Diagnostic Analysis and Numerical Study of Torrential Rain Associated with the Tropical Storm Fitow (0114)
热带风暴"菲特"(0114)特大暴雨的诊断研究

The strongest excessive heavy rainfall breaking the fifty\|two\|year historical precipitation records in Hainan Island is caused by the tropical storm passing through Qiongzhou Strait with a southwest turning. Satellite images, radar reflectivities and intensive observations demonstrate that the rain is associated with a mesoscale convective system (MCS) which forms in a convergent area between the Fitow and the mountain range in the middle of Hainan Island. A numerical simulation has been performed with PSU/NCAR mesoscale model MM5V3 to study the relationship between the heavy rain and the MCS. The diagnostic analysis is carried out so as to gain a better understanding of the heavy rain mechanism. The results show that the mountain range in the middle of Hainan Island is favorable for generating a MCS when a tropical cyclone cross the Qiongzhou Strait. The heavy rain does coincide with the MCS on the northwestern side of the mountain. The diagnostic analysis indicates that the convergence and vorticity are concentrative in the lower layer and extend to the middle layer when rain increases while the divergence field is strengthening in the upper level right above the MCS in the heavy rain period. It is also found that the moisture of the rainfall from the South China Sea transported by the southwest flow field around the right cycle of the vortex is drawn into the rainfall area. The study also shows that the vertical motion is quite strong in the MCS and the maximal upward speed appears in the height between middle and upper level due to the strong upper level divergence. The strong unstable stratification at the middle and lower layer of the MCS also offers a positive contribution to the heavy rain associated with the storm Fitow.