颱風侵台期間路徑發生打轉之研究

Abstract

本研究使用中央氣象局歷史颱風資料，分析侵台時具打轉路徑的颱風之特徵，並使用WRF模擬侵台期間發生打轉的辛樂克颱風（2008），探討導致打轉現象的可能原因。分析時為幫助了解打轉颱風的重要特徵，亦針對和打轉個案具相似路徑但並未打轉的颱風進行分析。結果顯示，西北行侵襲北台灣之颱風，若其生命期中曾發展至強烈颱風且移動速度較慢時，侵台期間發生打轉路徑的可能性較高。當颱風登陸台灣的緯度較高時，打轉的位置傾向先貼著地形邊界往南偏移；但若在較低緯度登陸時，開始打轉的位置約在23.6°N離地形0.4°經度。另一方面，曾發展至強烈颱風的打轉個案在打轉前，台灣東南部測站大都會觀測到強焚風；而侵台期間未打轉的相似路徑颱風於登陸前，東南部各測站之焚風發生頻率及強度均遠小於打轉個案。 控制組模擬（CTRL）結果顯示，當颱風剛登陸時，近中心強風速區由東北側移到西北側，之後颱風逐漸往西南方偏移。低層渦度收支分析顯示，打轉期間輻散項和剩餘項對於提供渦度變率的正貢獻多於渦度平流項。分析CTRL（最大風速半徑為70 km）和對照組實驗結果顯示，TR-60（最大風速半徑為60 km，未打轉）之颱風登陸前，台灣東南部海面上達36 ℃的暖空氣區域，其範圍較TR-80（最大風速半徑為80 km，打轉）或CTRL者為小，顯示颱風侵台期間台灣東南部海域的乾熱區似與颱風小範圍打轉現象有所關聯。

Purpose of this study is to examine the occurrence of looping track for typhoons affecting Taiwan such as Supertyphoon Sinlaku（2008）. All typhoons which invaded northern Taiwan in 1990∼2009 and experienced looping tracks are examined. In addition, all typhoons with similar tracks but did not have looping tacks are also examined. Results show that a typhoon which moves northwestward to affect northern Taiwan tends to have higher possibility to take a looping track if its intensity is stronger and the moving speed is slower. The position of the looping path also is related to TYs’ landfall position. On the other hand, significant foehn phenomenon often occurs near southeastern Taiwan before the looping track of typhoon. High resolution（4 km）simulation is conducted using WRF（Weather Research and Forecasting Model）V3.1.1 to investigate the possible mechanisms leading to the looping motion of Supertyphoon Sinlaku which affected Taiwan in 2008. In the control experiment, the typhoon intensity, structure and track especially the landfall point and the looping track are well-simulated. When Sinlaku just making landfall, its stronger wind field shift from northeast to northwest. Such change in wind field structure tended to drift typhoon toward the southwest. A detail vorticity budget analysis（η = 0.88∼0.993）reveals that the divergence term and residual term were enhanced significantly. They are the major contribution to the local change of relative vorticity and cause typhoon to change its moving direction from northwestward to southwestward. At low level（η = 0.993）, a large area of warm and dry air which is caused by the adiabatic warming of the downslope flow associated with Sinlaku after crossing over the mountains is observed at the southeast of Taiwan.