Investigations of the precipitation conditions in the central part of the Tianshan mountains

Transcription

1 Hydrology in Mountainous Regions. I - Hydrological Measurements; the Water Cycle (Proceeoings of two Lausanne Symposia, August 990). IAHS Publ. no. 9,990. Investigations of the precipitation conditions in the central part of the Tianshan mountains FELIX P. BLUMER ETH Zurich, Hydrology Section, Department of Geography, 07 Zurich, Switzerland ABSTRACT The climate of the Xinjiang Province in northwestern China is charaterized by low precipitation amounts and large seasonal temperature differences as a consequence of its innercontinental position. The flat areas are deserts (Tarim Basin) or prairies (Dzungarian Basin) with mm precipitation per year. For this reason precipitation falling in the mountainous regions of the Tianshans (about 00 mm/year) is very important for the people and economy in this region. Most of the annual precipitation falls during the summer months under the influence of the Indian monsoon system. At this time about 70% of precipitation is influenced by cyclone activity and only 0% results from local convection. Two zones with considerably more precipitation can be found, one at an elevation of about 2000 m a.s.l. and one at an elevation of about 00 m a.s.l. INTRODUCTION In the summers 9 and 97 extensive precipitation measurements at observation sites in the uppermost part of Urumqi River Valley were carried out by the Climatology and Hydrology Group of ETH Zurich and a Group from Lanzhou Institute of Glaciology and Geocryology of Academia Sinica. The Tianshan is a high mountain ridge more than 00 km long extending from 7 E to 9 E along 2 N latitude. It divides the Chinese Autonomous Province Xinjiang Uigur in two parts. In the south are the desert areas of the Taklamakan and in the north the great plains of the Dzungarian Basin. The Urumqi River rises in the mid-part of the Chinese Tianshan (90 km south of Urumqi, the capital of the province) and it runs from south to north. Mountains in this area reach an altitude of 00 m a.s.l. and glaciers are located in regions higher than 700 m a.s.l.. The climate of northwestern China The climate of the Xinjiang Province is characterized by its innercontinental location. For this reason temperature differences between summer and winter seasons are high and

2 Felix P. Blunter the yearly precipitation amounts are low. The corresponding values of the three stations, Kucha, Kashgar and Urumqi are given in Tab.. The pressure conditions are different between the summer and winter seasons. During the winter a cold anticyclone in the lowest layers covers the Asian continent north of the Himalayas and the Tibetan Plateau. It is formed already in September and does not normally disappear from the mean monthly pressure maps before May. During the summer months, the entire Asian continent is influenced by the seasonal depression of the Indian monsoon system. During this time most of the annual precipitation amount falls in the Tianshans. TABLE Temperature differences and annual precipitation of Kucha, Kashgar and Urumqi (Muller, 9). Station Kucha Kashgar Urumqi Altitude m a.s Geographical Site 0, N/ 0'E 9 2ÏJ/7 o 07'E 'N/7 2'E Mean Temperature uary July -l2.é o c 2. C -.-C 2.7 C -. C 2.9 C Annual Precipit. 9 mm mm 27 mm General precipitation conditions in the Xinjiang Province In general, precipitation amounts in the Xinjiang Province are very low. In Fig. annual precipitation results for some stations in the Xinjiang Province are given. FIG. Map of the Xinjiang Province with annual precipitation amounts (mm). Stations north of the Tianshans in the Dzungarian Basin have distinctly more precipitation than stations south of it in the Tarim Basin even if the altitude is higher in the south. Because of the shortness of the observation period the precipitation values are not totally reliable, but the differences in precipitation amounts can be also seen in

