梅雨季弱綜觀強迫下臺灣西南部沿海清晨對流發展之個案診斷分析研究

Abstract

2012 年6 月8 日清晨，臺灣西南部外海有對流移近沿海地區，過程中逐漸增強並組織成線狀對流型態。此系統移入陸地後在沿海區域産生降雨，其後側並有新對流激發，隨後在內陸消散。因此，當日清晨降雨集中在臺灣西南部沿海地區。 綜觀分析顯示，本個案期間屬於弱綜觀強迫之環境，並無顯著的綜觀環流系統影響臺灣。透過觀測資料的中尺度分析，顯示對流後側伴隨低層西南風的增強，低層速度輻合使對流在近海激發並移入陸地增強。對流登陸後，降水下衝流形成的冷池外流，亦使沿海輻合增強，因而在後側激發新胞維持對流系統發展。降水後之探空顯示，環境對流可用位能（Convective Available Potential Energy, CAPE）的減少與流抑制能量（Convective Inhibition, CIN）的增加、不穩度釋放等，會不利系統維持而使系統消散。此外，亦發現對流系統移入時，在陸風和環 境西南風間之局部輻合區內顯著增強。 利用雲解析風暴模式（Cloud Resolving Storm Simulator, CReSS）進行本個案模擬，在水平網格間距為500 m 之模擬實驗中，發現西南風輻合帶東移和陸風輻合帶西移，在兩輻合帶間造成有利對流發展的條件，使對流系統在該區有最顯著發展。並且，西南部沿海陸風前緣的舉升，也增強環境西南氣流前緣東移的對流，並導致西南部沿海的降雨。隨後，對流降水下衝流的外流，亦在沿海地區產生較大輻合，提供新對流的激發機制。顯然，本個案中之陸風在臺灣西南部沿海清晨對流之激發與增強，扮演重要角色。

In the morning of 8 June, 2012, convection approached and produced rainfall along the southwestern coast of Taiwan. During its approach, the system gradually organized into a linear structure and became more intense. After its landfall, new cells were triggered at its nearside near the coastline, and the rainfall was mainly over the coastal region. The case under study occurred during the Mei-Yu season under weak synoptic condition. Based on mesoscale analysis, the initial trigger of the convective system was mainly due to the convergence provided by the enhancing southwesterly winds to its rear. After the convective system made landfall, new cells were triggered by local convergence caused by the cold pool associated with convective downdrafts, and the system as a whole was maintained. Later, the convective system moved inland and dissipated because of a decrease in CAPE（Convective Available Potential Energy）and an increase in CIN（Convective Inhibition）. It was also found that the convective line strengthened near the coast over an area of local convergence between the environmental southwesterly winds and the land breeze. The CReSS（Cloud Resolving Storm Simulator, CReSS） model was utilized to investigate this case. The run using a grid size of 500 m revealed that a favorable condition for convection development was produced in an area between the convergence lines at the leading edge ahead of low-level southwesterly winds and land breeze. The simulated convection system exhibited a strong development in this area. In this case, the land breeze enhanced the eastward-moving convective system embedded in the environmental southwesterly winds. The outflow associated with convective rainfall on the coastal region produced local convergence and initiated new cells to the rear of old convection. Obviously, the land breeze was important to initiate and enhance morning convection over southwestern coast of Taiwan.