卡玫基(Kalmaegi)颱風於2008 年7月16至19日間影響台灣，在登陸台灣期間於中南部地區降下超大豪雨，造成嚴重水災及山區土石流，颱風離台後並引進西南氣流而造成豪雨。本研究使用WRF 模式模擬卡玫基颱風個案，探討中部地區豪雨之成因，並針對颱風離台後，台灣西南方海域上空大氣環境與西南氣流造成之降雨進行敏感度分析。模擬結果顯示，卡玫基颱風影響台灣期間，中央山脈的地形效應在對流發展及降雨方面扮演了重要角色。於颱風登陸後，來自颱風低層相對較乾冷之西來氣流受地形阻擋轉為朝向北至東北運動，並與西北方移入之暖濕對流帶於台灣中部地區輻合。輻合之氣流受到地形抬升，低層水氣舉升凝結，同時與不斷自海峽移入之對流帶於陸地上合併，而使對流系統發展更為旺盛，並在中部山區降下超大豪雨。此外，卡玫基颱風於出海前移速減慢，造成對流系統於中部地區滯留，也是豪雨成災的原因之一。本研究也利用FDDA nudging方法進行模擬。其中，在南海北部上空分別同化增加濕度及西南風速之虛擬探空資料，可改善西南氣流模擬偏弱之情形，並進一步使台灣西南部地區模擬之降雨較接近於觀測。分析結果顯示，濕度與風場調整後，台灣西南外海至西南部地區上空之低層位溫提高，水氣量增加，並伴隨較強之西南風。此環境有利於西南氣流伴隨之對流系統在通過台灣西南外海時持續發展，並藉由強西南風之吹送，將潮濕水氣及對流傳送至台灣地區。其後，受到地形效應之作用，造成台灣西南部地區顯著之降雨情形。

Typhoon Kalmaegi (2008) affected Taiwan during 17-19 July 2008. When Kalmaegi was located overnorthern Taiwan, heavy rainfall occurred over southern and central Taiwan, resulting in serious flooding andlandslides in many mountainous regions. When Kalmaegi was leaving Taiwan, it induced southwesterly flowwhich again produced heavy rainfall over southern and central Taiwan. In this study, we used the WRF modelto simulate Typhoon Kalmaegi for discussing the heavy rainfall in central Taiwan and the sensitivity tests ofthe precipitation produced by the southwesterly flow.The results show that Taiwan’s Central Mountain Range (CMR) played an important role in enhancingthe convective systems. After the typhoon made landfall on Taiwan, the dry and cold westerly flow from thelow level of the southwest part of typhoon circulation converged with the warm and moist flow from thenorthern Taiwan Strait. When the convergent flow was lifted by the CMR, the convective systems wereenhanced, resulting in heavy rainfall. Besides, as Kalmaegi moved slowly over northern Taiwan, the growingconvective systems were nearly stationary over central Taiwan, resulting in prolonged heavy rainfall.Furthermore, the sensitivity studies show that the simulated rainfall associated with southwesterly flowover Taiwan can be improved by assimilating the sounding bogus data that have southwesterly winds andrelative humidity increased over the South China Sea. The assimilation of the sounding bogus data helps toproduce stronger southwesterly winds, higher potential temperature, and more moisture in the low atmosphereover the ocean southwest of Taiwan and over southern Taiwan. These are favorable ingredients for convectiondevelopment and moisture transport near the Taiwan area, resulting in intense southwesterly flow and heavyrainfall in southern and central Taiwan.