High-Resolution Numerical Simulation of the Extreme Rainfall Associated with Typhoon Morakot. Part I: Comparing the Impact of Microphysics and PBL Parameterizations with Observations

Typhoon Morakot hit Taiwan the night of 7 August 2009 as a Category 1 storm and caused up to 3000 mm of rain, leading to the worst flooding there in 50 years as well as devastating mudslides. The Weather Research and Forecasting model (WRF) is used at high resolution to simulate this extreme weather event. The model results indicate that WRF is able to capture the amount and location of the observed surface rainfall and that the typhoon-induced circulation, orographic lifting and a moisture-abundant southwest flow are the main mechanisms that together produced the tremendous rainfall in this case. Furthermore, the model results suggest that the agreement with the observed rainfall is due to the simulated storm track and intensity being in relatively good agreement with the observed. Additional simulations were made to examine the sensitivity of this case to model physics (microphysics and planetary boundary layer or PBL). Both warm rain only as well as improved microphysics yield similar significant rain amounts at the same locations as the control case. The improved microphysics lead to a better storm intensity early on but later exceed the observed intensities by about 10 hPa. The stronger storm arises from less evaporative cooling from cloud and rain and consequently weaker simulated downdrafts. Warm rain results closely match the control (i.e., the track, intensity, and maximum rainfall locations/amounts), implying ice processes (i.e., additional heat release due to ice processes) have only a secondary effect on surface rainfall. Results are less sensitive to using different PBL schemes than different microphysics.