本文使用1948~2006年5、6月NCEP-reanalysis資料將西南氣流個案根據台灣西南海域之降水量分成有降水與無降水類型，利用合成分析法求取台灣西南方強西南氣流發生時各層的平均氣象場，討論兩類型綜觀天氣系統的差異；接著選定2008與2009年6月之強西南氣流有、無降水個案，利用WRF模式模擬進一步了解強西南氣流下有、無降水個案之中尺度現象，再與合成分析結果比較。合成分析顯示，西南氣流影響下之有降水類型中，台灣西南方低層具備暖濕水氣與高不穩定度之條件，且此區位於850hPa強西南風入區之北側，為潮濕並伴隨輻合上升運動之環境，利於發展對流並造成降水。反之，西南氣流影響下之無降水類型則由於太平洋高壓增強西伸，使梅雨鋒面雲系北抬，導致台灣西南方位於850hPa強西南風入區之南側，屬於乾燥下沉的環境，造成穩定的天氣型態。個案模擬結果可知，西南風增強且持續期間，華南有局部低壓，其氣旋式環流加強西南風風速；2008年有降水個案中，台灣西南海域及陸地低層之水氣由南至西南方而來，對流雲系不斷於海面上生成並移入台南和高雄，造成可觀地降水量，而對流雲系南方為強西南風區之北緣；反觀2009年無降水個案，台灣西南海域及陸地低層之空氣來自於南南東至南南西方，為太平洋高壓之下沉且乾燥的空氣，不利成雲且強西南風區的南緣位於台灣之西南方，其強風區之分佈相似於有降水個案後期因太平洋高壓西伸，導致強風軸北移，但無降水個案中之台灣西南方的風速未因而減弱，主要是受華南局部低壓強度較強且滯留較久之影響。

This study performs a composite study of southwesterly flow in the Taiwan area during 59 Mei-yu seasons using NCEP reanalysis data from 1948 to 2006. Strong southwesterly events were divided into two categories: one was associated with large rainfall southwest of Taiwan (the SW-R group), and the other was not (the SW-noR group). The composite analyses show that the SW-R group was associated with moist and unstable low-level flow and Taiwan was located at the rear-left side of the 850 hPa strong-wind region, which was in favor of convection development near the Taiwan area. On the other hand, in the SW-noR group the Pacific high extended westward and the major convective region moved northward, resulting in a stable condition near southern Taiwan. Two cases were chosen in this study for Weather Research and Forecasting (WRF) model simulations, in order to further investigate the mesoscale features associated with the two groups aforementioned. The 2008 case shows that moist air came from between the south and the southwest. Located in the leading edge of southwesterly flow, convective clouds developed and moved into southern Taiwan, resulting in heavy rainfall in Tainan and Kaoshung. As for the 2009 case, air flow in southern Taiwan originated from between the south-southwest and the south-southeast. Subsidence and dryness of the air was not favorable for cloud development. Similar to the SW-noR group, the leading edge of the strong southwesterly flow had moved to the north of Taiwan. The reason of the persistent strong southwesterly flow in southern Taiwan was related to the presentence of the low pressure system over the southeast mainland China.