| Based on NCEP reanalysis data, a heavy rain process occurred in Changjiang-Huaihe basins during the period from 20:00 BST July 9 to 20:00 BST July 10, 2005 is synoptic analyzed and diagnosed. It was found that the heavy rainfall occurred in a well weather situation with the low-level trough and subtropical high at 500hPa and the shear line at low level. The mesoscale numerical model MM5(V3.6) and WRF(V2.1) are used separately to simulate this process and the results show that this two model were basically possessed of a capability to reproduce the rainfall, especially the excessively heavy rain focus in the Xinyang. The comparison between the simulative rainfall of MM5 and that of WRF had shown that the latter was better, also the geopotential surface of 500hPa and the streamline field of 850hPa. The model outputs were used to diagnostic analysis and it was found that there were low-level vortex building and moving eastwardly in low-level convergence field above the rain-band. The low-level vortex could be maintained and developed due to the strong upward current which caused by interaction of the low-level jet inflow in its southwest and high-level jet outflow in its northeast. Above the rainfall area there were the instability area of MPV<0, and the negative focus of MPV located at the focus of heavy rainfall when MPV1<0, MPV2>0 and |MPV1|>|MPV2|. The positive focus of MPV slided toward low level along the iso-quasi equivalent potential temperature ribbon and leading to the enhancement the negative focus of MPV and convective instability in the low level. The air with high potential vorticity and low relative humidity intruced into the low level from the upper level along the iso-quasi equivalent relative humidity ribbon and inspired the fast development of the potential vorticity focus on account of the mass convergence and U-wind shear of upper level.
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