Abstract: Using the conventional meteorological observation data and ECMWF ERA-Interim data (0.125°×0.125°),the isentropic surface characteristic of two rainstorms,which were occurred in the west of south Xinjiang on May 21 to May 23,2012 and on May 26 to May 29,2013 respectively,were analyzed to establish the thermal dynamic model of the rainstorm process.The results indicated that the rainstorm came from a synthetic action of topographic forcing and jet streaming coupling among high,medium and low level jet streams when there was a Central Asian low vortex.At the high level of troposphere,the South Asia High appeared in a zonal distribution with its west part being northward raised obviously and its subtropical trough being extended southward to central Asia.Therefore a strong high-altitude southwest jet stream was formed over the west of south Xinjiang which intensified the chimney effect and caused a strong divergence formed at high altitude over the precipitation region.The cold east-deflected jet at the low level was also spread into the precipitation region which caused an intense convergence.This region has the terrain with high altitudes in its west side and low altitudes in its east side,westward airflow is lifted.The overlay of the dynamic effects from both high level and low level pushed strongly the ascending motion which provided the force condition for the rainstorm. The south-deflected airflow at middle level and the east-deflected airflow at low level carried abundant water vapor into the precipitation area which produced the vapor condition for the rainstorm.The cold air in the front of the Central Asian Vortex,which has high values of vortex,climbed over the west Tianshan adding more water vapor content at the middle level over the precipitation area.Part of the cold air slid along the geographic surface and joined the cold air at the low level and this convergence pushed the ascending motion further.The lifted cold air encountered the warm air at the middle level which made the water vapor coagulate into rain.There was a significant correlation between the potential vorticity of the isentropic surface and the heavy rainfall.The isentropic potential vorticity was increased when the precipitation began,and it was decreased when precipitation was over.Rainfall was triggered by the convergence of cold air and worm air.The intensity of the cold air was weaker than that of the warm air when there was a short-time strong rainfall,and it was the other way around when there was a persistent precipitation.

Key words: rainstorm, Central Asian Vortex, isentropic potential vorticity, thermal dynamic model, west of south Xinjiang