北行颱風伴隨西南氣流之研究

A Study of Southwesterly Flow Associated with Northward-moving Typhoons

簡芳菁(Fang-Ching Chien) ； 楊筑方(Chu-Fang Yang)

《大氣科學》 37卷1期 (2009/03) Pp. 27-48

摘要

敏督利(Mindulle)颱風於2004年6月底至7月初侵襲台灣。在離開台灣後引進之強烈西南氣流爲台灣中南部地區帶來豪雨，驚人的降雨造成了「七二水災」，在全台各地皆有嚴重災情傳出。此次的豪雨主要是由颱風離台北行後，牽引來自南海的潮濕西南氣流所導致。本研究爲了解敏督利颱風伴隨如此旺盛西南氣流持續影響台灣地區之原因，除了分析敏督利颱風離台前後，垂直分層的水平風場、重力位高度場、相對濕度及地面降雨量的時序變化，並對1950至2005年通過台灣北部外海之北行颱風作分類，進行合成平均後，以對照分析的方式探討台灣西南方海域及南海北部存在顯著西南氣流造成豪雨的有利條件包括有：1)南海及東南亞上空在颱風離台前已蘊含大量潮濕水氣。2)風東側向西南方延伸出呈東北－西南向且密集分佈的等重力位高度線，其間之強西南風速帶隨颱風的北移而向北延伸。3)颱風強度需較強，離台後移動偏慢，且朝向西北方行進，與華南低壓槽結合，使得台灣至南海北部間之低層西至西南風增強。因爲這些有利條件，南海上空的潮濕水氣透過強勁西南風的持續吹送，在台灣西南方及南海北部海域上空產生強西南氣流，並將潮濕水氣傳送至台灣地區，形成豪雨。

關鍵字

敏督利颱風 ； 西南氣流 ； 豪雨

並列摘要

Typhoon Mindulle (2004) affected Taiwan in late June and early July 2004. After it made landfall and moved northward away from Taiwan, its associated strong southwesterly flow brought heavy rainfall over southern and central Taiwan, resulting in serious flooding and mudslides in many mountainous regions. This study intends to examine the environmental conditions that favor for intense southwesterly flow and heavy rainfall near southern Taiwan after a northward-moving typhoon passed northern Taiwan. Besides Mindulle (2004), we also study all typhoons that advanced with a track between northwestward and northeastward directions after passing northern Taiwan from 1950 to 2005. We used reanalysis data from National Centers for Environmental Prediction (NCEP) and separated the cases in two groups, one with strong southwesterly flow and precipitation (SW), and the other without (noSW). We took composite means for these two groups and compared their difference in terms of geopotential height, winds, relative humidity, and precipitation. The results show that the important ingredients for a northward-moving typhoon to induce strong southwesterly flow and heavy rainfall near Taiwan area include: the low atmosphere is moisture abundant over the northern South China Sea and the southeastern Asia, there are compact isobaric lines extending from the east of typhoon center toward the southwest when the typhoon moving northward, and the typhoon has to be more intense to a certain level and move slowly toward the northwest, and merge with the trough over southeastern China. Under these environmental conditions, the moisture is thus transported northeastward by the strong southwesterly flow, resulting in heavy rainfall in southern and central Taiwan.

並列關鍵字

Typhoon Mindulle ； Southwesterly flow ； Heavy rainfall