梅姬颱風（2010）轉向及對台灣降雨的遠距影響－系集模擬與不確定性探討

Abstract

當梅姬颱風（2010）通過呂宋島後，距離台灣仍有一段距離，颱風於南海地區由原本的向西移動突然往北轉向，受到颱風外圍環流與東北季風影響，台灣東北部以及其外海地區於10月19日到23日降下豪雨。由於過去許多研究顯示颱風降雨預報技術高度地受到路徑預報準確度之影響，本研究利用Advanced Research WRF模式結合ensemble Kalman filter（EnKF）資料同化技術及系集模擬，探討梅姬北轉之路徑不確定性，以及梅姬對於台灣地區所造成之遠距降雨效應。 片段位渦診斷以及駛流分析結果顯示，副熱帶高壓之東退，加上季風槽影響範圍往西縮減，前者位於颱風東北側，後者位於颱風西南側，兩者皆提供利於颱風北轉之綜觀環境。此外，由熱帶地區逐漸發展且往北延伸至颱風東側之反氣旋系統，也提供颱風顯著的南風駛流。颱風北轉後一天，中緯度槽線系統也開始扮演增加颱風往北分量之角色。造成系集成員路徑差異之因素來自於模擬副熱帶高壓分裂時機之不確定性、反氣旋系統和季風槽勢力範圍變化，以及中緯度槽線系統之強度差異等，使得颱風駛流有所差異。 藉由公正預兆得分檢驗定量降水模擬技術，發現梅姬颱風所伴隨的遠距強降雨事件中，定量降雨模擬之優劣與路徑平均誤差無顯著相關，並非如物理直覺一般簡單，重點是還需考慮東北風勢力範圍、颱風環流大小、潛在降雨區域盛行風向及該區山脈走向、坡度等影響。分析結果顯示，颱風外圍環流與東北季風之水氣通量輻合提供一個相對潮濕的潛在發生降雨環境，但並不是造成台灣東北角陸地區域產生強降雨之主因。盛行風將水氣從外海地區平流至宜蘭平原南側較陡峭坡度之山區，透過強烈的地形抬升作用，在迎風面近地表產生輻合並伴隨強烈的垂直運動，進而產生強降雨。地形移除實驗也顯示，海面上之降雨主要為水氣通量輻合作用所致，而宜蘭地區的降雨則必須透過地形抬升之動力過程而增強。 藉由系集模擬分析，本研究一方面探討造成颱風北轉之成因，同時透過系集模擬檢驗在颱風遠距影響下，造成台灣東北部降雨事件的不確定性以及強降雨產生機制。此研究探討了包括颱風、季風、地形以及彼此微妙之交互作用與可能的不確定性，未來可提供在東北季風盛行季，颱風所伴隨遠距降雨預報之參考。

When Typhoon Megi (2010) was still distant from Taiwan, located to the west of Luzon islands, it took a sudden track change from westward to northward over the South China Sea, leading to heavy rainfall in northeastern Taiwan and the adjacent seas during 0000 UTC 19-23 October. Since previous studies often highlight the accuracy of the Tropical Cyclone (TC) track as a key factor for accurate forecast of typhoon-related rainfall, the uncertainties of 1) the sudden recurvature of Megi, and 2) precipitation over Taiwan under Megi’s remote effect are both examined with ensemble simulations based on ensemble Kalman filter data assimilation system and the Advanced Research WRF (ARW) model in this study. Based on the piecewise PV diagnosis and steering flow analysis, our results show that the eastward retreat of subtropical high (SH) and the westward retreat of monsoon trough (MT), the former located to the northeast and the latter located to the southwest of Megi, provide a favorable synoptic environment for Megi’s sudden recurvature. In addition, an anticyclone (AC) developing over the tropical area, expanding northward to the east rear of Megi, also contributes northward steering flow. After Megi’s sudden poleward turn, the approaching mid-latitude upper-level trough (UTR) helps enhance Megi’s northward movement. The diversity of simulated track among different members is affected by the uncertainty in simulating the expanding areas of AC and MT, the timing of a break of the band-like subtropical high pressure system, and the strength of UTR, all of which can further lead to different steering flows. The quantitative evaluation of precipitation forecast based on the equitable threat score (ETS) shows that the TC track is not the dominant factor affecting the performance of rainfall forecast in this remote rainfall event. The uncertainty of remote rainfall simulation is affected by the area covered by the northeasterly monsoon, TC size, the direction of prevailing wind, mountain orientation, and the slopes of windward mountains. The results show that the low-level moisture convergence of northeasterly monsoon and the outer circulation of TC provides a moisture-abundant environment, which is favorable for the occurrence of rainfall. Then, it is the prevailing wind that further advects the moisture inland, and causes strong low-level convergence and vertical motion over the steep mountain areas at the south side of Yilan plain, where stronger orographic lifting results in heavy precipitation. Based on ensemble simulations, our study not only investigates the dominant factors leading to Megi’s sharp recurvature but also evaluates the uncertainty in TC’s remote effect on precipitation and associated detailed mechanism, which are relatively limited in other studies. Our results also provide new insight into the precipitation under the typhoon–monsoon–terrain interaction, and can be applied in assessment of numerical products for real-time forecast, especially for the remote rainfall events occurring in northeasterly monsoon season.