梅雨季台灣中部豪雨個案分析與模擬

Abstract

2007年6月8日台灣地區因受梅雨鋒面之中尺度線狀對流與海峽中部新生對流的影響，全台各地均有降水，其中以中部地區最為嚴重，梧棲及台中測站之3小時累積雨量超過130 mm。高空天氣圖與探空資料顯示，當時大氣環境具有暖平流、高值對流可用位能、700∼550 hPa有噴流存在、500 hPa有短波槽及200 hPa有高層輻散等有利對流系統發展之條件。雷達回波觀測顯示，海峽北部線狀對流在7日1800 UTC達到成熟，呈東北－西南走向，並往東南移向台灣。在移動過程中，此對流帶與海峽中部的對流胞合併，在中部外海增強隨後移入台灣陸地，造成中部之豪雨事件。 本研究使用日本名古屋大學之非靜力雲解析風暴模式(Cloud-Resolving Storm Simulator，簡稱為CReSS)，模擬在個案期間台灣附近之天氣現象，並探討造成中部地區劇烈降水的原因。結果顯示，台灣海峽南部降水所伴隨的下衝流冷空氣在近地層往北輻散外流，疊加於盛行西南風之上。使被增強的西南氣流向北傳送至台灣海峽中部增加輻合，激發新生對流，並與向東南移動的線狀對流結合在登陸台灣前增強，最終導致台灣中部的豪雨事件。 此外模擬結果亦顯示，本個案之夫如數（Froude number, Fr）約0.3，環境氣流不易過山，在地形阻擋效應顯著情況下，低層盛行西南氣流在迎風面轉為偏南風，並與海峽上的西南西風造成額外輻合。此局部輻合，使上述海峽中部對流系統在登陸前增強，導致中部沿岸地區劇烈降水。在中部山區，海峽中部的西風氣流因地形阻擋產生回流效應，並與盛行之西南風輻合，激發新對流，最後造成顯著的累積雨量。

On 8 June 2007, the heavy rainfall in Taiwan was associated with meso-scale convective line of mei-yu and new cells at central Taiwan Strait especially in the central Taiwan region (Wuqi and Taichung were over 130 mm/3h). The high-altitude weather chart and sounding data shows that there had development conditions of convective systems, such as the warm advection, high convective available potential energy (CAPE), low-level jet between 700 and 550 hPa, short-wave through in 500 hPa and divergence in 200 hPa. Doppler radar observations show that mature convective lines of Strait were oriented northeast-southwest direction at 1800 UTC 7 June, then were merged with another new convection at central Strait. After moved to Taiwan, the mature convections were brought heavy rainfall (over 250 mm/24hr) at central Taiwan. The Nagoya University Cloud-Resolving Storm Simulator (CReSS, v2.2) is used in this study, and simulated the weather phenomena near Taiwan during the case period, to investigate the result of heavy rainfall in central Taiwan. The simulation result showed that precipitation in the southern Taiwan Strait and downstream associated with cold pool divergence outflow moved northerly near surface layer above the prevailing wind. These convective systems were enhanced before landing on Taiwan and led heavy rainfall in central Taiwan. The simulation results also show that the airflow didn’t across the mountain (Froude number~0.3). In the condition of terrain effect obviously, the southwest changed into the south wind in the low level, and made the additional convergence with southwest to southerly wind in the central Strait played the part of role and excited the convective in the Strait. After the convections of central Strait were organized, the north side changed west wind, and enhanced the convection systems, and cased obviously precipitation by terrain lifting when moved into central Taiwan region. In the central mountain, it produced the convergence by the backing effect when the west flow of central Strait with terrain blocked and inshore wind excited new cells, and made the heavy rainfall obviously.