Impacts of cyclogenesis and moisture transport by the enhanced southwesterly monsoon flow on heavy rainfall over southern Taiwan during SCSTIMX (2018)

During the 13–20 June 2018 South China Sea Two-Island Monsoon Experiment (SCSTIMX), two tropical disturbances formed along western end of a Mei-Yu front over the northern South China Sea (SCS) between the cold northeasterlies (> 15 m s ^− 1 ) coming from the Taiwan Strait and warm, moist southwesterlies. Both disturbances exhibit mixed baroclinic characteristics and an asymmetric tropical cyclone (TC) structure.The first disturbance formed around 1800 UTC 13 June and moved northeastward along the Mei-Yu front, deepened, and became tropical storm Gaemi before making landfall over southern Taiwan in the early morning on 15 June. With the deepening of Gaemi, the southwesterly flow on the southern/southeastern flank of the storm strengthened (> 20 m s ^− 1 ). During 14–15 June, heavy orographic rainfall occurred over southern Taiwan with a maximum (> 550 mm) on the windward side of the Central Mountain Range (CMR).The second tropical disturbance over the northern SCS made landfall over southern China around 0600 UTC 18 June. During 19–20 June, the southwesterly monsoon flow strengthened (> 20 m s ^− 1 ) due to relatively large pressure gradients between the remnants of the second tropical disturbance and the West Pacific Subtropical High (WPSH). During 19–20 June, the horizontal moisture fluxes associated with the enhanced southwesterly monsoon flow mainly occurred below the 850-hPa level. On 19 June, the maximum rainfall axis (~ 300 mm) occurred on the southwestern windward slopes due to orographic blocking and lifting of the warm, moist south/southwesterly flow under favorable large-scale settings. On 20 June, with the development of the cold pool (~ 25°C) and offshore flow over southwestern Taiwan due to rain evaporative cooling, the maximum rainfall (~ 300 mm) occurred along the southwestern coast.