A Simulation Study of Severe Afternoon Precipitation Events in Southern Taiwan During the Mei-Yu Front Period

On May 22, 2020, multiple areas in the western region of Taiwan, located in Southeast Asia, experienced intense rainfall, leading to widespread flooding and localized landslides. Observational data analysis indicate that Taiwan was influenced that day by the Mei-yu front, vigorous southwesterly winds, dynamic structures at both upper and lower atmospheric levels, and mesoscale vortices. These factors contributed to the development of two distinct mesoscale convective systems (MCS). After MCS moved into Taiwan from the west-northwest to the east-southeast, it brought severe rainfall to many areas of western Taiwan's plains and mountainous regions. In particular, the Qiaotou city in southern Taiwan recorded an astonishing daily rainfall of 416 mm, breaking the historical record for that weather station. This study uses the WRF model to simulate the area surrounding Taiwan. The results show that the terrain effects in southern Taiwan, the dynamic mechanisms of the environmental field, and the strong southwesterly flow transporting abundant moisture through a low-level jet to the southwestern region of southern Taiwan all influenced the intensity of the MCS.  The emergence of a mesoscale vortex led to the transport of moisture by the southwesterly flow on the southern side of the vortex. This moisture was lifted along the leading edge of the Mei-Yu front's low-level cold zone in northern Taiwan, enhancing convective activity. The intensified convection released latent heat through moisture condensation, thereby heating the atmosphere. As a result, the mesoscale vortex strengthened, allowing the southwesterly flow to carry even more moisture to the southwestern region of Taiwan. The terrain in southern Taiwan gradually descends from mountains over 2,000 meters high to complex terrain below 600 meters, playing multiple roles that favor precipitation, such as lifting and blocking moisture. These factors ultimately led to the intense afternoon rainfall event in southern Taiwan.