3 Precipitation in the central Tianshan mountains vegetation characteristics. The Tarim Basin is a desert and the Dzungarian Basin a prairie. In Tab. 2 the seasonal variation of precipitation amounts for different stations in the Xinjiang Province is given. TABLE 2 Monthly precipitation amounts (mm) for several stations in the Xinjiang Province: Urumqi 9 m a.s.l., Kucha 970 m a.s.l., Hero Bridge 920 m a.s.l., Houxia (Tianshan Glaciological Station) 20 m a.s.l. and Daxigau Valley (Meteorological Station) 0 m a.s.l.. Stations Urumqi Kucha Hero Bridge Houxia Daxigau Valley 2 Feb 0 9 b Monthly Precipitation Amounts Apr May Jun July Aug Sep ct Nov 2 Dec Observa ti. Period 7 years 2 years Stations in the centre of Xinjiang Province normally have their precipitation maximum during the three summer months (June - August) ; this is true as well for the Dzungarian Basin, the Tarim Basin and also in the mountainous regions of the Tianshan. During these months warm air currents are coming from the southeast in the lower atmosphere and cold and wet air masses are coming from the north and the northeast in the upper atmosphere. In autumn cold and wet air masses in the lower atmosphere moving from the arctic sea produce orographical rain showers on the northern slope of the Tianshans. At this time the winterly high pressure core over Central Siberia is only weakly developed (Grebner, 990 a). For this reason the annual precipitation maximum can exist in October for stations near the northern slope of the Tianshan Mountains. The year-to-year variability for precipitation in October for these stations is very high because precipitation depends on the exact position of the high pressure core and its development. This is the reason that Tab. does not show this maximum for October but in Tab. (NOAA, 9) this maximum can be found for a different time period. TABLE Monthly precipitation amounts Urumqi ( m a.s.l.) (NOAA, 9). Station Urumqi <eb Apr May 2 Jun Jul Aug Sep Oct 7 Nov 22 Jec 27 (mm) for Dbs. Per Stations near the western border of the Xinjiang Province also have different precipitation systems. Kashgar has nearly no seasonal differences in its precipitation. TABLE Precipitation conditions for Kashgar (0 9 m a.s.l.) for a years period. Precipitation Amounts (mm) Days f*eb < Apr May Jun fa Jul 0 Aug 2 Sep Oct < Nov Dec < Ob. Period years years

4 Felix P. Blumer The precipitation days are also divided over the whole year in Kashgar. Urumqi shows a slight maximum of days with precipitation (p > 0. mm) during the winter months even if at that time the seasonal precipitation amount has its minimum (Tab. ). In mountainous regions the maximum of days with precipitation is at the same time as the maximum of precipitation amount (Tab. ). TABLE Precipitation days in Urumqi 9). Precipitation Days 0 Feb 7 Apr May Jun Jul 7 \ug Sep ct 9 Nov Dec 2 9 (Mùller, Ob. Period 7 years TABLE Number of "days with precipitation for Hero Bridge (920 m a.s.l.), the Tianshan Glaciological Station in Houxia (20 m a.s.l.) and the Meteorological Station in Daxigau Valley ( 9 m a.s.l.). Station Hero Bridge Houxia Daxigau Valley Feb 0 0 Apr 9 0 May 22 Jun 2 Jul 20 2 Aug Sep 0 Oct 0 2 Nov Dec Ob. Period In flat areas year-to-year variabilities are much higher than in mountainous regions in the Tianshans where the annual precipitation amount is about 00 mm. TABLE 7 Annual precipitation at the Meteorological Station in Daxigau Valley (9 m a.s.l.) (Zhang, 9 and Blumer, 990). Precipitation (mm) Precipitation (mm) / / 77. 9/ / /. 70/7 0. /2.2 72/7. 2/ 9. 'fa/ti 90. /.2 7/77.0 / /7. / /7 2.0 aver.. For a seventeen year period at the Meteorological Station in Daxigau Valley (9 m a.s.l.) the annual precipitation average is. mm. The standard deviation is.9 mm or 0% which indicates that the year-to-year variability is less, especially with regard to the arid climate of the Tianshan Mountains. The amount for the three summer months in particular seems to be very constant, although there is a considerable variability between the different months. For the stations in Urumqi River Valley two thirds of the annual precipitation falls between June and August. METHODS observation sites including the existing permanent stations maintained by the Tianshan Glaciological Station were selected and analyzed to understand the characteristics of the precipitation regime in the Urumqi River Valley and especially on Glacier N at the source of the Urumqi River.

