66 Climate of 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 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 ob- tained 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-. 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- (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 ob- stacles. 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.

The high intensity of the insolation at Lhajung is clear from Fig. 2 a. The 10-day running mean of daily total insolation is close to the computed value at the top of the atmosphere except in the monsoon season. This becomes more clear com- pared with the data at low altitude at the same latitude. Data from Naze, Ryukyu Islands, (28°

23' N, 129•K30' E, 2 m) are also shown in the same figure. Both stations are situated in dif- Fig. 3. The relation between the attenuation ratio ferent climatic regions and have different rainy (A%) of solar radiation and mean cloudiness months. Although having a few exceptions in (C, C=0 for clear sky, C=10 for overcast). The black and open circles show the data at July for this reason, the solar insolation at Lhajung and Naze respectively. Ten-day Lhajung far exceeds that at Naze. The fact that mean values are used for both stations. At- more than half of the solar insolation is attenuated tenuation ratios at both stations under clear by monsoon clouds and arojund two thirds of sky and overcast are denoted by A0, A'0 and the solar energy passes under a cloudless sky in A10, A'10, respectively. Seppyo, 1976 J. Inoue 69

represents the absorption by the air layer effective terrestrial radiation becomes negligible

between the two stations. under such a foggy condition. RN at night is

ii) (A'10—A10), which is the difference of A very small due to the cloud ceiling.

under overcast, represents the sum of Generally speaking, in the monsoon season

(A'0—A0) and the absorption through the not only solar radiation but downward counter-

clouds above two stations. The mean radiation from the atmosphere increases due to

thickness of the clouds over Lhajung can the monsoon clouds. This results in a surplus

be regarded as much less than over Naze, in the radiation balance in summer. The

as otherwise (A'10—A10) would become Himalayas are supposed to be a great heat ab-

equal to (A'0—A0). sorber in summer and an appropriate radiator in

Sufficient data to consider the radiation balance winter. As a result, the Himalayas have a pos- were not obtained because of the difficulties in sibility of becoming a great heat engine in the the measurement of infrared radiation under the troposphere except for a short period in winter. conditions of frequent drizzle in the monsoon A further analysis of the radiation balance and season. Two examples of the radiation balance the accompanying heat exchange problem is given at Lhajung are shown in Fig. 4. The nocturnal in another paper by Inoue and Yasunari (1976). radiation in December, which is fairly constant with the value of -0.12- -0.15 ly/min., is 20% 5. Temperature more than the daytime net radiation in absolute Lhajung fits "the polar climate due to high value and results in a small deficit on balance. altitude" in Koeppen's classification, with the On the other hand, in September, the net radia- warmest month (July, 7.0•Ž in 1973 and 6.2•Ž tion (RN) in daytime far exceeds that in winter in 1974) well blow 10•Ž. The annual mean due to the increase of solar radiation (R5). The temperature was 0.2°C in 1974. The highest and observation of RN was stopped because of drizzle lowest temperature recorded are 12.2•Ž and in the afternoon. It can be supposed that RN 15.8•Ž, in July 1973 and January 1974 respec- in such a case is almost the same as Rs' because tively (Appendix II). The seasonal trend of the

10-day running mean temperatures at Lhajung is

illustrated in Fig. 5 a. Almost half of the year,

from the end of October to the beginning of

April, is below the freezing point. The annual

temperature pattern is characterized by little

variaton during the warmest months in the mon-

soon season, by a uniform increase and decrease

in pre- and post-monsoon and by rapid change

in winter with January being the coldest month.

