In this study we examine the role of typhoon spiral rain bands and the horizontal vorticity in a rainstorm case(typhoon Morakot,which occurred in Taiwan in 2009),using real-time observations and the Weather Research and Forecasting(WRF) model.The time period of our simulation is from 12:00 on August 6 to 00:00 on August 10,when there is typically heavy rainfall in Taiwan.Our results show that the maximum precipitation occurred on August 7 to 9,while at the same time the rainstorm in Taiwan was related to two spiral rain bands,the major spiral rain band of which was located in central Taiwan,while the other was located in southern Taiwan.At the low level,the rain bands consistent with the large values of the horizontal vorticity region pointed east,while at the top,the horizontal vorticity pointed west at the northern spiral rain bands.Rainstorms occurred near the vertical circulation,and moved with the large values of the horizontal vorticity region,showing that the two were closely linked.The large value of horizontal vorticity was related to the vertical vorticity,and the horizontal vorticity converted to vertical vorticity in the rainfall area.The positive curl of the horizontal vorticity corresponded with upward movement,as well as with the spiral rain bands and rainfall.When the horizontal vorticity reduced,the rain bands remained while the positive curl of the horizontal vorticity was present.