The impacts of Taiwan topography on the predictability of Typhoon Morakot's record-breaking rainfall: A high-resolution ensemble simulation

In this study, we examine the impact of Taiwan topography on the extreme rainfall of Typhoon Morakot and the predictability of this rainfall with a high-resolution (4-km) ensemble simulation using the Advanced Research WRF (ARW) model. Ensemble prediction with realistic topography reproduces salient features of orographic precipitation. The 24-h and 96-h accumulated rainfall amount and distribution from the ensemble mean compares reasonably well with the observed precipitation. When the terrain of Taiwan is removed, the rainfall distribution is markedly changed suggesting the importance of the orography in determining the rainfall structure. Moreover, the peak 96-h rainfall amount is reduced to less than 20%, and the total rainfall amount over southern Taiwan is reduced to less than 60% of the experiments with Taiwan topography. Further analysis indicates that Taiwan's topography substantially increases the variability of rainfall prediction. Analysis uncertainties as reflected in the perturbed initial state of the ensemble are amplified due to orographic influences on the typhoon circulation. As a result, significant variability occurs in storm track, timing and location of landfall, and storm intensities, which in turn, increases rainfall variability. These results suggest that accurate prediction of heavy precipitation at a specific location and at high temporal resolution for an event such as Typhoon Morakot over Taiwan is extremely challenging. The forecasting of such event would benefit from probabilistic prediction provided by a high-resolution mesoscale ensemble forecast system.