Numerical Simulation And Analysis On Rainfall Distribution Of Tropical Storm Bilis(2006) During Landfall

Firstly, surface observation rainfall data, surface rainfall rate retrieved from TRMM satellite observation and analysis data from NCAR/NCEP were used to compare and analyze the rainfall distribution characteristic of typhoon Haitang(2005) and tropical storm Bilis(2006). The result showed that though the tracks and landfall positions of these two tropical cyclones were similar, rainfall distributions of them distinguished each other obviously. The whole structure of Bilis was heavy in northern part and rare in southern part after landfall. And the rainfall structure showed wavenumber-1 asymmetric distribution.The past studies indicated that terrain, change of underlying surface properties during landfall, tropical cyclone motion and interaction between tropical cyclone and environmental flow were influence factors on rainfall distribution for landfall tropical cyclone. In order to study the dynamic mechanism to form wavenumber-1 asymmetric rainfall structure of Bilis in depth, the mesoscale numerical model WRF was used to simulate the landfall process of Bilis in high resolution. On the base of successful control experiment, two sensitivity experiments about terrain and underlying surface property were also carried out. Analyzing the results of these three experiments, we found that the rainfall in most part of Zhejiang and Fujian province was induced by the lifting convergence effect of Min-Zhe hill to tropical storm circulation. At the same time, the underlying surface properties change from ocean to land had some effect on rainfall in this region. The heavy rainfall in common boundary of Jiangxi, Fujian and Guangdong province did not showed clearly relation with Bilis motion speed change or underlying surface properties sharp change during landfall. So these two factors were not the dynamic factors to induce wavenumber-1 asymmetric rainfall distribution of Bilis. The mountain terrain and complicate fraction on land surface affected intensity of rainfall and vertical motion a certain extent, but they were not able to change the character of wavenumber-1 asymmetric rainfall structure. Similarly, these were not the main factors to form wavenumber-1 asymmetric rainfall structure. The direction of environmental vertical wind shear corresponded well with heavy rainfall in common boundary of Jiangxi, Fujian and Guangdong province. Strong rainfall and vertical motion located in downshear direction or downshear left mainly. In upshear direction, the rainfall and vertical motion were feeble. The change of wavenumber-1 rainfall amplitude lagged the change of environmental vertical wind shear magnitude two to three hours. As a result, environmental vertical wind shear was the dominant dynamic factor to form wavenumber-1 asymmetric rainfall structure of Bilis during landfall as compared to other factors. Under effects of vertical wind shear, convective updrafts were initiated in the downshear right side of vertical wind shear continually and moved cyclonically around the storm center. During this period, updrafts combined with each other and maximum updrafts located in the downshear left of vertical wind shear. So maximum precipitation located near downstream of maximum upward motion. Upward motions dispersed and weakened gradually a few hours later.