Measuring East Asian Summer Monsoon Rainfall Contributions by Different Weather Systems over Taiwan

Abstract

The east Asian summer monsoon (EASM) is characterized by a distinct life cycle consisting of the active, break, and revival monsoon phases. Different weather systems prevail in each phase following the change of large-scale flow regime. During the active phase, midlatitude cold, dry air moving equatorward into the tropics develops eastward-propagating fronts and rainstorms. The western ridge of the North Pacific subtropical anticyclone, which leads to the break phase, suppresses the development of synoptic-scale weather systems but enhances the diurnal heating. In the revival phase, the monsoon trough displaces the anticyclonic ridge northward and increases typhoon activity. This study examines quantitative measurements of climatological rainfall contributed by these weather systems that will help to validate simulations of the EASM climate system and facilitate water management by government agencies. To accomplish this goal, rain gauge measurements in Taiwan were analyzed. It was found that in the active phase (late spring?early summer), mei-yu rainstorms forming over northern Indochina and the South China Sea contribute one-half of the total rainfall, and cold-frontal passages account for about 15%. During the synoptically inactive break spell (midsummer), rainfall is produced mainly by diurnal convection (51%) along the western mountain slopes of the island. In the revival phase (late summer?early autumn), the impacts of diurnal convection and typhoons become comparable, with each accounting for about 40% of the total rainfall. The diurnal and typhoon contributions are separated by the north?south-oriented mountain range, with more than 50% of diurnal (typhoon) rainfall over the western (eastern) half of the island. Rainfall contributions from diurnal convection over the entire summer and mei-yu rainstorms in the active monsoon phase are also significant.