Diagnostic and Predictive Studies of Torrential Rain Location Associated with Landfalling Typhoon Morakot (0908) Using Multi-Dynamical Parameters

Typhoon Morakot (0908), the most wide-ranging, disastrous typhoon to make landfall in China during 2009, caused flooding from rainfall in southern Taiwan and in parts of Fujian and Zhejiang provinces. Because the location of torrential rain is the focus of forecasters, the Weather Research and Forecast Model (WRF) is used in this study to simulate Typhoon Morakot at high resolution with triple two-way nesting in which the finest grid size is 2 km. This simulation agrees well with track, intensity, and precipitation distribution observations. Diagnostic analyses included the simulation result and reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The southwest monsoon was determined to be the dominant contributor of water vapor. In addition, a strong updraft spreading through the entire troposphere resulted in heavy rain. On the basis of the multi-dynamical parameters forecasting method, generalized moist potential temperature, the vertical component of convective vorticity vector and moisture divergence flux were chosen to perform diagnosis and forecasting of this heavy precipitation event. The results reveal funnel-shaped areas of generalized moist potential temperature isolines in the vertical sections, which corresponds well with the strong precipitation region. By using four-times-daily NCEP-Global Forecast System (GFS) forecast products, the vertical integration of the vertical component of convective vorticity vector and moisture divergence flux in the middle and lower troposphere were calculated; both show some significance in the prediction of rainfall areas. The high-value region of convective vorticity vector integration gradient is in fair agreement with the rainfall area, and the time evolution of convective vorticity vector integration is rather coincident with the observed precipitation. The forecast made by moisture divergence flux is better capable of both locating the rainfall area and tracing the torrential rain center. All of the parameters show indicative signals for the heavy rain location of Typhoon Morakot, and they are of certain significance in the diagnosis and prediction of typhoon precipitation regions.