東亞颱風轉變為溫帶氣旋之個案診斷分析研究

A diagnostic study of extratropical transition over East Asia

劉志信(Chih-Shin Liu)

指導教授 ： 陳泰然

國立臺灣大學/理學院/大氣科學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.02750

摘要

本文利用非線性平衡流場架構下的位渦反演工具，探討颱風 Winnie登陸後的轉變過程、氣旋自我增強過程及所伴隨之對流層頂折疊過程。颱風Winnie 1997年8月18日由中國大陸浙江登陸後迅速減弱，隨後北移進入斜壓區，透過自我加強機制與中緯度系統交互作用，再增強 (地面中心氣壓在12小時內下降10 hPa)並轉變為一具有暖心結構之溫帶氣旋。
氣旋轉變由低對流層伴隨原熱帶氣旋的對稱結構，轉變為具有冷暖鋒的非對稱結構開始。伴隨氣旋轉變的雲系發展特徵，與Klein et al. (2000) 針對西北太平洋上颱風ET概念模式中的雲系發展特徵大致類似，但仍因陸地的影響而有些差異。此個案中冷鋒的發展機制與一般伴隨溫帶氣旋的氣團鋒面發展機制並不相同，顯然於冷空氣區的降水蒸發冷卻效應對於冷鋒鋒生，可能扮演相當重要角色。
氣旋登陸後，便與一高層噴流條的入區右側發生偶合，噴流條應有助於氣旋再發展前環流強度的維持。氣旋的再增強由暖鋒區發展劇烈的對流，在氣旋中心東北形成一個新的位渦中心開始，隨後對流不但對氣旋的發展扮演重要角色，也明顯影響氣旋的移動方向。當氣旋進入中層斜壓區，伴隨氣旋環流(主要來自潛熱釋放)的強氣旋式風場，在斜壓區形成明顯之冷暖平流，再加上潛熱釋放作用加強高層的脊場，使得高層槽脊系統振幅增大。隨後被加強的高層擾動與中低層系統達到適當的相位，首先透過垂直偶合過程增強低層氣旋，隨後再透過環流的疊加使低層氣旋達到最強。不同於許多以前其他關於ET的研究是類似Petterssen type B 的發展形式，強調高層槽在氣旋發展中的角色，本個案中氣旋的發展為一自我增強過程，即氣旋本身扮演了決定性的角色。
一般而言對流層頂折疊通常伴隨著高層鋒生，且發生在冷季的中高緯度。本個案中伴隨颱風 Winnie 的ET，出現了相當罕見的夏季中緯度對流層頂折疊現象。伴隨氣旋區中潛熱釋放效應增強的上升運動與伴隨高層槽的下沉運動，使對流層頂傾斜，伴隨潛熱釋放效應的氣旋位渦擾動風場產生的合流效應，增加了對流層頂的斜度，最後對流層頂折疊的機制則由伴隨氣旋的風場引發的垂直風切而來。顯然轉變中的氣旋和其伴隨的潛熱釋放效應，對於本個案對流層頂折疊現象，扮演相當重要角色。

關鍵字

氣旋變性 ； 自我增強 ； 垂直偶合 ； 環流疊加 ； 對流層頂折疊

並列摘要

The extratropical transition of Typhoon Winnie, the self-amplification process of the cyclone, and the tropopause fold accompanying the extratropical transition are diagnosed using potential vorticity inversion. Typhoon Winnie weakened rapidly, re-curved, and transformed into an extratropical cyclone after making landfall over mainland China on 18 August 1997. The cyclone transformed from a vertically deep system into a shallow one limited in the lower-to-middle troposphere. The initially symmetric structure associated with a tropical cyclone also changed into an asymmetric one with cold and warm fronts in the lower troposphere. Analyses suggest that the mechanism responsible for the development of this cold front was very likely evaporative cooling, which strengthened the cold tongue at the western section of the cyclone. Such a frontogenetic process is very different from that for an ordinary mid-latitude cold front associated with airmasses.
The cyclone coupled with the upper-level jet streak shortly after making landfall, and the jet streak seemed to play some role in maintaining the circulation strength of the cyclone. The re-intensification started with a burst of convection over the warm-front area. The latent heat was not only very crucial for cyclogenesis, but also affected the movement of the cyclone. The PV anomaly associated with the cyclone induced warm and cold advections at mid-levels when the cyclone migrated into a mid-level baroclinic zone. The dipole structure of the temperature advections, as well as a diabatic contribution through latent heating caused an amplification of the initially weak upper-level wave. With proper phasing with the upper-level system, the cyclone intensified first by vertical coupling and then by superposition. Many previous studies of ET suggested that the re-intensification process of the cyclone was similar to the Petterssen type B scenario, and the role of the cyclone was secondary. In contrast, the re-intensification of Winnie was a manifestation of self-amplification process and the role of the cyclone is primary.
Strong tropopause fold events are usually associated with upper-level frontogenesis during cold seasons. In this study, a rather rare strong mid-latitude tropopause fold event in summer was found to be associated with the extratropical transition of Typhoon Winnie. The vertical motions associated with the cyclone and the mid-latitude trough tilted the tropopause, the horizontal confluence caused by the cyclone further steepened the slope of tropopause, and finally the vertical wind shear induced by the cyclone produced a fold.

並列關鍵字

extratropical transition ； self-amplification process ； vertical coupling ； superposition ； tropopause fold

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