非對稱潛熱效應對離台期間颱風運動之影響

Abstract

本論文以離台時期速度減慢的莫拉克(2009)、凡那比(2010)與海棠(2005)颱風個案進行研究，探討颱風不對稱降雨結構如何影響並減慢颱風移速。此三個被探討的颱風皆屬於中央氣象局分類中的西行颱風第2、3類(穿越台灣中北部地形)，衛星與雷達資料顯示，此三個颱風皆有明顯的不對稱降雨結構。以莫拉克颱風來說，離台期間觀測到的颱風移速(5 km h-1)比綜觀尺度資料推估的駛流速度(10 km h-1)慢了許多，顯示除了弱駛流因素外，亦存在其它使颱風減速的機制。由於離陸期間的颱風移速相較先前時段來得緩慢，我們提出研究假說：受西南季風氣流與台灣地形影響而於東南側產生的颱風不對稱對流結構，可以產生波數1的位渦傾向，進而減慢颱風向西北方運動之速度。 我們利用雲解析風暴模式(CReSS)，並搭配歐洲中長期天氣預報中心熱帶對流年計畫(ECMWF-YOTC)或美國國家環境預報中心(NCEP)提供的分析資料，針對此三個颱風進行模擬。首先，三個颱風都有不錯的雨量與路徑模擬結果，模式能掌握颱風離台時的減速情形及不對稱降雨結構，與觀測資料相符。接著，利用位渦傾向診斷分析，確認不對稱降雨結構是影響颱風運動的因素之一；控制實驗的非絕熱項向量指向颱風中心東南側(運動方向後側)，這個因波數1位渦傾向所產生之與颱風運動反向的作用力，會抵銷部分環境駛流的牽引，導致離台期間向西北方運動的颱風移速減慢。此外，透過水氣含量的敏感度測試，亦證實不對稱對流中潛熱效應的強弱對颱風移速之影響性；將颱風減速期間控制實驗與水氣含量減少實驗的平均運動向量相減，所得向量差指向颱風運動方向後側，顯示離台期間的水氣含量越少，潛熱效應越不顯著，以致颱風移速越快，減速現象越不易發生。以上結果在三個颱風個案中皆得到相同結論。 本研究顯示，因西南季風氣流或台灣地形所導致的颱風不對稱降雨結構，尤其是位於颱風中心東南側、因台灣地形而產生的強降雨區，由於強潛熱效應所產生的正位渦傾向，使整體具有明顯的波數1位渦傾向，颱風受此波數1位渦傾向的影響後，將產生一指向東南方的運動分量，進而減慢颱風離台期間往西北方向的移行速度，從而產生更多降雨。由此可知，颱風的運動與潛熱釋放之間存在非線性與複雜的交互作用。

This thesis studies the slowdown of typhoon motion by the asymmetric typhoon (TY) structure for typhoons classified as the tracks 2 and 3 by Central Weather Bureau (CWB). Specifically, we study the post-landfall TY motion slowdown of Morakot (2009), Fanapi (2010), and Haitang (2005). Satellite and radar data indicate that these TYs are with significant asymmetric convection structure in the post-landfall period. In particular, the post-landfall speed of TY Morakot is 5 km h-1, which is much slower than the environmental deep-level mean flow speed of 10 km h-1. Since the post-landfall speed of these TYs are slower than that before, our hypothesis is that the asymmetric convection structure to the southeast of the TY, induced by the interaction of TC circulation with topography of Taiwan or southwesterly monsoon flow, may generate wavenumber 1 (WN1) potential vorticity (PV) tendency to slowdown the northwestward moving storm. We use Cloud-Resolving Storm Simulator (CReSS) with analysis data for the Year of Tropical Convection from European Centre for Medium-Range Weather Forecasts (ECMWF-YOTC) or from National Center for Environmental Prediction (NCEP) to simulate the three TYs. The CReSS model produces rainfall and track simulations that are in general agreement with observations. With PV tendency diagnosis, we investigate the impact of convection and latent heating effect on the storm motion slowdown for the three TY cases. The diagnosis suggests that the asymmetric convection indeed contributes to the WN1 PV tendency and leads to the slowdown of TY motion. In addition, we perform the water vapor sensitivity experiments to substantiate the importance of the water vapor supply in the asymmetric convection and latent heating. By reducing water vapor content in the period immediately after TY’s departure from Taiwan, the model experiments with less water vapor are with faster TY motion. The PV diagnosis of the water vapor reduction experiments indicates the decrease of the latent heating effect and the WN1 PV tendency leads to the increase of TY motion. Our research highlights the importance of the asymmetric convection structure on the TY motion in the post-landfall period. The asymmetric convection in general may be caused by the interaction of TY circulation with Taiwan topography or southwesterly monsoon flow. The PV diagnosis is effective in identifying the contributing factors for the TY motion.