The Remote Effect of Typhoon Megi (2010) on the Heavy Rainfall over Northeastern Taiwan

Abstract

The goal of this work is to improve understanding of the mechanisms leading to a heavy rainfall event under the combined influences of the outer circulation of Typhoon Megi (2010), the Asian monsoon, and the topography of Taiwan. Megi is a case featuring high forecast uncertainty associated with its sudden recurvature, along with remote heavy rainfall over northeastern Taiwan (especially at Yilan) and its adjacent seas during 19–23 October 2010. An ensemble simulation is conducted, and characteristic ensemble members are separated into subgroups based on either track accuracy or rainfall forecast skill. Comparisons between different subgroups are made to investigate favorable processes for precipitation and how the differences between these subgroups affect the rainfall simulation. Several mechanisms leading to this remote rainfall event are shown. The northward transport of water vapor by Megi’s outer circulation provides a moisture-laden environment over the coastal area of eastern Taiwan. Meanwhile, the outer circulation of Megi (with high [Formula: see text]) encounters the northeasterly monsoon (with low [Formula: see text]), and strong vertical motion is triggered through isentropic lifting in association with low-level frontogenesis over the ocean northeast of Yilan. Most importantly, the northeasterly flow advects the moisture inland to the steep mountains in south-southwestern Yilan, where strong orographic lifting further induces torrential rainfall. In addition, the analyses further attribute the uncertainty in simulating Megi’s remote rainfall to several factors, including variations of storm track, strength and extension of the northeasterly monsoon, and, above all, the impinging angle of the upstream flow on the topography.