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

Abstract

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 (∼60%) and rainfall intensity (∼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.

A Modeling Study on the Impacts of Typhoon Morakot's (2009) Vortex Structure on Rainfall in Taiwan Using Piecewise Potential Vorticity Inversion

“…Corresponding to this PV structure, the azimuthally-averaged wind speed distributions computed from the ECMWF-YOTC analysis and QuikSCAT data (the latter at 1200 UTC 6 August) indicate a radius of maximum wind (RMW) of about 150-200 km near the surface (Fig. 3), consistent with Wang et al (2012Wang et al ( , 2013b and Chen et al (2017). Thus, the eye region of Morakot is very large and atypical among most TCs.…”

“…Obviously, the role played by the topography of Taiwan was not unique to Morakot, as noted previously on many occasions (e.g., Chang et al 1993;Wu et al 2002;Cheung et al 2008), but the precipitation efficiency was particularly high in this case with the moisture-rich southwesterly monsoon surge (Chien and Kuo 2011;Chen et al 2017). Thus, a number of favorable factors from large to convective scales interacted together and worked in synergy to produce the extreme rainfall in Morakot (2009), where its slow translation speed and large size appear to be a deadly combination (cf.…”

“…8g-i) also resemble those observed (Figs. 8d-f), with the highest daily rainfall (of about 1560 mm) recorded on 8 August upon the departure of Morakot (Wang et al 2012;Chen et al 2017). Over the SCMR (near 22.3-22.7N), the model appears to produce more rain than observed, but this might be linked to an issue of under-sampling since very few rain gauges are installed in this remote mountainous area, as discussed by Wang et al (2012).…”

“…In the previous studies of Wang et al (2012Wang et al ( , 2013bWang et al ( , 2015 and Chen et al (2017) on TY Morakot (2009), control experiments were also carried out using CReSS and validated against observations. Starting from 0000 UTC 7 August for three days, the CTL here is quite similar to those experiments and its TC track and intensity are compared with the best-track data in Table 2).…”

“…This additional slow-down was attributed to the effect of asymmetric rainfall and, thus, latent heating that generated potential vorticity (PV) to the rear side of the TC motion during this period (Wang et al 2012;Hsu et al 2013). Recently, Chen et al (2017) further demonstrates that it was the east-west rainband over the Taiwan Strait, rather than the north-south topographic rainband at the windward slopes of the SCMR, that contributed to the additional slow-down of Morakot during its departure.…”