5 7 Precipitation in the central Tianshan mountains TABLE Network of the observation sites. A B C D E F G H I K L Abrevc H. Sfc Observation Site Accumulation area of Glacier N Equilibrium line of Glacier N Near the tongue of Glacier N Moraine on the northside of Glacier N Ablation area of Glacier N Daxigau Valley Empty Cirgue Tianshan Glaciological Station in Houxia Runoff Station Urumqi River Runoff Station near Glacier N Runoff Station in the Empty Cirgue itions of the precipitation gauges: Altitude 020 m a.s.. 90 m a.s.. 70 m a.s.. 0 m a.s.. 0 m a.s.. 0 m a.s.. 2 m a.s.. 20 m a.s.. 0 m a.s.. 70 m a.s.. 00 m a.s.. Precipitation gauge H.St H, Ch. UR H. 2 Ch, St H, Ch St St St Ch Ch Ch Ch Hellmann gauge Ch: Chinese Standard gauge Storage gauge UR Universal Recording raingauge (Belfort type) The Hellmann gauges were cleared every morning at six o'clock Local Standard Time (LST). The Chinese Standard gauges were cleared after each precipitation event. This was necessary to impede evaporation because there was no funnel in the gauge. The Swiss storage gauges were measured twice a month, on the st and the th day of the month. The Universal Recording raingauge (Belfort type) operates on the weighing principle. In this precipitation gauge the beginning, the end and also differences in the cumulative precipitation are recorded. The resolution of the records is 0. mm/h. The capacity of the gauge is 00 mm. The observation sites were chosen in such a way that the precipitation characteristics on the whole glacier should be well represented. Sites A, B and C are considered to represent the middle axis of the glacier. Together with the other permanent stations, they supply good information about the precipitation conditions along the upper part of the Urumqi River Valley on the north slope of the Tianshan Mountains. Results of the precipitation measurements in Urumqi River valley The precipitation conditions in summer 97 in the mid-part of the Tianshans were characterized by a normal June (values near the year-long average). Strong cyclonic activities were responsible for the above-average precipitation in July, while August was very dry, influenced by anticyclonic circulation (Grebner, 990 b). In an analysis of the precipitation regime of the Xinjiang Province and especially the regime of high mountain areas in Tianshan, it is important to differentiate between convective precipitation and cyclonic precipitation. Using the structure of the ciouds, the form of precipitation and air pressure values, it was possible to determine the type of precipitation for the period from June th to August st 97. Tab. 9 gives information about the cyclonic precipitation events. Precipitation events were classified as cyclonic according

6 Felix P. Blumer to the synoptic weather conditions even if the structure of precipitation seemed to be convective. TABLE 9 Contribution of cyclonic precipitation to total precipitation in 97. Month June 7 July 7 August 7 Total Précipitât. Amount (mm) Cyclonic Precipitation Amount (mm) (%) Précipitât. Events (count) Cyclonic Precipitation Events (count) (%) In the summer months 97 only 0-0% of the precipitation events were of cyclonic origin, but 70-9% of the monthly precipitation amounts were produced by cyclonic influence. This accords well with the literature. Lu (9) (Watts, 99) found that only 0% of the annual precipitation is brought by the local convection. The high percentage values for August are also a result of low moisture in low evening convective activity. For the same reason the number of days with precipitation was decreasing in August but the percentage value of cyclonic events was higher than in the other months. At the same time convection was dominant for stations in the lower part of Urumqi River Valley. Convective clouds climbed only up to an elevation of about 2000 m a.s.l.. On Glacier N 0 isolated precipitation events with 09 precipitation hours were observed for the period from June th to August st. About 0% of these precipitation events took not more than hours. This is the result of the dominance of the convective typus. Precipitation intensities in the Tianshans are very low. The highest intensity was measured with. mm/h. Nearly half of all 09 hours with precipitation had intensities of less than 0. mm/h, although 70% of the precipitation events have a cyclonic origin. This is a result of low moisture content in the air Dependence of precipitation on the altitude In Tab. 0 the measurement results of different altitudes are compared. TABLE 0 Precipitation amounts for the various observation sites in the Urumqi River Valley in summer 97. Observation Site A B D L C I H Altitude June (mm) July (mm) Aug. (mm) Total (mm)

