NUMERICAL SIMULATION AND DIAGNOSTIC ANALYSIS OF A RAINSTORM DURING THE ANNUALLY FIRST RAINY SEASON OF SOUTH CHINA

With the outputs from the WRF model and the WRF 3DVAR model(version 3.2),this study gives detailed diagnosis and study of the thermal and dynamical conditions of a heavy rain that happened from June 18 to 20 during the annually first rainy season of South China.First,the simulation assimilated the data from regular observation stations and the automatic weather stations.The results showed that the distribution and intensity variations of the torrential rainstorm from the WRF model outputs were consistent with the observation.Second,a stable large-scale circulation was the background condition of this rainstorm.Third,an upper-level jet stream(ULJ),a lower-level jet stream(LLJ) and a vertical secondary circulation driven by them were the main dynamical mechanism of this process.Fourth,an ascending wet jet was induced by the continuous accumulation of convective available potential energy(CAPE) on the surface,and warm water vapor and unstable energy were transported by a southwest jet stream.This process was driven by strong vertical motion.Accompanied by dry intrusion driven by a dry cold from the mid-and upper-level invading the low level,the process served as the thermodynamic condition of this rainstorm.Finally,diagnose of the moist potential vorticity(MPV) and frontogenetical function indicated that the dry cold air from the mid-and upper-levels of the troposphere that invaded the lower level due to the secondary circulation interacted with the ascending warm,wet air flow from low levels,being conducive to the release and development of baraclinic instability and convective instability energy,which may have triggered the rainstorm.