| The characteristics of the mesoscale convective systems of a torrential rain case occurring over Yangtz River area on 9-10 July 2005 (" 05-7") are investigated by using mesoscale numerical simulations and observations data. The diagnostic analyses show that the heavy rainfall are produced together by trough, shear line and South-west vortex. The nonhydrostatic version of mesoscale numerical mode MM5 was used to simulate this case .The simulation results indicate that there are mesoscale weather systems which occurred near Xinyang in Henan province during 1200 UTC 9 July to 1200 UTC 10 July in 2005. Simulated results indicated that in the process of the mesoscale system development, a train of meso-P systems inspired. They will attenuate while they are apart from their parent. Meanwhile, the strong southwest jet and the northeast flow on low levels increased the convergence on low levels, and the westerly jet and the easterly jet on high levels increased the divergence on high levels. This collocation from low levels to high levels urged the mesoscale system to develop. By sensitivity test and analysis, the local mesoscale terrains have a certain extent influence on the location and intensity of rainfall. The relative location between terrains and the shear line is the key of these influences.
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