Impacts of Typhoon Track and Island Topography on the Heavy Rainfalls in Taiwan Associated with Morakot (2009)

Cloud-resolving ensemble simulations and sensitivity experiments utilizing the Weather Research and Forecasting model (WRF) are performed to investigate the dynamics and predictability of the record-breaking rainfall and flooding event in Taiwan induced by Typhoon Morakot (2009). It is found that a good rainfall forecast foremost requires a good track forecast during Morakot's landfall. Given a good track forecast, interaction of the typhoon circulation with complex topography in southern Taiwan plays a dominant role in producing the observed heavy rainfalls. The terrain slope, strength of the horizontal winds, and mid lower-tropospheric moisture content in the southwesterly upslope flow are the primary factors that determine the rainfall location and intensity, as elucidated by the idealized one-dimensional precipitation-rate forecast model. The typhoon circulation and the southwesterly monsoon flow transport abundant moisture into southern Taiwan, which produces the heavy rainfall through interactions with the complex high terrain in the area. In the meantime, as part of the south China monsoon, the southwesterly flow may be substantially enhanced by the typhoon circulation.