夏季時，西行台灣北部的颱風偶會受到西南氣流影響，將南海海域暖濕水氣帶至台灣西南方。由於台灣山脈高聳，水氣經地形抬升作用凝結成雨，導致西行颱風離開時於中南部地區造成嚴重豪雨災情。在颱風監測上，中央氣象局將颱風歸納 9 種主要行進路徑，其中 2 號路徑颱風最容易引入西南氣流，進而在台灣南部輻合水氣而產生豪大雨；另外 1 號路徑行進路線颱風雖然沒有登陸台灣，但與 2 號路徑相似，其外圍環流可能受南海地區季風作用，而有機會發展成西南氣流共伴現象。 本篇研究探討西行颱風與環境場相互作用的現象。主要利用 NCEP FNL 再分析資料和 GSMaP 資料分析 2000 至 2014 年間行進於 1 號與 2 號路徑共 13 個颱風個案。分析結果顯示：西南氣流共伴的西行颱風登陸台灣東海岸前，南海海域的風場大多是西風或西南風，且太平洋高壓較為西伸，於颱風東側呈東北─西南走向的等重力位高度線，西行颱風經過常會加強南海海域的西南風，若此時水氣較高，則隨西南氣流北上至台灣西南方輻合，此為颱風與南海季風交互作用形成降雨的機制。 本研究亦測試改良式颱風降雨潛勢預報模式 (I-TRaP) 對前述 13 個 1、2 號路徑颱風個案預報降雨分布的潛力，結果發現：在台灣西南部之降雨有低估的情形，此因I-TRaP只計算颱風在台灣區域累計的降雨量，並未考慮季風與環境場的加乘影響。所以在預報降雨潛勢時，需再考慮環境場及太平洋高壓的西伸情形，並將南海海域的風向、風速及水氣納入計算。 ;Typhoons are one of the most disastrous weather systems and frequently bring strong winds and heavy rains to Taiwan in summers. And some typhoons, which tracks in Type-I and Type-II track categories, passed westward around northern Taiwan region often cause the southwesterly flow from South China Sea (SCS) into Taiwan. Then the water vapor through the terrain uplifting, causing heavy rains and severe disasters in central and southern Taiwan regions. In this study, the aim is to investigate the southwesterly flow effect on typhoon rainfall patterns. Mainly the NCEP FNL Analysis data and GSMAP data are used to analyze Type-I and Type-II typhoon cases during 2000-2014. The result shows that the SCS monsoons modulate the relationship with typhoons and southwesterly flow. For instance, the active monsoons provide evidently favorable conditions for prolonging heavy rainfall while the typhoons passed. There are compact isobaric lines extending from the east of typhoon centers toward the southwest when the typhoons to the northward, and enhance the southwesterly flow in SCS. Under such environmental conditions, rich moisture is thus transported northeastward by the strong southwesterly flow, resulting in heavy rainfall in southern and central Taiwan. The study also delineates the rainfall distributions with the Improved TRaP (I-TRaP) when Type-I and Type-II typhoons around Taiwan. By comparing with gauge rainfall, the patterns show that I-TRaP underestimates rainfall amounts in southern and central Taiwan. Therefore, the quantitative precipitation forecast (QPF) models must further consider that the westward extension of the Pacific high pressures, wind fields, and water vapors in South China Sea to obtain more accurate rainfall potential over Taiwan.
