season. The mean annual temperature at Lhajung is nearly 0•Ž. The weaker monsoon in 1974 than in 1973 appeared also in the temperature and humidity patterns. Diurnal and daily variations of temperature or humidity are very small in the monsoon season, while they are large, having a periodic change, in dry season. A very stationary valley wind governs the diurnal changes of meteorological parameters at Lhajung through out the year. However, the valley wind is confined to a thin layer of about less than 1500 m thick in the main valley of the Himalayas. at Lhajung (27•K53' N, 86•K50' E, 4.420 m), which 1. Introduction is situated on the old moraine terrace elevated During the past decade climatological data in 200 m from the floor of the U-shaped valley of the Nepal Himalayas have been widely reported Lobuche Khola. Although the station is sur- by many mountaineering expeditions. However, rounded by many high peaks, little of the sky is since the activities of these parties were con- blocked as illustrated in Fig. 1. The highest centrated in the few months of the premonsoon season, climatic features of the highland through- out the year have remained unfamiliar. Khumbu Himal, which lies in the eastern part of Nepal and contains the world's highest peak is one such area where shortterm climatological data have often been given. The long-term climatological observations were commenced in April 1973 in Khumbu Himal and continued through October 1975. In this report, the seasonal trends of meteorological elements obtained through our observations through December 1974 are briefly discussed and the climate of Khumbu Himal is outlined. Although the data are confined to surface observations, they are sufficient to outline the climate over this highland. 2. The observations The main station in Khumbu Himal is located Fig. 1. Surrounding views from Lhajung station. A- Ama Dablam, T-Tauche, P-Pokalde. Eleva- * Glaciological Expedition to Nepal , Contribution tion is shown by the concentric circles with No. 16 values in radians: O is the horizon, 0.4 ** Disaster Prevention Research Institute , Kyoto corresponds to roughly 22•K. (Original figure University, Uji-shi Kyoto, 611 was drawn by S. Iwata). Seppyo, 1976 J. Inoue 67 obstacles are Mt. Tauche, Ama Dablam and which are situated immediately south of Lhajung. Pokalde with elevation of 25 degrees of arc. Further discussion of the precipitation distribu- Conventional instruments are used for the tion in this region is given by Ageta (1976). meteorological observation at Lhajung; air tem- The difference in the amount measured by perature, humidity, wind velocity, precipitation, an ordianry rain gauge and by a tipping-bucket ground temperature and radiation are recorded gauge is caused by the difference in the location and checked every 3 hours. Visual observations where each gauge is exposed. Comparing the of cloud, evaporation and present weather are data obtained by tipping-bucket gauge in 1973 also taken in daytime. Our daily observations and 1974, it can be said that the monsoon was started at 0600 NST (Nepal Standard time) then more active in 1973 than in 1974 (total rainfall were altered to 054ONST adjusted to the observa- over 4 months during the monsoon was 409 mm tion time of the Nepalese Meteorological Office in 1973 and 280 mm in 1974). On the contrary, since May 1974, because Nepal Standard Time is the numbers of rainy days were almost the same GMT plus 5 h 40 m. The seasonal trends of the in 1973 (103 days) and in 1974 (116 days). Among principal meteorological elements are described them the number of days of precipitation more in the following section. than 25.0 mm in 1974 was only one (4 days in 1973), but those of less than 1.0 mm in 197 3. Precipitation exceed those in 1973. Thus it can be said that Since the Nepal Himalayas are affected by the monsoonal rainfalls in 1974 were mainly drizzle Indian summer monsoon (called merely 'monsoon' and light shower, while there was relatively more hereafter in this paper), there are marked rainy heavy rainfall in 1973. and dry seasons. In Khumbu Himal, the onset The drizzle or light shower which occurs most of the summer monsoon comes in the beginning frequently at this altitude is associated with the of June and the monsoonal rainfall continues development of local cumulus clouds. This type through the end of September. The season from of precipitation begins early in the afternoon October to May is a relatively dry season and and usually ceases in the evening around Lhajung. has fair weather except for periodic light showers Another type of precipitation which is a rather in winter and a spell of showers before the sum- heavy rainfall in this area is likely to be associated mer monsoon. with larger scale circulation and usually occurs As shown in Appendix I, most of the precipita- at night. Further discussions of the development tion occurred in the four months from June to of local clouds and of these two types of pre- September. The percentage of monsoonal to cipitation are given by Nakajima (1976), Ageta anual precipitation is 75% in 1973 and 81% in (1976) and Yasunari (1976). 1974 (the rather big amount in October 1973 is due to the snowfalls followed by developed 4. Radiation westerly disturbances). Monsoonal precipitation In many cases, the local circulation has its is characterized by frequent light showers or origin in the different sunniness of each place. drizzle (the daily amount is below 10 mm) and The insolation observed at Lhajung is illustrated rather heavy precipitation (up to several tens of in Fig. 2. The annual variation of the 10 day millimeters). The latter occurs several times in running mean insolation is shown in Fig. 2 a. a season, mainly in July and August. In Fig. 2 b, the 10-day running mean of insola- The precipitation data measured by rain gauges tion at each observed time during the poriod is distributed in the Khumbu region show that the plotted and the isopleths are drawn. Since the precipitation at Lhajung is representative for abscissa and ordinate of this figure show date this area except for the gauges installed at the and time respectively, the annual course of the head of the valley. It can be said that the annual daily variation of the element is expressed in this precipitatation at around 4000-5000 m in Khumbu figure. Himal is several hundred millimeters. This value The insolation reaches its maximum in May. is almost half of the annual precipitation of The hourly maximum in one day exceeds 1.6 ly/ nearly 1000 mm at Thyangboche (3867 m), min., while it decreases in June due to the begin- Khumjung (3790 m) and Namche Bazar (3450 m) ning of the rainy season. Days with their 68 Climate of Khumbu Himal the day season causes the annual variations of insolation at Lhajung to be less than those at Naze. This is due to the different amount of obstacles along the path of the solar beam between 2 stations. Cloud droplets and atmospheric suspen- sions (dust, water vapor, etc.) are two such obstacles. Attenuation by them can be distinguished qualitatively as follows; A = (expected value) - (observed value) (expected value) x 100(%) Fig. 2. a) (above) Annual variation of 10-day run- A for both stations is plotted against mean ning mean of Insolation (1y/min) in 1974 at cloudiness (C) in Fig. 3. Attenuation increases Lhajung (4420 m, 28•K23' N) shown by con- with cloudiness. The rate of the increase at nected bars and at Naze (2 m, 28•K23' N) by Naze is larger than at Lhajung. The tendency isolated bars. The daily solar radiation at of Lhajung is less attenuation and almost in- the top of atmosphere is also shown. b) (below) Annual course of diurnal insola- dependent on cloudiness than at Naze. Since tion change. Time is taken as ordinate and insolation is not only reflected upward at the top date is taken as abscissa. Isopleths are of of the cloud but is absorbed in the cloud, the hourly mean values at intervals of 0.2 ly/min difference in the rate of increase is caused by the interval. The areas exceeding 1.0 ly/min are difference in the cloud properties above both shown by point stipples. The straight line stations. Denoting the points at which the two near noon shows the time of the local solar curves cross the ordinates C=0 and C=10 as meridian. Ao, A'0 and A10, A'10 respectively, the following statements can be made; maximum value below 1.0 ly/min., appear only i) (A'0—A0), which is thedifference of A in January. between two stations under a clear sky, The straight line in the isopleth is the time when the sun is due south. The isopleths are expected to be symmetrical around this line. The center is shifted below the line from March to September due to the frequent occurrence of convective clouds in the afternoon.

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