2018年9月8日臺北豪雨之觀測研究

2018年9月8日傍晚在臺北市發生的大豪雨事件，造成臺北地區多處的都市閃洪與積淹水災情。當日雙北地區有多個地面雨量站達到24小時累積降雨超過200毫米的記錄（即中央氣象局的長延時豪雨定義）。其中值得注意的是，在豪雨期間總共有9個雨量站觀測到60分鐘累積雨量超過100毫米的驚人降雨強度，最高的記錄為信義區挹翠山莊測站所量測到的138毫米。這次豪雨事件雨勢相對集中於北市南區，並顯示出極具短延時豪雨的特性。本論文利用五分山氣象站的雙偏極化雷達與中尺度站網資料進行中尺度分析，探討造成9月8日傍晚臺北市豪雨的強對流天氣系統，其發展移動、結構特徵、以及形成機制等。再分析資料顯示，北臺灣周邊的低層環境存在較多水氣，在此豪雨個案的對流胞形成之前，北部外海鋒面帶有零星回波移入陸地，但最初並未有完整組織，而在鋒面南移、東北風風速增加後伴隨著中低層環境的渦度，才使北部沿岸的對流得以在大屯山東北側迎風面地形上舉升，而後逐步發展、整併，並隨著鋒後東北風越過地形進入臺北盆地。衛星雲頂影像及雷達資料顯示，此中尺度對流系統在進入盆地之後出現明顯旋轉滯留特徵。由雙偏極化雷達風場及渦度分析則顯示，此個案在盆地內形成中尺度渦旋，伴隨強回波並滯留二小時左右。渦旋的旋轉性質使其中心兩側呈現反向偶極的風場特徵，並存在垂直風切。當渦旋發展至成熟期的時候移至市中心，造成傍晚臺北市南區的短時豪雨。雷達雙偏極化參數則在對流系統南緣反映了入流區的特徵，低層西風進入系統的輻合位置出現了ZDR的極值，並有部分垂直結構，但發展高度低於融解層，顯示此對流的垂直速度並不大，是一個以暖雨過程為主的劇烈降水個案。

關鍵字

臺北豪雨；短（長）延時豪雨；雙偏極化雷達；中尺度渦旋

並列摘要

Heavy rain on the late afternoon of September 8, 2018 caused flash floods in many parts of the Taipei City. A number of rainfall stations in the area reached the record of accumulated rainfall of more than 200 mm in 24 hours (i.e. the Central Weather Bureau's definition of long-duration heavy rain event). It is worth noting that a total of 9 rainfall stations observed an astonishing rainfall intensity of more than 100 mm in 60 minutes during the heavy rain period, with the highest recorded being 138 mm measured by the Station at Tsu-I Mountain Village in Xinyi District. This heavy rain event is relatively concentrated in the southern part of Taipei City, and shows the characteristics of very short-duration heavy rain event. This paper makes a mesoscale analysis using the polarimetric radar and mesoscale network to explore the development, movement, structural characteristics, and formation mechanism of this severe convective weather system. Reanalysis data marks abundant water vapor around northern Taiwan in the low-level environment. Before the convective cells formed, echoes from the frontal region moved onto land without well organized. While the cold front moved southward and the northeasterly enhanced along with vorticity in the low- and mid-level, convective cells were initially formed on the northeast side of Datun Mountain and then gradually developed, integrated, and crossed the terrain into the Taipei Basin. Cold cloud top IR images showed rotational signature after the mesoscale convective system entered the basin. Radial velocity of Doppler radar and vorticity analysis results showed a mesoscale vortex formed in the basin accompanied by strong echoes and stranded for about two hours. Cyclonic characteristic led to dipole pattern of radial velocity, and there were also vertical shears in the system. Analysis suggest that formation of mesoscale vortex is a manifestation of the convergence of northeasterly wind and the southwesterly wind in the Taipei basin. The existence of the mesoscale vortex provided the dynamical forcing for the short-duration heavy rain in Taipei when it developed to mature stage and moved to the city center. Radar polarimetric parameters indicated the inflow in the southern region of the system. ZDR maximum appeared in the position of convergence in low-level and showed vertical structure. Nevertheless, it developed under melting level and indicates a smaller vertical velocity. This is an extreme heavy rain case with warm rain process dominant.