本研究針對2021年6月4日臺北都會區午後大豪雨個案發生下爆流(downburst)進行雷達及地面觀測資料的中尺度分析，以了解下爆流發生的過程、物理結構、以及在整起強降雨事件中扮演之角色。
我們發現此個案中對流胞發展初期(1130~1230 LST)的中尺度現象為上坡風、盆地內風場輻合、以及冷池的發展，與過去北臺灣發生午後雷暴豪雨之發展歷程相似。對流發展至1230~1300 LST在臺北都會區的南側發生下爆流。初期雷達特徵(1230~1235 LST)有雲內的風場輻合和強回波區邊緣出現回波缺口，對應近地面明顯的線狀輻散區域，輻散強度達10 × 10-3 s-1以上。漸趨增強的外流風場使得對流胞附近的上升運動更加顯著。於12:35 LST，2~4公里高度出現一降水核心。透過雙偏極化參數分析，降水核心之Z_DR數值達3dB以上，表示內含粒徑較大的雨滴，其位置與Z_DR柱重疊。12:38~12:50 LST此降水核心降落至近地面，外流最大徑向風速由10.2 m∙s^(-1)逐漸增強至14.8 m∙s^(-1)。下爆流的發生增強了冷池內的外流風速，造成對流胞西及北側的冷池前緣外流邊界向西、北方推移，使對流胞發展的位置快速北移進入臺北盆地內。這個過程可能是導致此次臺北都會區60分鐘雨量達137.5毫米最大強度降雨主要關鍵。
On June 4, 2021, before the heavy rainstorm occurred in the Taipei metropolitan area, wet downburst was observed in the south of Taipei City. The development of convective cells changed significantly after the downburst occurred. This study analyzed data from radars and ground stations to understand the development process and characteristics of wet downburst in convective cells.
The mesoscale phenomena in the early stages of convective cell development (1130~1230 LST) in this case were upslope winds, wind convergence in the Taipei Basin, and the development of cold pool, which were similar to the results from earlier studies. Wet downburst was observed in the south of Taipei City between 1230 and 1300 LST. In the early stage of downburst (1230~1235 LST), the radar characteristics were convergence wind above the cloud base and the reflectivity echo notch appearing at the edge of the strong reflectivity area, corresponding to obvious linear divergence areas near the ground and the divergence intensity achieve to 10×〖10〗^(-3) s^(-1). The increasing outflow enhanced the updraft near the convective cells, and a reflectivity/precipitation core appeared at 2 to 4 kilometers height near the convective cells (12:35 LST). The precipitation core contains raindrops with larger particle sizes (Z_DRvalue achieve to 3dB), and their positions colocation with the Z_DR column. From 12:38 to 12:50 LST, this precipitation core descended to near the ground, and the maximum radial wind velocity of the outflow increased from 10.2 m∙s^(-1) to 14.8 m∙s^(-1). The occurrence of downburst enhanced the outflow wind velocity in the cold pool, accelerating the cold pool outflow boundary on the west and north side of the convective cell to the west and north, causing the development position of the convective cell rapidly moved northward towards the Taipei Basin, and produced the record-breaking intense rainfall in the Taipei metropolitan area, i.e., 137.5 mm in 60 minutes.
