Causes Analysis and Numerical Forecast Test of a Heavy Rainstorm in Changsha on May 21 in 2010

By using NCEP reanalysis data,an extremely heavy rainstorm process occurred in Changsha on May 21,2010 was made,which contain the analysis of vapor condition,unstable condition of thermal,ascending motion,dynamical condition etc. and the test of medium-low layer wind field,physical quantities of T639,precipitation forecast of RJ model. The study results show that the main causes that forming heavy rainfall is the oscillation of West Pacific Subtropical High(WPSH) occurred in South China and South China Sea and the collective activity of high trough,southwest vortex shear in medium-low level,surface inverted trough and cyclonic wave. The low southwest jet lying on medium layer is well corresponding to heavy rainfall. The good moisture condition including enough moisture transportation and moisture convergence condition reflect on heavy rainfall. With embedding of cold air,the water vapor flux and its divergence have a development trend from low level to high level. The profound mixed-level of infirm divergence and thin convergence is used to forming rainstorm. The forecast of EC real-time wind field,jet and low vortex shear is relatively exact. It is just that forecast heavy precipitation. The vertical motion forecast is inaccurate,but moisture flux is in good accordance with real. The RJ model is a good forecast index in the rainfall area and intensity.