Rainfall over the arid area in the Northwestern China - An analysis during HEIFE

Synoptic characteristics of precipitation over the HEIFE area in Northwestern China are investigated. The results show that rainfall in the HEIFE area is caused by disturbance on the polar frontal troughs, and the rain area covers the whole HEIFE area of 200 x 100 km. However, the amount of rainfall depends on the altitude: e.g. 600 mm/year in the Qilian Mountains at 3000 mASL and only 100 mm/year or less at the bottom flat of the valley called ''Hexi Corridor'' at around 1300 mASL. During summer, the water vapor content is rather large and stratification of the air over the desert shows potential instability. However, because of the high lifting condensation level (LCL) and the free convection level (LFC) over the desert, isolated cumulus convections do not develop, in spite of the large buoyancy induced by high ground temperatures. As a result, rainfall over the HEIFE area occurs when the disturbances on the polar frontal trough move into the area, The total amount of rainfall observed in the lower land of the HEIFE area, however, seems to be significantly reduced due to the evaporation of raindrops within the deep and dry boundary layer. Meanwhile, in winter, the water vapor content is quite small and stability is nearly neutral, which accounts for the low precipitation in this area.