The annual course of the diurnal temperature

change is illusstrated in Fig. 5 b, which is drawn

in the same manner as Fig. 2 b. The concentra-

tion of the isotherms in winter is caused by the

above-mentioned annual temperature pattern and

by the increase of the diurnal temperature range

in winter. The temperature range, defined here

as the difference between mean daily maxima and

mean daily minima, is more than twice as large

in midwinter as in summer. Namely, the mean

temperature ranges in summer (June-September)

were 4.6•Ž (1973) and 6.3•Ž (1974), but those Fig. 4. Examples of radiation balance in winter in midwinter exceeded 12•Ž. (above) and during the monsoon (below) at Lhajung. The black and open circles show In the isopleths of Fig. 5 b, almost the same solar radiation (Rs) and net radiation (RN) patterns appear in both years except in July, respectively. September and October. The rather big warm 70 Climate of Khumbu Himal

Fig. 5. a) (above) Two year variation of 10-day mean air temperatures (at Lhajung).

b ) (below) Two year variation of daily temperature change (drawn in the same manner as Fig.

2 b ). Isotherms are at 2°C intervals. The areas above 0•Ž are shown by point stipples. Those

above + 8•Ž and below -8•Ž are shown by fainter stipples. cell enclosed by the 8°C isotherm also expressed in summer of 1973 (Appendix II). This may be by fainter stippling in July 1973 does not appear related to the fact mentioned in Section 3 that but breaks into smaller cells in 1974. Tem- the monsoon was weaker in 1974 and afternoon peratures below freezing point were not observed drizzle occurred more frequently than in 1973.

Fig. 6. a) (above) Two year variation of 10-day mean relative humidity (R.H.) and vapor pressure (V.P.) at Lhajung. b) (below) Two year variation of daily variation of relative humidity (drawn in the same manner as Fig. 2 b). Humidity isopleths are at 20% intervals. The areas above 90% are shown by point stipples and those below 30% are shown by fainter stipples. The broken line shows the 100% isopleth. Seppyo, 1976 J. Inoue 71

posed to be related to frequent drizzle in 1974. 6. Humidity As can easily be supposed from the climatic 7. Wind situation of the Nepal Himalayas, the annual It was pointed out in the previous sections change of humidity at Lhajung is very marked. that the role of the local wind circulation is very As shown in Fig. 6 a, there is no significant important in determining the diurnal variation change of the relative humidity in the monsoon of the other meteolological parameters. The season, with a value of nearly 100%, but it annual pattern of mean and maximum hourly decreases rapidly after the end of the monsoon wind speed in 10 days are shown in Fig. 7 a. and shows a large variation throughout the dry The mean wind speed was very stationary with season. The vapor pressure in winter is less than a value of around 5 m/s throughout the period. one fifth of that in the monsoon (Fig. 6 a). The maximum wind speed was also stationary The annual pattern of diurnal change is shown except for the extraordinary peak in the middle in Fig. 6 b. The tendency of low humidity in of March 1974. This is caused by the fact that the morning, increase in the afternoon and the wind pattern at Lhajung has the form of a maximum in the evening persisted throughout stationary mountain and valley wind, though the the year except in the summer of 1974. This is former is obscured in summer night. The valley related to the developemnt of the valley wind wind in the daytime is extremely distinct. This which is dominant throughout the year in this appears in the distribution of wind-shaped dwarf region, as described in the following section. shrubs in this area (Fujii, 1976). The diurnal range of relative humidity exceeds The stationarity of the valley wind is clearly 40% in winter, but it is less than 20% in the shown in the isopleths of the mean wind in Fig. monsoon season. 8 b with a clear maximum belt in daytime The difference in the pattern of the monsoon throughout the year. That the width of this belt season in 1973 and 1974 also appears in these becomes more narrow toward winter is explained isopleths. Namely, the atmosphere was more by the fact that the duration of sunshine, namely humid in 1974 than in 1973, which can be sup- the prevalence of the valley wind, is shortened.

Fig. 7. a) (above) Two year variations of mean and maximum hourly wind speed during each 10 days at Lhajung. b) (below) Two year variations of daily variation of wind speed. Isopleths are at 2 m/s intervals. The broken line is 5 m/s. The areas below 4 m/s and those above 8 m/s are shown by point and fainter stipples respectively. 72 Climate of Khumbu Himal

Fig. 8. Hourly wind speed at Lhajung (above) and on the peak of Mt. Pokalde (5057 m) (below) from 20th Nov. to 15th Dec. 1973.

