| 摘要: | 2014年8月12日在臺灣西南部地區最大累積雨量達345.5mm，接近中央氣象局超大豪雨標準(350mm/day)，且降雨集中在臺灣西南部沿海平地，是台南、高雄人口稠密之地區，造成市區橋梁、道路封閉、居民撤離。臺灣夏季除了颱風帶來的降雨，主要受到西南季風影響，在中央山脈迎風面西南部地區有較明顯地降雨，另外中尺度對流系統也容易沿著盛行風，由西南部外海向臺灣西南部移動帶來降雨(Chen and Chen 2003)。為了研究個案在沿海平地地區的降雨機制，本文利用European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim資料、雷達回波、氣象局局屬測站與自動測站以及Weather Research and Forecasting (WRF) Model數值模擬，對於8月12日大豪雨個案進行討論。分析ERA-Interim在2014年8月5～14日平均綜觀環境與氣候值的差異，顯示在2014年有較大的底層水氣通量(150g/kg·m/s)，850hPa在日本南方有低壓環流，中層500hPa中緯度槽向西南延伸，台灣附近上升區有利雲系發展。在8月12日的綜觀環境，顯示大陸低壓向東擴張，台灣西南部盛行西南西風，低層950hPa水氣輻合顯著，500hPa在南海北部、巴士海峽為低壓環流，上升運動有利於雲系發展。研究發現8月12日期間臺灣西南部近海持續有對流系統以及潮溼空氣向臺灣西南部地區移動。且西南部上游低層的風速並不大(6～9m/s)，moist Brunt-Vaisala frequency約為0.017/s，Froude number(F_r)約為0.22。在低F_r情況下，盛行西南西風受臺灣南部地形影響產生阻塞、地形回流以及偏轉的東南風，與原本的盛行風輻合造成降雨。另外降雨蒸發冷卻產生的外流冷空氣(離岸風)以及夜晚地面冷卻造成的下坡風與盛行風的輻合有助於對流系統在沿海地區的發展。;The southwestern Taiwan heavy rainfall event, on 12 august 2014, with a maximum rainfall of 345.5mm, and moreover, the rainfall are distributed over lowland and coast area. In summer, usual rainfall are produced on windward side of southwestern Taiwan, come from southwesterly monsoon and convective systems follow monsoon flow. In order to identify the synoptic environments associated with the 12 August 2014 heavy rainfall event and its mechanism, this study will perform observational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) data, satellite imagery, radar reflectivity, rainfall data, and Weather Research and Forecasting (WRF) Model. The simulation results from the WRF model will compare with observation data to identify the mechanisms resulted to this localized heavy rainfall.
Through the analysis of ECMWF/ERA-Interim data shows that there were more water vapor flux in 2014 average than ten years (2005~2014) average synoptic environments, and there is a low pressure over southern of Japan on 850hPa, and monsoon trough extended from southern China to east of Taiwan, as a result, raising motion area over Taiwan conduce clouds developed. On 12 august 2014, there is low-level water vapor convergence on 950hPa, and cyclone circulation over northern South China Sea and Bashi Channel, it’s also promote clouds developed. We found that the mainly heavy rainfall areas in 12 August 2014 are in the lowland area and the stronger convergence induced by southwesterly monsoon and down-slop wind and southeastern wind which the monsoon flow deflected by orographic effect, and the convective system follow monsoon flow moved in lowland area from southwestern Taiwan offshore region.
In the simulation result, it can also see the same heavy rainfall mechanisms which the convergence was produced by the convergence between the prevailing wind and the down-slop wind, and the deflection wind. But the offshore convective systems did not move in to southwestern Taiwan, the rainfall in early day did not simulate well. Through by 27km grid WRF simulation result, we can see on 700hPa lacking east wind resulted in less convective systems had been enhanced. |
