Rainfall over the Arid Area in the Northwestern China

Abstract

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 100km. However, the amount of rainfall depends on the altitude, e.g. 600mm/year in the Qilian Mountains at 3000mASL and only 100mm/year or less at the bottom flat of the valley called "Hexi Corridor" at around 1300mASL.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.

Evaporation of Rain Falling Below a Cloud Base Through a Deep Atmospheric Boundary Layer Over an Arid Region

“…6. This rain intensity corresponds to 23.3 mm day-1; this value is almost the same as a rainfall event at the Qilian Mountains area (about 20 mm day-1) in August 1991 analyzed by Sahashi (1995) and Itano (1997). In the present calculation for this light rain, the strong dependence of F on height is also clearly seen (Fig.…”

“…Chen et al (1991) showed, in their study on the cyclogenesis over East Asia, that the peak frequencies of cyclogenesis over the Gobi Desert and Mongolia Plateau occur from April to May and from August to September. Consistent with their analysis, Itano (1997) showed that in summer frontal cloud systems frequently pass over the desert area. In spite of the frequent passages of frontal cloud systems, the desert surface receives only a small amount of rain compared with the adjacent mountains.…”

“…Rain intensity is very weak, i. e., a few mm h-1, and the surface precipitation continues for about one day (Sahashi, 1995;Itano, 1997). Thus, a horizontally uniform, steady rain within the range of stratiform-type weak intensities was assumed in the present model, and the model configuration was one-dimensional (1D).…”

“…Sahashi (1995) analyzed the surface data of HEIFE in summer and showed that the rainfall was mainly observed within a surface high pressure region: the rainfall began more than 1 day after a sudden pressure rise that indicates a frontal passage. However, using satellite data, Itano (1997) showed that this rainfall results mainly from synoptic-scale precipitating cloud systems associated with a polar trough. The cloud systems had a band structure whose width extended several hundred kilometers, and covered the whole HEIFE area for one or two days.…”

“…The surface elevation of the desert is about 1400 m above mean sea level (MSL), while that of the mountains (the Qilian Mountains) is higher than 3000 m above MSL. Thus, precipitation depends strongly on elevation (Itano, 1997). Sahashi (1995) analyzed the surface data of HEIFE in summer and showed that the rainfall was mainly observed within a surface high pressure region: the rainfall began more than 1 day after a sudden pressure rise that indicates a frontal passage.…”