A Comparison Study of Mei-Yu Front Structure and Its Impacts on Extreme Precipitation over Taiwan

The Mei-Yu front is an annual stationary boundary that can produce large amounts of rain and damaging floods in Eastern Asia. This boundary forms in late spring and early summer when the winter northeast monsoon weakens and the summer southwest monsoon strengthens; neither monsoon can displace each other thus, they create a stationary boundary, the Mei-Yu front. Two recent Mei-Yu front events produced significant rainfall: approximately 600 mm was observed in Taipei on June 2, 2017 and roughly 300 mm was observed in Nantou County on June 11, 2019. We used the Weather Research and Forecasting (WRF) model to simulate each case and compare the structure and large-scale environment associated with each front. Available radar reflectivity data from June 2017 show that the simulation of the 2017 event does a reasonable job of representing the overall structure and location of the front. The results show that in 2017, high θe values and strong winds at lower levels produced strong moisture advection over Taiwan. Frontal convergence as well as convergence over the island provided lift which supported the moisture transport. In the 2019 case, the θe gradient and wind speeds were weaker. The frontal convergence was roughly the same magnitude; however, the strongest convergence was located just off the northern shore of Taiwan which kept the heaviest rainfall offshore. We conclude that the 2017 case had a stronger and more defined frontal structure than the 2019 case. The placement of non-frontal convergence, higher availability of moisture along the front, and strength of low-level winds contributed to the high rainfall amount in the 2017 case.