本篇統計了近20年(1993-2012)梅雨季(五月中旬到六月中旬)在台灣西北部較低地區(地形高度低於250m)造成豪大雨(日雨量>200mm)的事件。結果共有13天的大豪雨事件。其中在大台北地區只有兩個個案，本研究主要探討其中之一的2012年6月10-12日超大豪雨(日雨量>350mm)的有利條件。 由中央氣象局各雨量站可得知，在北部豪雨發生之前兩日，在6月10日及11日台灣中南部山區先發生了超大豪雨，主要是因為台灣受到由印度洋傳送到台灣西南部上游地區以及台灣的較強水氣通量的影響，所以可降水量、水氣通量以及水氣輻合都有高值。由The European Centre For Medium-Range Weather Forecasts (ECMWF)- Re-analysis資料得知，台灣海峽在11日可降水量日平均達67.5kg m-2及850hPa以下的水氣通量日平均高達312 g kg-1 m s-1可知這是有利於中南部豪雨的發生(Chen et al. 2007b)。10、11日兩日南部的降雨型態為海上的水氣輻合及地形抬升使降雨增加。直到6月11日晚間鋒面接近，台灣北部開始降雨且在12日造成超大豪雨事件。6月12日北部的降雨，是過去20年來較罕見的較低地區(小於250m)降雨。造成了6死1失蹤14傷。高層輻散；低層輻合；低層有高相當位溫；豐沛的水氣；有東北東-西南西走向的鋒面滯留在台灣北部長達近12小時。雷達回波圖顯示較大的回波(大於40dBz)持續出現在台灣北部與鋒面滯留的位置相當。同時豪大雨的測站與強回波常出現的區域一致。 利用WRF模式3.4.1版(Weather Research & Forecasting Model version 3.4.1)模擬地形在台灣北部造成強降雨的重要性。模擬結果顯示，近似滯留的鋒面前後較強的回波[圖3.2.15a]在西北外海的西南風與地形激流輻合區[圖3.3.13c]加強後往內陸移動。在11日夜間及12日清晨，不斷有強回波在鋒前及鋒後，經過低層輻合由西北近海移入台灣北部，是造成豪雨的原因。模擬回波之變化特性與觀測的雷達時序圖一致。除此之外鋒前的西南風遇到台北盆地南方地形抬升，以及對流尺度的冷空氣外流與地形激流輻合，亦是有助於降雨的發生。在移除台灣地形的模擬，無地形激流與較快速南移的鋒面輻合作用使得降雨沒有集中在北部地區。 除了有豐沛的水氣，地形激流加強由中國東南沿海移出的對流胞，台北盆地南方的地形抬升，再加上地形激流與在台灣北部移動緩慢的鋒面的輻合，在鋒面移進北部陸地後對流尺度的冷空氣與地形激流輻合，是產生此超大豪雨的有利條件。 An unusual heavy rainfall event (> 350 mm day-1) occurred in Taiwan from 10 to 12 June 2012. The favorable conditions for the occurrence of heavy rainfall are investigated using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-analysis grid data, observational data from Central Weather Bureau of Taiwan, satellite images, radar reflectivities, and simulations from the Weather Research and Forecast (WRF) model. This event occurred under a favorable large-scale environment including: a quasi-stationary upper-level East Asia trough upper-level divergence middle-level short-wave trough low-level high equivalent potential temperature low-level high moisture flux a low level of free convection (LFC) a quasi-stationary surface front over northern Taiwan and subsynoptic-scale ascending motion over the northern Taiwan Strait and northern Taiwan. On 10 and 11 June, high moisture flux was transported from Indian Ocean and Bay of Bangle through Indochina Peninsula, South China Sea to Taiwan. The low-level moisture air was lifted and produced heavy rainfall in the windward side of central and southern Taiwan and mountainous areas. In the late night of 11 and the morning of 12 June, a surface front moved from southeastern China coast to northern Taiwan and became quasi-stationary. The convergence between the southwesterly barrier jet and the westerly/northwesterly wind behind the windshift line associated with the low-level trough axis occurred over northeastern Taiwan Strait and northwestern coast of Taiwan. Because of the quasi-stationary surface front was present over northern Taiwan, intense radar reflectivities frequent occurred over the convergence area and moved into northwestern Taiwan and Taipei Basin. In addition, the orographic lifting of the prevailing wind over southern Taipei Basin enhanced rainfall. As a result, heavy rainfall occurred in the low land of northern Taiwan.
