尼莎颱風（2022）之大台北地區遠距降水事件探討：雷達雙偏極參數分析、系集模擬與敏感性實驗

Abstract

熱帶氣旋（Tropical cyclones, TCs）相關的降水可以分為兩種類型：直接降水和遠距降水，前者主要是由TC內核主環流所主導，而後者則是透過TC外圍環流與環境流場（如冷鋒或季風）交互作用所產生。遠距降水對預報與防災更具挑戰性，其所帶來的經濟損失與社會衝擊亦經常被低估。2022年10月中旬，與尼莎颱風相關的遠距降水事件具有三個不同於過往TC遠距降水文獻的特徵。首先，降水熱區位於大台北都會區（Greater Taipei Area, GTA）而非宜蘭，且12小時累積雨量超過300毫米。再者，與大陸冷高壓相關的冷鋒鋒面在事件發生時尚未抵達北台灣。最後，當時綜觀系統顯示有一獨立於尼莎颱風環流的次大水氣輸送位於季風槽（Monsoon trough）邊緣，且輸送方向指向北台灣。 在本研究中，我們使用五分山雷達的雙偏極參數與徑向風場分析該事件降水的時空分佈與特徵；此外，也利用Advanced Research WRF模式進行系集模擬和敏感性實驗，前者探討在與前人研究不同的特徵下，GTA發生遠距降水的關鍵因素，後者旨在探討TC的壯度（Strength）對本次遠距降水事件的影響。依五分山雷達資料分析結果將當天早上與下午分別定義為雨帶時期（Rainband stage, R-stage）與輻合時期（Convergence stage, C-stage），經雙偏極變數和徑向風場分析後，顯示兩時期造成強降水的機制明顯不同，前期為颱風外圍雨帶，後期為GTA東側的輻合機制。針對C-stage輻合所致的強降水進行系集模擬，結果顯示，台灣北部附近的鋒生（Frontogenesis）為GTA出現強降水的關鍵，當台灣海峽低層的東北風和水氣梯度越強時，GTA的降水越多，同時在台灣北部以東的海域，季風槽邊緣處獨立於颱風環流的深厚水氣輸送亦為強降水發生的另一重要原因。最後我們以敏感性實驗探討TC壯度對北台灣附近鋒生強度的影響，結果發現當TC壯度越大時，季風槽邊緣的氣壓梯度越大，增強東南東風，更強的東南東風挾帶暖濕空氣在GTA遇到近地層東北風，造成更強的水氣通量輻合，進而增強舉生運動以及降水強度；此外，對流非絕熱過程增強的低層位渦（Potential vorticity）亦可能進一步加強鋒生，再次增強降水，形成正回饋作用。綜合研究結果顯示，即便北台灣附近的水氣通量並非直接來自颱風環流，颱風加強之季風槽邊緣東風分量仍可與台灣海峽東北風於北台灣造成劇烈遠距降水。

Remote precipitation associated with tropical cyclones (TCs) is one of the challenging weather events. Such events are caused by interactions between TC outer circulation and environmental factors such as cold frontal systems or monsoon flow. The remote precipitation event related to Typhoon Nesat (2022) is characterized by notable differences (e.g., rainfall hotspots, Siberian high and secondary moisture transport) from other cases previously discussed in literature. Based on the above unique features, the objective of this study is to investigate the mechanisms that lead to torrential and persistent rainfall in Greater Taipei Area (GTA). We use dual-polarization variables from the Wu Fan-San radar (RCWF) to analyze precipitation pattern and temporal distribution. Weather Research and Forecasting (WRF) model is also utilized in ensemble simulations and sensitivity experiments to explore the mechanisms contributing to heavy rainfall and the influence of TC strength on southeasterly flow in monsoon trough (MT). The RCWF analysis reveals significant differences in both dual-polarization variables and the wind field between the rainband stage (R-stage) and convergence stage (C-stage). Profiles of ZH, ZDR, KDP, ρhv ¬¬¬¬and hydrometeor identification demonstrate that convection during the R-stage was deeper and contained larger particles than the C-stage. However, high concentrations of small particles at lower-altitude and the enhanced convergence observed in the wind field analysis also resulted in significant precipitation during the C-stage. The WRF ensemble simulations show that the frontogenesis near northern Taiwan is crucial for determining whether GTA experiences heavy rainfall. The stronger the northeast winds and moisture gradient in the Taiwan Strait, the heavier the rainfall in the GTA. Over the ocean east of northern Taiwan, both the intensity and vertical extent of moisture transport along the edge of the MT are positively correlated with rainfall in the GTA. Meanwhile, the secondary moisture transport separate from the circulation of Nesat is a distinctive feature compared to previous studies. Understanding the frontogenesis which the warm and humid southeasterly flow converges with northeast winds is a crucial mechanism for lifting. In light of this, we conducted sensitivity experiments to investigate the impact of TC strength on the frontal structure. The results reveal TC with larger strength increases the pressure gradient along the edge of the MT, leading to the intensification of southeasterly flow. Stronger southeasterly flow encountering northeast winds in northern Taiwan results in more intense water vapor convergence, leading to more severe precipitation. Moreover, potential vorticity contributed by diabatic processes may also enhance frontogenesis.