中層環流對熱帶氣旋形成影響之探討 －以桔梗颱風（2013）為例

Abstract

桔梗颱風（2013）為少數於臺灣鄰近地區形成的颱風，其形成過程與康芮颱風（2013）殘餘之中層氣旋式環流有關。康芮颱風於2013年8月28日向北移動至臺灣東部近海時，受強烈垂直風切影響，其低層與中層環流出現分離現象；低層環流持續往北移動，而中層環流則向西北移動通過中央山脈。此中層環流移至臺灣海峽北部後，於該處滯留了一至二天；之後桔梗颱風初始渦旋之低層環流在此地區形成。本研究之目的在探討上述中層環流對桔梗颱風形成過程之影響，及其中之重要機制。 本研究採用片段位渦反演方法，診斷中層氣旋式環流在準平衡條件下對低層風場的貢獻，並進行移除不同倍率中層環流之數值敏感度實驗。片段位渦反演結果顯示，在桔梗颱風初始渦旋之低層環流形成前，中層正位渦距平對低層正渦度有最大的貢獻。移除部分中層環流的敏感度實驗結果顯示，隨著移除倍率的增加，模擬的低層擾動強度有系統性減弱的現象，且伴隨中層環流區域的對流強度亦有減弱並延遲發生的現象。而在完全移除中層環流之模擬實驗中，皆無顯著對流系統發展，因此無低層擾動形成。綜合分析結果顯示，中層渦旋能提供適合對流發展的環境，促使桔梗颱風形成；因此，中層環流在桔梗颱風的形成過程中，扮演關鍵角色。

Tropical Storm Toraji (2013) is a rare case that formed near Taiwan and is involved with a mid-level cyclonic circulation which is a remnant of Tropical Storm Kong-Rey (2013). When Kong-Rey (2013) was moving northward to the offshore of the eastern Taiwan on Aug 28, 2013, its low- and mid-level cyclonic circulation decoupled due to the strong vertical wind shear. The low-level circulation kept moving northward, while the mid-level circulation moved west-northward and across the central mountain range of Taiwan. The mid-level circulation moved to the northern Taiwan Strait and had become stationary for 1 to 2 days. After then, the incipient vortex of Tropical Storm Toraji formed in the region. The major purpose of this study is to disuss the influence of the mid-level circulation on the formation of Tropical Strom Toraji and the mechanism. The piecewise potential vorticity inversion (PPVI) method was applied to figure out the contribution of the mid-level cyclonic circulation to the low-level wind field under quasi-balanced condition. In addition, sensitivity numerical experiments designed to remove the mid-level circulation by different removing factors had also been conducted. The results of PPVI showed that the mid-level positive PV anomalies had the most contribution to the low-level positive vorticity before the formation of the low-level circulation of Toraji’s incipient vortex. Besides, the sensitivity experiments showed that when the removing factor was increased, there was a declining trend of the intensity of low-level perturbation, and the intensity of convection systems in the region of the mid-level circulation also became weaker and developed late. Moreover, the simulations whose the mid-level circulation was totally removed showed that no significant convection system developed, so that no low-level perturbation formed. The analyses showed that the mid-level circulation provided a favorable environment for convection and led to the formatio of Toraji. As a result, the mid-level circulation played a critical role in the formation of Toraji.