西南共伴型颱風引發澎湖地區強降水事件之數值模擬研究

Numerical Study on the Heavy Rainfall over the Penghu Area Associated with the Co-Movement Effect between the Southwesterly Flow and the Typhoon Circulation

蔡明憲(Ming-Hsien Tsay) ； 汪建良(Jian-Liang Wang) ； 侯昭平(Jou-Ping Hou) ； 何台華(Tai-Hwa Hor)

《大氣科學》 42卷3期 (2014/12) Pp. 253-272

摘要

2008年卡玫基（Kalmaegi）颱風與西南氣流共伴在台灣中南部引發豪雨成災，更在澎湖創下了最高日雨量紀錄。本研究使用WRF（Weather Research and Forecasting Model）模式V3.1.1版,以數值模擬方式進行研究,探討卡玫基颱風影響台灣期間與西南氣流有何交互作用，在無地形舉升作用的澎湖地區如何產生強降水系統。中央氣象局雷達回波合成CV（column vector，垂直最大回波）圖顯示，澎湖地區發生強降水期間內，有一東西向強線狀回波於澎湖地區滯留長達6小時。模擬結果亦顯示，颱風中心出海時，西南氣流與颱風外圍環流在澎湖地區輻合，形成一條東－西走向的對流雨帶在澎湖地區造成強烈的上升氣流，並進而形成強降水。在卡玫基颱風中心持續向西北移動的同時，此雨帶並未立即向西北移動，而是逐漸轉成東北－西南走向；約6小時後，雨帶減弱，同時向北移動離開澎湖地區。以氣塊軌跡分析發現，不同高度的輻合氣流其來源有所不同，低層為颱風環流與西南氣流輻合；而高度3000 m以上，則是颱風環流與來自大陸上空之西風氣流輻合。颱風環流與西風氣流的熱力秉性不同，前者較暖濕、後者較冷乾，於是西風氣流扮演「障壁（barrier）」的角色迫使颱風環流舉升，而形成雨帶對流降水。此西風氣流在大陸上空時，風速便逐漸向下游增強，在到達輻合區強迫颱風環流舉升後，仍持續加速，但轉向沿台灣西側北上，風速最大達36 m s^(-1)以上。

關鍵字

卡玫基颱風 ； 西南氣流 ； 輻合 ； 強降水 ； WRF模式

並列摘要

Typhoon Kalmaegi (2008) not only caused heavy rainfall in the central and southern Taiwan, but also made a new record of maximum daily rainfall over the Penghu area. The Weather Research and Forecasting (WRF) model (version 3.1.1) with four nested domains was used to perform numerical experiments to investigate how the long-lasting heavy rainfall (more than 40 mm/hr for 5 hours) occurred over the Penghu area. It is found that the record-breaking daily rainfall was associated with a long-lasting rainband. Simulation results show that, as seen from radar observations, just after the center of Typhoon Kalmaegi passed by the Taiwan inland, the typhoon circulation and the southwesterly flow formed a confluent zone over the Penghu area, and thus maintained an east-west oriented convective rainband. While the center of typhoon kept moving northwestward, the rainband did not move away simultaneously. Instead, it gradually turned its direction into the northeast-southwest orientation. This rainband lasted about 6 hours before weakening and moving northwestward. Based upon the analysis of air parcel tracks, it is found that the sources of air flows resulting in the convergent zone were different at different heights. At low levels, the convergence was formed by the typhoon circulation and the southwesterly flow. Above 3000 m, the typhoon circulation and the westerly flow coming from Mainland China converged. As the air in the typhoon circulation was warmer and moister than the westerly flow, the high-speed southwesterly flow made a barrier to force the typhoon circulation to ascend, and then determined the formation of the rainband. The southwesterly flow accelerated downstream over Mainland China, and after reaching the convergent zone, it kept accelerating but turned northward along the west coast of Taiwan. Its maximum speed was above 36 m s^(-1). The mechanism of the evolution of this southwesterly flow needs further investigation.

並列關鍵字

Typhoon Kalmaegi ； Southwesterly flow ； Convergence ； Heavy rainfall ； WRF model