7 9 Precipitation in the central Tianshan mountains It is obvious that the general precipitation conditions do not change with the altitude under the aspect of the short horizontal distance and the dominant influence of cyclonic events to precipitation amounts. For all stations the maximum value is in July and the minimum in August ? 000 B_ 200. f S AJuna BJuly OAugusI x?^ o],,,,,,,,,,.,,,, i, Monthly precipitation amounts (mm) in 97 FIG. 2 Dependence of monthly precipitation on the altitude in Urumqi River Valley in 97. Fig. 2 shows that the maximum precipitation amount for all three summer months is not over 900 m a.s.l.. Similar results were found by Yang (99) for summer 9 and by Wang and Zhang (9) for the summer months 99. This result is very remarkable because the measurement sites in the upper Urumqi River Valley in 9 and 9 9 are not identical with those of 97. Wang and Zhang gave a result for the south slope of the mountain ridge as well, where the maximum precipitation amount was also below 900 m a.s.l.. For summer 9 and summer 99 a second maximum can be found at an elevation of about 2000 m a.s.l.. This peak can also be seen for August 97. It seems to be a result of convective precipitation events, reaching up to only about 2000 m a.s.l.. For the same reason it could not be seen in June and July 97 because cyclonic influence was extremely dominant. In August 97 this result could be analyzed very well. On Glacier N at 700 m a.s.l. precipitation was falling in the first half of the month as a result of slight cyclonic activity. At the Glaciological Station in Houxia (20 m a.s.l.) precipitation days appeared in the second half of August and were a result of some convective events. This zone of maximum precipitation for convective events is at about the same altitude as precipitation maximums that can be found in other convective regions on earth (Lauscher, 97). CONCLUSIONS About 70% of the annual precipitation in the Tianshan Mountains falls during the three summer months under the influence of Indian monsoon system. During this time about 70% of the precipitation amount is of cyclonic origin and

8 Felix P. Blwner 0 only 0% is from local convection. The altitudinal distribution of precipitation through the Tianshan Mountains shows two zones with relative high precipitation amounts, one at an elevation of about 2000 m a.s.l. and one at an elevation of about 00 m a.s.l.. The lower zone seems to be primarily influenced by local convection, while the upper zone, where mountains reach up to 00 m a.s.l., seems to have a maximum of cyclonic precipitation amount. ACKNOWLEDGEMENTS Thanks are extended to the Lanzhou Institute of Glaciology and Geocryology for scientific cooperation and logistic support. At the Geography Department of ETH Zurich my thanks are also extended to H. Lang, A. Ohmura and L. N. Braun for their valuable comments and Mrs. S. Braun-Clarke for proofreading. REFERENCES Blumer, F. (990) Precipitation Conditions in Xinjiang Province. Glacial Climate Research in the Tianshan. Ziircher Geographische Schriften,, Grebner, D. (990 a) Climate in Northwestern China. Glacial Climate Research in the Tianshan. Ziircher Geographische Schriften,,, 2-2. Grebner, D. (990 b) Synoptical Conditions during the Summers 9 and 97. Glacial Climate Research in the Tianshan. Ziircher Geographische Schriften,, 29-. Lauscher, F. (97) Weltweite Typen der Hôhenabhângigkeit des Niederschlages. Wetter und Leben f 2/2, Lu; A. (9) Chinese climatology. Collected Sci. Papers (Meteorology) Academia Sinica, Peking, 9, -. Miiller, M. J. (9) Handbuch ausgewâhlter Klimastationen der Erde, Trier. NOAA, (9) U. S. Department of Commerce. World Weather Records 9-70, Vol,, Asia. Wang, D. & P. Zhang (9) On the valley climate of Urumqi River in the Tianshan Mountains. Journal of Glaciology and Geocryology, 2, Watts, I. E. M., (99) Climates of China and Korea. Climates of Northern and Eastern Asia. World Survey of Climatology, H. Arakawa, Ed., JL, -7. Yang, D., Y. Shi, E. Rang & Y. Zhang (99) Research on analysis and correction of systematic errors in precipitation measurement in Wulumqi River Basin, Tianshan. Precipitation Measurement. WMO/IAHS/ETH Workshop on precipitation measurement St. Moritz, Dec. 99, 7-79.