Table 1. Frequency of wind direction at Lhajung in 1974

Frequency is expressed by the number of the days. Small monthly frequencies are due to lack of observation. Directions are expressed with N-S component. Seppyo, 1976 J. Inoue 73

The strength usually reaches its maximum in The gale, which is possibly a foehn wind from April and May. This is correlated with the oc- the Tibetean plateau to the north occurred in currence of an insolation maximum in the same the morning and prevailed almost half a day, months, beacuse the upslope wind, which is the ceasing late at night on the 20th. This is as- driving force of the valley wind, becomes sronger sociated with the extraordinary change of the flow in proportional to the incoming solar energy. pattern of the subtropical jet stream around the The frequency distribution of wind direction Himalayan range. Namely, the jet stream flowed is shown in Table 1. Directions are projected from north to south crossing over the Himalayan on N and S which means directions of the range of the 20th, though it usually flows along mountain and valley wind respectively. The the contours of the range. This meridional flow, valley wind reaches a steady state within half an which is the direction along the main valleys of hour after sunrise and and ceases with sunset. the Himalayas, is not weakened or blocked by The mountain wind, on the other hand, is not the main ridges. A further discussion of this well-developed except in the winter season. There sporadic phenomenon is given in another paper are many cases of nocturnal valley wind, mainly (Inoue, 1976). in summer, but the velocities are small as shown As described in the previous sections, climato- in Fig. 8 b. The mountain wind is clearly de- logical features around 5000-6000 m in the veloped in winter. Khumbu region are governed by the valley wind. According to the theory of the valley wind it Namely, cumulus convection followed by this is a phenomenon confined to the air layer in the valley wind controls every climatic feature shield- valley and it disappears at the hight of the sur- ing the solar radiation, decreasing temperature rounding ridge. Observations at the peak of Mt. as a result, bringing more moisture and causing Pokalde (5075 m), which is elevated 600 m above precipitations in many cases. The structure of Lhajung, shows that the valley wind disappeared the local wind circulation and its relation to the at this altitude. In Fig. 8 the hourly wind speeds general wind field must be analyzed further to at the two places are shown by the width of the improve our understanding of this phenomenon. band. A clear contrast is apparent between the The development of local clouds above the ridge two stations. Neither valley wind nor mountain or peaks of 5000-6000 m high can be a good wind is disappeared at the higher station. It indicator for this study. may be concluded that both wind circulations are limited to the shallow layer along the main valley. References The origin of intermittent gales at Mt. Pokalde Ageta, Y. (1976): Characteristics of precipitation is not clear. That they are not correlated with during the monsoon season in Khumbu Himal. the wind at Lhajung suggests that those gales are in this issue. caused by the upper wind. However, the wind Fujii, Y. (1976): Distribution of prevailing wind at Mt. Pokalde is also unlikely to represent the direction in the monsoon season inferred from general wind field above the Himalayas, because the wind-shaped trees in the Khumbu reregion, the Khumbu region is surrounded by much more Nepal Himalayas, unpubulished. elevated terrain, i.e., many Himalayan mountains Inoue, J. (1976): The extraordinary gale at the end of winter in the Himalayas, in this issue. 7000-8000 m high. Inoue, J. and Yasunari, T. (1976): Surface heat The strong wind which occurred only in the exchange in the Himalayas, unpublished. middle of March 1974 was due to the gale on Nakajima, C. (1976): Movement and development of 20th March. A daily mean wind speed of 15.2 the clouds over Khumbu Himal in winter, in this m/s, which is 3 times as large as the ordinary issue. mean value, was measured on this day and the Yasunari, T. (1976): Some characteristics of seasonal hourly maximum wind speed reached 25.0 m/s. weather change in Khumbu Himal, in this issue,