本研究使用MM5（The Fifth-Generation PSU∕NCAR Mesoscale Model ）模式第3.7 版模擬2009 年8 月莫拉克颱風個案，探討台灣西南部地區極端降雨之原因。控制實驗模擬的72 小時累積降雨量在台灣中央山脈南部約2200mm，接近觀測但降雨分布稍微偏南。利用四維變分同化方法(4DVAR)，同化虛擬渦旋，加強颱風初始場結構。經過同化虛擬渦旋之結果顯示，不管是颱風強度、最大風速、路徑方面皆得到明顯改善，而降雨分布從原本台灣南部山區，移至台灣中部地區。為了進一步了解極端降雨機制，也分別對西南季風之水平風場增量、移除柯尼颱風以及降低西南季風之水氣含量做敏感度測試。由西南季風增量實驗得知，西南季風增量對台灣影響時間主要在8月8日，而西南季風強度對降雨量影響並不顯著，但對降雨位置有明顯差異，增強西南季風實驗降雨量分布在台灣中部山區；反之，減弱西南季風實驗降雨量仍維持在台灣南部地區。另一方面，當莫拉克颱風登陸台灣後，移動速度減慢，而台灣西南方位於海南島附近的柯尼颱風，正逐漸減弱為熱帶性低氣壓，所以移除海南島附近的柯尼颱風當作初始場，模擬結果發現台灣西南方的平均水平風速明顯減少，且台灣西南部山區8月8日最大累積降雨量減少約三分之一，表示柯尼颱風引導暖濕的西南氣流在台灣西南部與莫拉克外圍環流輻合，進而導致台灣地區極端降雨。最後，降低西南季風區域之水氣含量敏感度測試，當台灣西南方大尺度的水氣含量減少後，透過水氣場隨時間變化可知，西南區域水氣傳送至台灣西南部和莫拉克颱風外圍環流，進而導致減少台灣西南部地區之累積降雨，由此顯示西南氣流區域之水氣含量亦是造成莫拉克颱風劇烈降水的原因之一。This study used the PSU/NCAR MM5 Model to simulate Typhoon Morakot(2009), and investigated the mechanisms of the torrential rainfall in southwestTaiwan and the role of the southwest monsoon flow. The results indicated that themodel is able to capture the observed 72-h accumulated rainfall over 2000 mmafter applying bogus vortex data assimilation (BDA) based on 4DVAR that helpsto improve the initial intensity of the typhoon. The simulated track near and afterlandfall is close to the best track, resulting in a good agreement of accumulatedrainfall with the observations.In the sensitivity experiments for monsoon intensity, the 72-h accumulatedrainfall shows only some differences but have significant changes in rainfallpatterns with respect to the intensity of southwest monsoon. For stronger monsoon,the 72-h accumulated rainfall tends to be enhanced in central Taiwan, whileweaker monsoon induces more rainfall in south Taiwan. In the experiment with theinitial field where Goni vortex is deactivated by BDA, rainfall maximum isreduced by about one third on 8 August in southwest Taiwan. When the Gonicirculation and strong moisture-laden southwesterly flow converge to thesouthwest of Taiwan, it causes extreme rainfall in southwest Taiwan. On the otherhand, as the relative humidity of southwest flow is reduced, the rainfall intensityalso decreases. It appears that the water vapor transmission from southwest flow isalso an important factor for heavy rainfall in this event. Consequently, southwestmonsoon intensity, Goni Typhoon, and southwesterly moisture are integrated toproduce the tremendous rainfall in Taiwan.
