卡努颱風(2017)對台灣東部降雨的遠距影響-系集模擬與不確定性探討

Abstract

颱風所帶來的強降雨為台灣嚴重災害之一，而其又能分為直接與遠距效應。在遠距效應中，主要可歸因為由颱風環流與東北季風交互作用而生的季風mode，以及外圍環流受地形舉升的地形mode。2017年的卡努颱風 (Typhoon Khanun) 是個同時存在兩種降雨mode的個案，其中心平均與台灣距離500公里以上，強度僅達中颱下限，且未發布海上和陸上颱風警報，但卻在10月12日到15日在台灣東部降下豪雨。因此本研究使用Advanced Research WRF模式結合Ensemble Kalman Filter (EnKF) 資料同化系統，並利用台灣東北部海域之東北/東南風水氣通量配置來客觀區分季風mode與地形mode，對卡努颱風遠距降水事件中的兩個mode做不確定性分析與探討。 透過公正預兆得分檢驗定量降水模擬技術，發現無論是季風mode還是地形mode，降雨的模擬能力與路徑誤差在各累積雨量門檻中，皆沒有顯著關係。各系集成員的季風mode能根據降水極值位置分為東北 (NE)、東南 (SE) 及兩者 (ALL) 三類型，並發現鋒生和地形舉升是導致台灣東北部三種類型強降水的主要機制，而地形阻擋效應和颱風外圍環流之間的交互作用則導致台灣東南部強降雨。而地形mode部份則發現，颱風外圍環流與台灣地形間的入流角和降雨的累積頻率 (cumulative frequency) 有顯著關係，且颱風路徑的傾斜角度也與降雨累積頻率存在高度相關性。敏感性實驗也顯示，減少颱風初始場水氣使台灣山區平均累積雨量減少40%，而移除台灣地形不僅影響地形上的降水 (約減少90%)，雨區的分布以及強降雨位置也連帶受影響而改變。綜合上述，本研究主要有三個發現：1) 造成台灣東部強降雨事件的季風mode不僅歸因於颱風外圍環流與東北季風之間的共伴效應，地形舉升與水氣平流亦扮演重要的加持作用；2) 遠距降水事件的地形mode則與登陸的颱風有類似的強降雨型態，凸顯颱風外圍環流相較於山脈入流角度的重要性；3) 有多種因素 (如：水氣通量輻合位置、潛在降雨區的盛行風向、東北風的勢力範圍…等) 皆可能影響遠距降雨的不確定性，然而在卡努颱風的個案中，地形的舉升效應是最為主要的機制。

Indirect rainfall related to tropical cyclones (TCs) constitutes a major flooding hazard in Taiwan. Such events can be attributed to interaction between the northeasterly monsoon and TC circulation (hereafter monsoon mode), and topographic blocking and lifting effects associated with the Central Mountain Range (CMR), (hereafter topographic mode). Typhoon Khanun (2017) is a particular case where rainfall is forced by both of these factors. The objective of this study is to understand the key factors resulting in uncertainty in the TC-induced remote rainfall associated with the monsoon effects and topographic interaction. The Advanced Research Weather Research and Forecasting model based ensemble Kalman filter (EnKF) data assimilation system is used to conduct simulations of Typhoon Khanun. The time-varying northeasterly and/or southeasterly moisture fluxes near the northeastern Taiwan are used to attribute the rainfall event to either topography or monsoon interaction. The ensemble members related to the monsoon mode are classified into three types: northeast (NE), southeast (SE) and both (ALL) based on the geographic location of the precipitation maxima. The results demonstrate that frontogenesis and terrain-induced uplifting are the main mechanisms leading to the heavy precipitation in northeastern Taiwan for all three types, while the orographic blocking and the interaction between the TC circulation and the topography result in the heavy rainfall in southeastern Taiwan for ALL and SE types, respectively. For the topographic mode, high correlations are found between the inflow angle of the TC circulation and the cumulative frequency (CF) of the rainfall. Track direction is also shown to be closely related to the rainfall CF. Analyses from the sensitivity experiments with TC-related moisture reduced (MR) and the terrain of Taiwan removed (TR) are also conducted. In MR, the average of the 3-day accumulated rainfall is reduced by 40% over the mountainous area; however, the precipitation in TR is reduced by more than 90 %. In summary, this study presents three key findings: 1) heavy rainfall over eastern Taiwan associated with the monsoon mode in Khanun is not only attributed to the interaction between the northeasterly monsoon and Khanun’s outer circulation, but also the orographic lifting and moisture advection; 2) the remote rainfall associated with topographic mode is similar to that in landfalling TCs, which highlights the importance of the impinging flow field relative to the terrain/mountain geometry; and 3) in Khanun, multiple mechanisms (such as the location of the low-level moisture convergence, the prevailing wind direction) contribute to remote rainfall processes, particularly the orographic forcing.