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Spatial and Temporal Distribution of Water Vapor and Its Variation Trend in Atmosphere over Northwest China

YU Ya xun 1, WANG Jin song 1, LI Qing yan 2 (1. Lanzhou Institute of Arid Meteorology, Chinese Meteorological Administration, Lanzhou Gansu 730020, China; 2. Lanzhou Center Meteorological Observatory, Lanzhou Gansu 730020, China)  
The interannual and intermonthly variation of the water vapor and its mean transfer in the atmosphere over Northwest China are calculated and analyzed by using the NCEP/NCAR global reanalysis grid data(2.5°×2.5°Lat/Lon) for 43 years (1958-2000). The results show that: (1) The climatic averaged water vapor in whole air column over Northwest China is concentrated in the east and west areas, respectively, with the maximum in summer and the minimum in winter. In the east areas water vapor is most abundant and stable, while in the west areas, such as the Tarim Basin and north part of the Tianshan Mountains, water vapor prominently varies with course, which changes seasonally. The climatic moisture over the middle of Northwest China is the least, especially over the west and north parts of Qinghai Province. (2) The water vapor over Northwest China mainly comes from the southwest warm and wet air flow over South China Sea, Arabian Sea and Bay of Bengal, subjected to the influence of the Tibetan Plateau's lifting force, forming three vapor transfer centers over eastern, middle and western parts of the plateau, respectively, with the strongest one located in eastern part of the plateau along the gorge of Yarlung Zangbo River. Another one comes from southeast warm and wet airflow over the Qinling Mountains in eastern Sichuan Province and southern Shaanxi Province, and the third one is the northwest moisture from Siberia and Mongolia during spring and summer. (3) On the average, the water vapor over Northwest China decreases obviously during the late 1950s to the middle 1980s, but it has being increasing since the late 1980s. The vapor increasing areas are mainly located in Tarim Basin, Northern Xinjiang Region and eastward along Hexi Corridor to the middle Qilian Mountains, while the vapor decreasing areas are located in the middle and eastern parts of Northwest China. (4) For the decadal scale of climate change, the water vapor in most parts of Northwest China decreased during the 1960s to 1970s, but increased obviously during 1980s to 1990s, especially in western parts of Northwest China, where the increasing rate of vapor reached 0.8~1.6 mm·10 a -1 . (5) Finally, the climatic dynamical factors influencing vapor distribution and transfer over Northwest China are discussed. Subjecting to the impact of increasing Mongolia anticyclone located in north side of Northwest China, the westerly at 500 hPa decreases and the southerly increases obviously over western parts of Northwest China, but the northerly at 500 hPa increases and the southerly decreases obviously over eastern parts of Northwest China. This may be one of the causes resulting in interannual variation and transfer of water vapor in atmosphere over Northwest China.
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