Typhoon Fanapi (2010) and its Interaction with Taiwan Terrain – Evaluation of the Uncertainty in Track, Intensity and Rainfall Simulations

Using special data from the field program of “Impact of Typhoons on the Ocean in the Pacific” (2010) and an ensemble Kalman filter–based vortex initialization method, this study explores the impact of Taiwan terrain on the uncertainty in forecasting track, intensity, and rainfall of Typhoon Fanapi (2010) based on ensemble simulations. The results show that the presence of Taiwan topography leads to rapid growths of the simulation uncertainty in track and intensity during the landfall period, particularly at the earlier landfall period. The fast moving ensemble members show an earlier southward track deflection as well as weakening of intensity, resulting in a sudden increase of standard deviation in track and intensity. During the period of offshore departure from Taiwan, our analysis suggests that the latitudinal location of the long-lasting and elongated rainband to the south of the tropical cyclone (TC) center has strong dependence on the latitude of the TC center. In addition, the rainfall uncertainty in southern Taiwan is dominated by the uncertainty of the simulated TC rainband, and the latitude of the TC track can be regarded as a good predictor of the rainband's location at departure time. It is also found that the rainband develops farther to the south as the topography is elevated. Considering the fact that the rainband impinging the high mountains in the southern Central Mountain Range generates the greatest accumulated rainfall, positions where the rainband associated circulation and its interaction with topography appear to offer an explanation on the uncertainty of the simulated rainfall.