The Numerical Simulation And Diagnostic Analysis Of A Storm Rainfall Caused By Typhoon Morakot

First, we analyzed the activities of eighth Morakot typhoon on August 2009 by using the real-time observations data, the results show that the typhoon was formed in the South China Sea monsoon trough, developed and moved under the joint action of subtropical high, Goni typhoon, Etau typhoon and South Asia high pressure system. The Morakot typhoon caused heavy rainfall in the island of Taiwan, heavy rain occurs mainly in central and southern regions of Taiwan. Typhoon hovered in the Taiwan Strait for a long time after across the Taiwan is an important factor that caused the storm.We examine the role of typhoon spiral rain bands and the horizontal vorticity in a rainstorm case (typhoon Morakot in Taiwan in 2009) using the Weather Research and Forcasting (WRF) model. Our simulation is from 12:00 on August 6 to 00:00 on August 10, when there is heavy rainfall in Taiwan. Our results show that the maximum precipitation occurred on August 7 to 9, in the meantime the rainstorm in Taiwan is related to two spiral rain bands, the major spiral rain bands was located in central Taiwan, another branch is located in southern Taiwan. In low level, rain bands consistent with the large values of vorticity region, positive vorticity greater, indicating that the rain bands stronger. Spiral rain bands have lower convective unstable stratification, conducive to stimulate convective clouds.Rainstorm occurred near to the vertical circulation, and moved with the large values of horizontal vorticity region, shows both closely linked, The large value of horizontal vorticity was related to the vertical vorticity, horizontal vorticity converted to vertical vorticity in the rainfall area, because of the non-uniform distribution of vertical movement, lead to the horizontal vorticity tube distorted. when precipitation enhancement, transformation strengthened. The positive curl of horizontal vorticity was corresponded with upward movement, it was also corresponded with spiral rain bands and rainfall,when the horizontal vorticity reduced,rain bands remained as the positive curl of horizontal vorticity existed.Between Goni and Morakot Typhoon, horizontal vorticity convergence zone exists on which to form a cloud band, horizontal tube as a channel to promote the typhoons’water vapor and energy transport.Horizontal helicity can react typhoon spiral rain band inflow horizontal airflow strength and size along the inflow horizontal vorticity. After horizontal vorticity input into spiral rain bands, gradually transformed into vertical vorticity, which is conducive to enhancing the rotation of the spiral rain bands, making the storm even more intense. Horizontal helicity on 750hPa had a better correspondence with the precipitation, the positive helicity corresponds to the center of the precipitation.Over the precipitation area, the positive horizontal helicity was larger in the lower, and decreased with height. When the stronger horizontal helicity in low level, and the positive zone extends higher, the rainfall would be stronger.