伴隨高層冷心低壓之熱帶氣旋形成之分析

Abstract

本研究藉由分析伴隨與未伴隨高層冷心低壓（Cold Core Low, CCL）之熱帶氣旋形成個案，比較其環境特徵及變化情形之差異，並且針對CCL有利熱帶氣旋之位置個案，選擇2001年Man-Yi與2003年Dujuan作進一步分析，以探討CCL在熱帶氣旋形成過程中，可能扮演的角色。 本研究分析2000至2004年6至9月美國聯合颱風警報中心（JTWC）發佈之西北太平洋熱帶氣旋，挑選21個形成期間伴隨CCL個案與31個形成期間未伴隨CCL個案，分析其環境參數及熱帶氣旋之四個象限對流與深對流變化情形。結果顯示，伴隨與未伴隨個案形成前一天之環境參數與對流變化明顯不同；相較於未伴隨個案，伴隨個案形成前一天，5°半徑內之200 hPa散度、850 hPa渦度及西北方對流增加相當顯著，平均高於未伴隨個案兩倍以上。 進一步分析伴隨個案顯示， CCL多半伴隨於熱帶氣旋之東北與西北象限，少部份個案伴隨期間跨越至其西南象限。當CCL位於熱帶氣旋東北象限時，CCL相對熱帶氣旋距離與系統低層渦度值變化具相關性，即熱帶氣旋低層渦度隨CCL移近而增加，推測CCL位於熱帶氣旋東北方，為利於熱帶氣旋發展之位置。 由CCL位於東北象限個案中，挑選Man-Yi與Dujuan作進一步分析，結果顯示，兩個案伴隨CCL前後低層環流皆不明顯，且其綜觀環境流場變化也不大；雲圖顯示，兩個案形成階段中心雲簇並未發展，至伴隨CCL後，其周圍消散之雲系受CCL影響往東北方移動，而中心西北方新生之對流發展成為熱帶氣旋。由於CCL對於熱帶氣旋形成之影響可能較複雜，且影響熱帶氣旋形成的因素是多重的，故分析伴隨與未伴隨個案特徵時，雖然顯示兩類型個案明顯不同，但進一步分析伴隨個案，則較難從各參數變化量上推測其影響之物理過程。

The role of the upper level cold core low (CCL) played in the formation of a tropical cyclone is analyzed based on the characteristic of environment. To this end, 21 named Western North Pacific tropical cyclones form with CCL from June to September during 2000-2004 and 31 those form without CCL are analyzed. The results obtainded reveal that the 850 hPa vorticity, the 200 hPa divergence, and the (deep) convection area in the northwestern quadrant of the tropical cyclones form with CCL increase at least two times as those of the tropical cyclones form without CCL. CCL is found to be either in the northeastern or northwestern quadrant of most of the tropical cyclones form with CCL. However, some of these cyclones move to the southwestern quadrant. If CCL is in the northeastern quadrant of a tropical cyclone, its 850 hPa vorticity increases with decreasing separation distance between the CCL and the tropical cyclone. This implies that the presece of CCL in the northeastern quadrant of a tropical cyclone is advantageous to the development of the tropical disturbance. Among the Typhoons with their CCLs positioned in this quadrant, Typhoon Man-Yi (2001) and Typhoon Dujuan (2003) are examined in detail. The low level circulations of these Typhoons are unclear. The cloud images of those Typhoons reveal that the cloud clusters of tropical disterbance dose not develop until a CCL is present in the northeastern quadrant. The cloud clusters are driven by the southwesterly of CCL toward the northeast. At the same time, a new cloud cluster is developed in the northwestern quadrant. In general, the influence of CCL on the formation of tropical cyclone is of complicatd nature, the data gathered so far are insufficient for the interpretation of the mechanism of tropical cyclone formation.