On the extreme rainfall of Typhoon Morakot (2009)

Typhoon Morakot (2009), a devastating tropical cyclone (TC) that made landfall in Taiwan from 7 to 9 August 2009, produced the highest recorded rainfall in southern Taiwan in the past 50 years. This study examines the factors that contributed to the heavy rainfall. It is found that the amount of rainfall in Taiwan was nearly proportional to the reciprocal of TC translation speed rather than the TC intensity. Morakot's landfall on Taiwan occurred concurrently with the cyclonic phase of the intraseasonal oscillation, which enhanced the background southwesterly monsoonal flow. The extreme rainfall was caused by the very slow movement of Morakot both in the landfall and in the postlandfall periods and the continuous formation of mesoscale convection with the moisture supply from the southwesterly flow. A composite study of 19 TCs with similar track to Morakot shows that the uniquely slow translation speed of Morakot was closely related to the northwestward-extending Pacific subtropical high (PSH) and the broad low-pressure systems (associated with Typhoon Etau and Typhoon Goni) surrounding Morakot. Specifically, it was caused by the weakening steering flow at high levels that primarily resulted from the weakening PSH, an approaching short-wave trough, and the northwestward-tilting Etau. After TC landfall, the circulation of Goni merged with the southwesterly flow, resulting in a moisture conveyer belt that transported moisture-laden air toward the east-northeast. Significant mesoscale convection occurred on a long-lived east-west-oriented convergence line and on the mountain slope in southern Taiwan. This convective line was associated with large low-level moisture flux convergence caused by the northwesterly circulation of Morakot and the southwesterly flow. It is thus suggested that the long duration of Typhoon Morakot in the Taiwan area, the interaction of southwest monsoon and typhoon circulation, the mesoscale convection, and the presence of terrain are the key factors in generating the tremendous rainfall.