| By using the conventional observational datasets, the causes of the typhoon rainstorm process occurred on during 8-14 Aug 2004 in Zhejiang Province were studied. The primary diagnostic analyses show that the Continental High/ subtropical high in Japan Sea on the west/east of the shallow slot though in the northeast China is the mainly large-scale circulation systems. It is found that the deep convective system was the direct cause of the rainstorm; the dense structure of helical ribbon was the impact factor whether the typhoon can cause the severe rainstorm; the favorable terrain may enhance precipitation; the interaction between mid and low latitude circulations is the great trigger mechanism of the typhoon rainstorm and sever convective weather.Based on the two-way nested non-hydrostatic mesoscale numerical model (MM5V36), a series of sensitive experiments are carried out to simulate 24-hour precipitation caused by the landfall typhoon "RANANIM" on 12 August 2004. The results show that the simulated rainfall area and the precipitation intensity are excellent agreement with the observations, especially for the position of the typhoon rainfall center and the precipitation rate. The analysis of the simulated 850hPa flow field shows the center position and moving track of typhoon are also very closely with observations. Terrain sensitive experiments proved that: (1) the increase of rainfall near the center of the typhoon has a close relation with the terrain; (2) terrain tendency and height can result in more uniformity of the precipitation distribution; (3) except for the precipitation on the west of periphery of the typhoon, the precipitation enhancement extent appears counter of phase relationship with that of the typhoon center or near the typhoon, especially on the region of the East Sea; (4)The distribution of typhoon precipitation presents the positive ad negative change as the gravity wave column wise under the influence of terrain.
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