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Measuring East Asian Summer Monsoon Rainfall Contributions by Different Weather Systems Over Taiwan 2068 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47 NOTES AND CORRESPONDENCE Measuring East Asian Summer Monsoon Rainfall Contributions by Different Weather Systems over Taiwan SHIH-YU WANG AND TSING-CHANG CHEN* Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa (Manuscript received 2 July 2007, in final form 24 October 2007) 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 sub- tropical 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 anticy- clonic 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. 1. Introduction ery and the western tropical Pacific (WTP) monsoon trough along its southwest rim, composes the lower- Monsoon rainfall in eastern Asia—in particular, dur- tropospheric circulation of the EASM. Seasonal north- ing summer—is beneficial and affects nearly one-quar- ward progression of the EASM circulation results in ter of the world’s population. The east Asian summer sequential passages of the mei-yu rainband, the ridge of monsoon (EASM) is unique for its distinct active– the anticyclone, and the WTP monsoon trough. These break–revived life cycle of precipitation (Ramage 1952; periods cause a rapid increase in rainfall (active phase), Matsumoto 1989; Chen et al. 2004, hereinafter CWHY). a synoptically inactive dry spell (break phase), and an- The western North Pacific subtropical anticyclone, be- other period of large rainfall caused by the northwest- tween the mei-yu rainband along its northwest periph- ward propagation of tropical cyclones (revival phase), respectively (CWHY). The evolution of this monsoon life cycle is depicted in Fig. 1a by a latitude–time cross * Current affiliation: Visiting Scientist at National Central Uni- versity in Taiwan sponsored by a National Science Council Chair. section of the Climate Prediction Center Merged Analysis of Precipitation (CMAP; Xie and Arkin 1997) data between 120° and 125°E. Corresponding author address: Tsing-Chang (Mike) Chen, At- With the passage of each monsoon phase, a change in mospheric Science Program, Department of Geological and At- mospheric Sciences, Iowa State University, 3010 Agronomy Hall, the flow regime results in different weather systems Ames, IA 50011. affecting eastern Asia. During the active monsoon E-mail: [email protected] phase (also known as mei-yu or baiu), deep cumulus DOI: 10.1175/2007JAMC1821.1 © 2008 American Meteorological Society JAMC1821 JULY 2008 NOTES AND CORRESPONDENCE 2069 rainfall is closely related to frontal activity in the active phase and tropical disturbances in the revival phase but have not categorized rainfall contributions from all weather systems during the monsoon season. Quantita- tive measurement of the monsoon rainfall produced by different weather systems will facilitate water manage- ment and land-use planning by the local government. For example, new reservoirs should be constructed over areas where rainfall amounts are largest and yet least hazardous (Lin 2005). In addition, rainfall mea- surements will help in the understanding of the EASM climate system and will validate regional climate mod- els. Isolating the contributions of rainfall from different weather systems is generally complicated. Nevertheless, with proper criteria, numerous studies over different regions have shown that reasonable estimates of pre- cipitation amounts produced by different weather sys- tems can be obtained. For instance, it was found that mesoscale convective systems (MCSs) contribute one- quarter of the rainy season precipitation in Sahelian Africa (Adelekan 1998; Laing et al. 1999) and about 75% of the summer rainfall in the central United States (e.g., Changnon 2001; Fritsch et al. 1986) and Spain (Llasat and Puigcerver 1997). In Israel, previous studies have shown that cold fronts produce up to 45% of the cold-season rainfall while other cloud systems contrib- ute another 40% (Goldreich et al. 2004). In addition, Indian monsoon depressions have been found to bring FIG. 1. (a) Latitude–time cross section of pentad-mean CMAP in about one-half of the monsoon rain over India (Yoon (Xie and Arkin 1997) rainfall averaged over the longitudinal zone and Chen 2005), and satellite observations indicate that of 120°–125°E (adopted from Fig. 3a of CWHY). (b) Half-month tropical cyclones during the hurricane season contrib- rainfall averaged over the 1993–2005 period estimated with 427 ute 10%–20% of the total rainfall over the North Pa- rain gauges over Taiwan as shown in Fig. 2, below. The three EASM phases (active, break, and revival) are defined in (a) and (b). cific (Rodgers et al. 2000) and the North Atlantic (Rodgers et al. 2001) Oceans. Surprisingly little effort has been made to measure convection frequently occurs over the region from rainfall contributions by eastern Asian weather sys- northern Indochina and southwest China to the north- tems, although previous studies have analyzed the ern South China Sea and quickly develops into orga- physical nature of individual rainfall events in this re- nized convective storms. Mei-yu rainstorms (the defini- gion (e.g., Li et al. 1997; Johnson and Ciesielski 2002; tion is given in section 2, called rainstorms hereinafter) Wang 2004, and many others). Using a field experimen- frequently propagate eastward and often lead to disas- tal dataset during May–June 1987, Yeh and Chen trous floods in Vietnam, southern coastal China, and (1998) discussed rainfall characteristics associated with Taiwan. In the revival monsoon phase, frontal activity various weather events over Taiwan, but the analysis is generally restricted to higher latitudes and allows for was confined to a short season in an unusually weak west-northwestward-moving tropical cyclones to bring monsoon year (Krishnamurti et al. 1989). Therefore, heavy rainfall to eastern Asia (CWHY). In addition to their results were likely not climatologically represen- these synoptic disturbances, topographic heating in tative of the entire EASM period. Assessing rainfall Asia provides forcing for diurnally varying rainfall (e.g., contributions from different weather systems in eastern Chen et al. 1999; Krishnamurti and Kishtawal 2000; Asia is complicated because of inhomogeneous observ- Chen 2005). The variety of weather systems in eastern ing locations and highly variable terrain. However, Tai- Asia complicates the understanding of the monsoon wan, a subtropical island located between the eastern rainfall characteristics. CWHY noted that the EASM Asian continent and the western North Pacific, has a 2070 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 47 FIG. 2. Orography of Taiwan and distribution of 427 rain gauges administered by the Environ- mental Protection Administration (EPA; open circles), the Weather Bureau (asterisks), the Automatic Rainfall and Meteorological Telemetry System (ARMTS; filled circles), and the military (plus signs) in Taiwan. The scale of the orography is given in the lower-left corner. fine-resolution observational network with 427 rain half-month mean rainfall measured by this network gauges in an area of 36 000 km2. Administrated by sev- (Fig. 1b). Because the EASM produces, on average, eral governmental agencies, this rain gauge network over 1600 mm of rainfall during the warm season (May– consists of the Automatic Rainfall and Meteorological September) in Taiwan, it is a very important part of the Telemetry System (ARMTS) of more than 300 rain climate and inspires us to understand further the gauges (Chen et al. 1999), 25 conventional stations weather systems involved in its production. managed by the weather bureau of Taiwan, 72 stations The objectives of this study are to measure quantita- monitored by the Taiwan Environmental Protection tively the EASM rainfall produced by various weather Administration, and several military-based
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