Influence of southwest monsoon flow and typhoon track on Taiwan rainfall during the exit phase: modelling study of typhoon Morakot (2009)

This paper studies the dramatic decadal increase of Taiwan rainfall in the typhoon exit phase. The exit phase is defined as the time interval between a typhoon centre leaving the Taiwan coast to reaching 100 km away from the nearest coastline. Typhoons which move across Taiwan's northern (track N), central (track C), or southern (track S) areas are selected for a statistical study. Taiwan hourly rainfall data at 21 surface gauge stations from the past 57 years are divided into two periods: 1960-1989 (P1) and 1987-2016 (P2). From P1 to P2, there are decadal increases of rainfall (approximate to 60%) and rainfall intensity (approximate to 30%, mm h(-1)) in the exit phase. There is also a decadal increase of the track C typhoons. The southwest monsoon water vapour flux (SWF) in a local region southwest of Taiwan, as computed from the JRA-55 dataset, is substantially larger in the track C typhoons than that in the track N typhoons. Our analysis indicates that the increase of SWF leads to the increase of rainfall intensity. Moreover, both the enhanced SWF and the prolonged duration time contribute to the increased rainfall in the exit phase. Typhoon Morakot (2009), a track C typhoon with extremely slow speed in the exit phase, produced the record-breaking rainfall. Model experiments and potential vorticity tendency diagnosis of typhoon Morakot are used to understand the dynamics of the increased duration time. The slowdown of typhoon motion is shown to be due to the asymmetric convection in the Taiwan Strait, which is produced by the interaction between typhoon circulation and southwest monsoon flow. The enhanced SWF and the prolonged duration time may explain the observation that the decadal rainfall increase is much larger than that of rainfall intensity in the exit phase.