西南季風對莫拉克颱風降雨模擬之影響

Rainfall Simulation Associated with Typhoon Morakot (2009)

黃清勇(Ching-Yuang Huang) ； 趙子瑩(Tzu-Ying Chao)

《大氣科學》 41卷2期 (2013/12) Pp. 91-115

摘要

本研究使用MM5（The Fifth-Ge nerati on PSU/NCAR Mes oscale Model）模式第3.7版模擬2009年8月莫拉克（Morakot）颱風個案，探討台灣西南部地區極端降雨如何受到西南季風強度及濕度變化之影響。為強化初始場的颱風結構，本文利用四維變分同化方法（4DVAR）同化虛擬渦旋。同化虛擬渦旋實驗的模擬結果顯示，不管颱風路徑及72小時累積降雨量（超過2200 mm），與觀測均相當一致。為了進一步了解此極端降雨與季風變化的關係，本文分別進行移除柯尼熱帶氣旋、改變西南季風強度以及水氣量等敏感度實驗。西南季風強度敏感度實驗顯示，72小時累積降雨量及降雨分布受到西南季風強度影響，增強西南季風為原來的1.5倍，使台灣中南部山區及西側增加降雨量（約可達500 mm）；減弱西南季風為原來的0.5倍，則台灣中央山脈玉山南側及西南平地普遍減少降雨量（約可達600 mm）。另一方面，在海南島附近的柯尼（Goni）熱帶風暴以較弱之虛擬渦漩代替，台灣西南方海上的風速明顯減少，使中央山脈最大降雨量減少約三分之一（約600 mm），此結果與吳等人（2010，大氣科學）一致。柯尼熱帶氣旋可增強暖濕的西南氣流，在台灣西南方與莫拉克外圍環流輻合增強，進一步強化台灣南部地區極端降雨。降低西南季風區域之水氣量敏感度實驗顯示，將同化虛擬渦旋的實驗之西南季風區域的水氣量分別減少為原來的0.75倍、0.5倍、0.25倍，導致傳送至台灣西南部的水氣量及濕度減少，亦使台灣南部地區之降雨減少，72小時累積降雨量分別減少約為40%（可達920 mm）、43%（可達1010 mm）、44%（可達1020 mm）。

關鍵字

西南季風 ； 莫拉克颱風

並列摘要

This study used the PSU/NCAR MM5 model to simulate Typhoon Morakot (2009), and investigated the mechanisms of the torrential rainfall in southwest Taiwan and the role of the southwest monsoon flow. In this study, bogus vortex data assimilation (BDA)based on 4DVARhas been employed to help improve the initial structure of the typhoon. The simulated track and 72-h accumulated rainfall (over 2200 mm) are in good agreement with the observations.In the sensitivity experiments for monsoon intensity, the 72-h accumulated rainfall in Taiwan is sensitive to the intensity of southwest monsoon. For stronger monsoon, the 72-h accumulated rainfall tends to be enhanced (by about 500 mm) west of and in central and southern Taiwan, while weaker monsoon induces more rainfall (about 600 mm) south of central CMR and in the southwest plains. In the experiment with the initial field where Goni tropical storm is deactivated by BDA, the rainfall maximum is reduced by about one third in southwest Taiwan. As the Goni circulation and strong moisture-laden southwesterly flow converge to the southwest of Taiwan, it causes enhancement of heavy rainfall in southwest Taiwan. On the other hand, as the relative humidity of southwest flow is reduced, the rainfall intensity also decreases, indicating that transport of moister water vapor from the southwesterly flow also contributes to the production of heavy rainfall.

並列關鍵字

MM5 ； Southwest Monsoon ； Typhoon Morakot