Mechanisms of a Record-Breaking Extreme Mei-Yu Rainfall Event in Taiwan: Synoptic-Scale and Mesoscale Forcings

Abstract

This study investigates the dynamical mechanisms responsible for a historically extreme mei-yu rainfall event over Taiwan during 8–10 June 2006. The event produced the highest Taiwan-averaged 6-h precipitation intensity and 72-h accumulated rainfall within the 44-year mei-yu climatology (1979–2022), with six consecutive 6-h periods exceeding the 99th percentile. The maximum 3-day accumulated rainfall surpassed 1500 mm over southern Taiwan, highlighting its extreme nature. Climatological analyses indicate that this event occurred under anomalously strong low-level dynamical conditions rather than extreme moisture availability. A markedly intensified western North Pacific subtropical high and a southward-extending midlatitude low-pressure system established exceptionally strong and persistent southwesterly flow, generating extreme low-level convergence and frontal vorticity over the Taiwan Strait. Ensemble simulations reproduced the observed dual-peaked rainfall structure and its temporal evolution. Vorticity budget diagnostics reveal that the first rainfall peak was associated with frontal stagnation caused by a near balance between negative tilting contributions and positive divergence and vertical advection terms. During the second peak, the eastward intrusion of a mesoscale convective vortex from South China restructured the weakening mei-yu front by enhancing low-level convergence and increasing the vertical vorticity gradient, thereby amplifying vertical advection and sustaining slow frontal migration. The resulting broadened and northward-tilted frontal structure expanded the spatial extent and prolonged the duration of heavy rainfall. These results demonstrate that interactions between extreme synoptic-scale circulations and mesoscale vortex–front dynamics can substantially amplify mei-yu rainfall extremes, underscoring the importance of monitoring upstream mesoscale convective vortices for improving heavy rain forecasts over Taiwan.