A Case Study Simulation of the Heavy Rainfall Event in Southwestern Taiwan during the Mei-Yu Front on May 22, 2020

Abstract

On May 22, 2020, Qiaotou Meteorological Station in Kaohsiung City of southern Taiwan recorded 415.5 mm of daily rainfall. This heavy rainfall event was driven by the Mei-yu front, a strong and moist southwesterly flow, a mesoscale convective system (MCS), and the complex terrain of southern Taiwan. The study indicates that on May 22, the low-level jet (LLJ) intensified to southwesterly flow strength, rapidly bringing abundant moisture to southwestern Taiwan, resulting in higher equivalent potential temperatures in the lower atmosphere. This study, through observational data analysis and case simulation, found that the first important cause of heavy rainfall in southwestern Taiwan is the presence of a strong southwesterly LLJ in an unstable atmospheric environment, which transports abundant moisture to the land in southwestern Taiwan. At the same time, the lifting mechanisms ahead of the Mei-yu front and low-level convergence over the ocean contribute to the intensification of the MCS. The second factor is Taiwan’s complex terrain effects, which functioned as a barrier to moisture transport and enhanced orographic lifting on the windward side, further intensifying the rainfall when combined with the strong southwesterly flow and MCS. Numerical simulations show that under the influence of complex terrain, mean precipitation over the southern section of Taiwan’s Central Mountain Range increases from north to south as elevation decreases, the average precipitation gradually decreases when the terrain is below 1100 m. For mountains between 2000 and 2500 m, maximum precipitation occurs on the windward slopes or in front of mountains, while rainfall and moisture sharply decrease on the leeward side. For elevations between 1000 and 1500 m, mean precipitation on the windward slopes increases, with the peak shifting closer to the mountain tops. Below 1000 m, mean precipitation decreases but aligns with the terrain, with the maximum near the peaks. At elevations below 600 m, leeward mean precipitation was above average.