- 學位論文
應用雷達觀測追蹤分析臺北盆地夏季午後雷暴雨胞
Tracking and Analyzing Afternoon Thunderstorm Cells in Taipei Basin Using Radar Observation
摘要
本研究探討臺北盆地午後雷暴的生命週期，主要內容可分為三部分：(1) 利用2015年6月14日強降雨事件的WRF模擬資料對午後雷暴發生前後的環境進行討論；(2) 對雨胞辨識、追蹤的方法進行回顧，並以案例資料進行測試與改良，提出一套辨識、追蹤與連結的演算法；(3) 將該演算法應用於2021年6月4日（0604降雨）與7月31日（0731降雨）兩場事件上，並對兩事件進行分析比較。 在臺北盆地形成午後雷暴所需要的水氣主要來自海面，從WRF近地表的風場資料可以看到在日出後風向轉為海風，並且在西南風尾流的強化之下，來自海上的氣流從淡水、基隆河谷進入臺北盆地。富含水氣的空氣塊在進入盆地後受到都市熱島效應加熱，其增溫程度會反映在相對濕度的降低與虛溫的增加上。在加熱過程中空氣塊同時往南邊移動，被推送至盆地南側山坡後受地形抬升而產生降雨。降雨後冷池隨之產生，從位溫的變化可以看到冷池範圍逐漸往北擴張。冷池的外流風場與海風輻合，在冷池邊緣水平渦旋受上升氣流與下衝流翻轉，從WRF模擬資料中可以觀察到此處有垂直渦旋的存在。 在雨胞的辨識、追蹤與連結方面，本研究利用集水區法，將垂直分層的雷達水平回波切割成若干雨胞單元，將每一至數個雨胞單元組合為雨胞，作為追蹤與連結的目標，再以ABLER目標函數值以及反射率因子加權質心作為相似度指標，連結不同高度的雨胞，並追蹤雨胞隨時間的空間移動。 利用上述演算法，本研究分析0604和0731降雨個案中的雨胞。在兩場降雨中，都有來自盆地南方山坡的雨胞和盆地上空的雨胞產生合併。兩案例事件的主要差異是0604降雨中西南風勢力較弱，和海風在盆地南方山坡輻合生成雨胞後才往盆地方向傳播，之後向東北方向移動，追上盆地上空的雨胞並與之合併；在0731案例中，則有較強勢的西南風，因此與海風的輻合位置更偏向東北方（接近盆地），在山坡與盆地之間有各自生成的獨立雨胞，最後相連而合併成單一雨胞。除此之外，在兩事件中也可觀察到降雨前的低相對濕度區域在空間分布上的差異，和雨胞合併過程的差異相同，認為和近地面西南風的強弱有關。關於雨胞合併對於增強、減弱變化的影響，在0604降雨案例中，觀察到相鄰雨胞是先在低層連結，透過連結區的交換，乃出現此消彼長的反向強度變化情形。
並列摘要
The aims of this thesis are to investigate into life history of thunderstorms at Taipei basin, and develop an algorithm of identifying, tracking, and connecting thunderstorm cells. This thesis consists of three main parts. The first part is devoted to the analysis of the environments before and during thunderstorm using WRF simulation of the rainfall event on 14 June 2015. From the simulated wind field we could see that water vapor needed for the development of thunderstorm mainly entered through Tamsui and Keelung river valleys. The vapor was then heated by urban heat island effect in Taipei basin, which could be seen with the change in relative humidity and virtual temperature. The heated air then converged toward mountains at the south of basin and the rainfall started. The cold pool appeared after the rain started and spread north toward basin. Vertical vortices were generated from the tilting of horizontal vortices by updraft at the edge of cold pool. At the second part we tested some methodologies of cell identification and tracking, and proposed our own algorithm. Watershed method is used in the algorithm to segment the radar echo into rain cell unit. Rain cell units are then combined, connected between neighboring levels and tracked in different times, using the object function of ABLER and the reflectivity-weighted centroid as indices of similarity. At the third part two rain events in 2021 were analyzed. It was found in the first event that two cells connected with each other at the lower level and interchange energy through the connection, and thus showed an opposite behavior in growth and decay. Comparing two events, we also found that the airflow through Dahan river Valley might play an important role in the rainfall events. The strength of the airflow might not only affect the location but also the mechanism of the merging of cells.
參考文獻
延伸閱讀
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