| Heavy rainfall event during 4th-5th July 2003 is diagnosed by using the conventional observational datasets and infrared satellite pictures , and it occurs under the influence of the westerly trough at 500hPa level, the shear line at 850hPa level, the mesoscale low vortex system and the stationary front on surface while large-scale circulation is stable.Based on the mesoscale model MM5, the numerical simulation of the rain gush is carried out. Simulated results show that the development of the mesoscale rainstorm system is well described. By analyzing high resolution model output, the structure, evolvement of meso- β -scale system along with rainstorm and its mechanism are investigated. It shows that : MCSs propagated eastwardly, which developing on Meiyu front, is the direct influent system of the rainstorm and Coupling of high and low level winds is the main reason of the event., symmetric stability is the possible mechanism of meso- 0 -scale system. Over the convectively unstable area, baroclinic item of moist potential vorticity (mpv2) is the dominant factor. At the early stage of the development of MCS, secondary circulation forced by frontogenesis intensifies the convergence of low level air over the frontogenesis region and it make a positive feedback on convective activity. The essential reason that causes this precipitation process terminated is not variety of uprising condition, but gradual substitution of dry air for wet air at low-level over torrential rain area under the influence of in the mid-low-level horizontal circulation and secondary circulation over Meiyu front, vapor sensitive experiments proved that vapor play an essential role in the evolvement ofMCS.
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