哈隆颱風(2014)通過琉球海域期間臺灣西南部沿海豪雨事件之研究

Abstract

哈隆颱風(2014)於日本南方海域北上期間，颱風南側伴隨的西南氣流使臺灣西南部沿海地區連日出現豪雨，並造成淹水、海水倒灌等災情。由於西南氣流影響下強降雨集中於沿海的個案較少，且強降雨發生期間哈隆颱風距臺仍有1000公里，與過去討論之伴隨旺盛西南氣流的侵臺颱風環境不同，因此本研究之目標在分析導致臺灣西南部沿海地區強降雨的原因，並探討颱風與臺灣地形在此強降雨事件中所扮演的角色。 分析行經日本南方海域、且臺灣南部發生強降雨事件的颱風個案之綜觀環境顯示，此類個案皆於南海北部伴隨較強且較為潮濕的西南風。分析WRF模擬結果與觀測資料顯示，哈隆颱風通過日本南方海域期間，臺灣南部外海的西南風逐漸增強，並分別與近地面之離岸風、受地形阻擋而產生的繞流輻合產生上升運動，導致平行海岸線的對流發生。而移除颱風後之敏感度實驗顯示，受大範圍低壓帶影響，移除颱風後西南風仍供應大量水氣，臺灣西南部沿海地區之環境仍有利對流發展，但對流發展的位置因流場改變而有所不同。另外，改變臺灣地形高度的模擬實驗結果則顯示，地形高度越高，盛行風受地形阻擋而偏折的效應越大，此可能亦有利於底層輻合之增加。綜合模擬與分析結果顯示，哈隆颱風的出現使強盛之西南氣流向臺灣南方海域接近，潮濕西南氣流與離岸風的輻合，導致對流於臺灣西南部沿海地區發展並造成強降雨；哈隆颱風雖不是造成強降雨的主要原因，但卻影響對流出現之區域，而臺灣地形對西南氣流的阻擋作用，在此降雨事件上亦似扮演一定之角色。

A heavy rainfall event occurred over the southwestern coast of Taiwan when Typhoon Halong (2014) was located at the sea of southern Japan. While most heavy rainfall events due to southwesterly flow caused maximum precipitation on windward side of mountain ridge, heavy rainfall occurred along the coast of southwestern Taiwan and caused some damages. The purpose of the study is to analyze the synoptic environment and triggering mechanisms of the rainfall event. Humid and strong southwesterlies in the northern South China Sea accompanies typhoon at the sea of southern Japan with heavy rainfall event in southern Taiwan. Three major numerical simulations using the Advanced Research Weather Research and Forecasting model (ARW-WRF) are performed. The control experiment indicates that southwestrelies is the major moisture source of heavy rainfall zone. A convergent line parallel to the coast forms due to the offshore flow near the surface and deflection flow which is caused by the blocking of the topography. Then convections develop along the convergent line and bring heavy rainfall around southern Taiwan. In the sensitivity experiment the typhoon is removed at the initial time. Southwesterlies still bring abundant moisture to the southwestern coast of Taiwan. However, where convection develops shift to offshore due to the difference of wind field between two experiments. The heavy rainfall zone shifts as well but the accumulated precipitation is as much as the control experiment. The impact of topography is also tested by sensitivity experiments that change the altitude of Taiwan. Results show that the higher the altitude, the more precipitation due to blocking of topography and the offshore flow. In sum, southwesterlies which brings abundant moisture triggers heavy rainfall event due to the convergent line because of the convergent line caused by the offshore flow and blocking of topography. The heavy rainfall event occurs no matter typhoon Halong exists or not, but the location of heavy rainfall zone may change through the modification of wind field by the interaction between typhoon and southwesterlies.