| Zhejiang province is one of the most-hit areas by typhoon in China every year. In order to further understand the origin and development of the short-time rainstorm process caused by landfalling typhoon impacting Zhejiang province seriously, discover the ground trigger mechanism of the rainstorm and provide some scientific references for the refined short-range forecast, the advanced research WRF modeling system is used to simulate this kind of rainstorm processes in recent years. With the WRF-Var 3-D assimilation system, conventional surface, upper air data and radar reflectivity as well as radial wind are assimilated into WRF every 6 hours. Results show that the typhoon track and intensity development and precipitation are well simulated by WRF, and the assimilation of observed data into model can to some extent improve the simulated results.Results reveals that there's confluent stream in the low level and effluent stream in the upper level before the rainstorm happens, which provides favorable environment for the rainstorm. The water vapor comes from the low-level southeast jet stream to the northeast of typhoon center. When the jet stream is close to the rainstorm zone, the water vapor will be transferred there directly by the jet stream, while the jet stream is far away from the rainstorm zone, the water vapor will be transferred by other streams. During the rainstorm process, there's a meso-β-scale convergence belt appearing within the planetary boundary layer which can be observed by radar. There will be strong precipitation occurring along the convergence belt in the next hour. The distribution of precipitation is similar to the convergence belt, traveling and developing with the convergence belt. The convergence belt develops upward and inclines to the outside of typhoon, but the height is not fixed. However, the strongest convergence center always locates within the planetary boundary layer. On the top of the convergence center, there's a low-level jet stream. Below the jet stream, there's stream conflux and ascent in the convergence zone. The vertical distribution and development of convergence belt is up to the vertical convergence line, which appears in the area with horizontal wind gradient less than -1.2×10-4 s-1. The vertical convergence line first appears on the surface and then develops upward, either remaining upright or inclining to the outside of typhoon. Sometimes the vertical convergence line stops developing when it reaches the top of boundary layer, sometimes it keeps developing upward to the middle level of troposphere. The vertical convergence line is mainly caused by the direct confluent of streams from different directions and the velocity degression in the same stream. The streams come mainly from the low-level southeast jet stream to the northeast of typhoon center.The analysis also indicates that there're generally three types of trigger mechanisms of the rainstorm. In the first type, there's weak cold wet air intruding into the warm wet air zone and causing convergence, which forces the warm wet air to ascend and release vapor and energy rapidly, and finally trigger convection. Besides, the cold wet air and the warm wet air below make the air unstable in the zone. Once the convergence formed, it will also lead unstable energy to release and intestify the rainstorm. In the second type, there's a strong cold dry stream butting up against the warm wet stream in the rainstorm zone which causes an energy front. When the convergence forms. The warm wet air will ascend along the front and release vapor and energy quickly, which can bring about convection. After the precipitation produced, the air cools down and descends to the surface, which strengthens the strong cold stream and maintains the energy front. Then desending air flows back to the ascending area and makes confluent with the warm wet air, causing convergence and vertical circulation, which could intestify the ascending air and strengthen convection. In the third type, the structure in the rainstorm zone is stable and all the air in the planetary boundary layer are warm and wet. When convergence forms, it forces the warm wet air to ascend, which starts the convection and precipitation. After the air ascends beyond the planetary boundary layer, the latent heat released by solidification warms the air, which intestifies the ascending movement and forms unstable structure, then makes the precipitation stronger. Besides, there's strong positive relative vorticity appearing in the rainstorm zone, which will make the precipitation persistent and developing. Finally, the structure of short-time rainstorm process caused by landfalling typhoon is summarized, and some forecast factors are extracted to build a forecast model for this kind of rainstorm and provide some scientific references for future forecast.
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