| Abstract: | 本研究利用2006至2020年中央氣象局QPESUMS資料，針對台灣地區中尺度對流系統(Mesoscale Convective System, MCS)進行辨識以及分析，探討在不同季節間MCS結構特性，包含面積、形狀、長短軸比、發展高度及強度等。以ECMWF ERA5再分析資料及板橋探空資料探討MCS發生時的綜觀環境及熱力條件變數。以JMA 颱風資料濾除受TC影響事件。分為五個季節:春季(3月~5月中)、梅雨季(5月中~6月中)、夏季(6月中~8月)、秋季(9月~11月)以及冬季(12月至隔年2月)進行MCS的探討。
研究結果發現，MCS發生在梅雨季的比例最高(31.5%)，次高為春雨季(30.0%)，最少的是冬季(7.0%)。在春季時，高層中緯度的西風帶的影響逐漸減小，MCS主要是來自於春雨或冷鋒事件，由低層的西南風與冬季季風輻合所形成，以北台灣為主。梅雨季為MCS最活躍以及強度最強的季節，MCS的環境與梅雨鋒面息息相關，有著高層分流、加深的中層槽線與低層噴流。夏季的熱力環境最好，但垂直風切弱，因此對流面積較梅雨季小，MCS環境受西南氣流以及副熱帶高壓影響，當高壓東退時，西南氣流增強，使MCS容易在台灣西南部生成。
秋季之後開始轉為冬季季風，MCS以東部、東南部為主。高層開始受到中緯度的西風帶的影響。由於冬季季風會帶來較為乾冷的空氣，因此MCS事件發生時，北風分量會減小，由東北風轉為東風甚至東南風(11月後尤為明顯)，並開始有冷鋒訊號出現於台灣北部。冬季則接續秋季11月後的情形，MCS事件以台灣北部為主的冷鋒事件以及東部的線狀對流為主，但由於熱力條件較差，因此MCS在發生頻率、強度上皆較低。
;This study uses CWB QPESUMS radar, rain gauge, sounding, and ECMWF reanalysis data to investigate (Mesoscale Convective Systems) MCS events and analyze their synoptical environment characteristics in the Taiwan area from 2006 to 2020. The selected MCSs of five seasons, namely Spring, Mei-yu, Summer, Autumn and Winter, were analyzed.
The results show the Mei-yu season has the most MCS events (31.5%), the second most is spring (30.0%), and the least is the winter season (7.0%). In spring, MCS mainly associated with cold frontal system and located in northern Taiwan. The environment shows the southwesterly wind converged with winter monsoon in the lower level. The MCS events of the Mei-yu season are the most active with deepest convections. The environment shows there are upper-level divergence, deeper mid-level trough, and low-level jet. The MCS are located around retire Taiwan. The thermal condition is the most unstable to convection in Summer, the vertical wind shear is the weakest. Thus, Summer has lesser total number and weaker MCS events than Mei-yu season. The MCS events are mostly located in southwestern Taiwan and triggered by the southwesterly monsoon.
The instability starts to decrease after the winter monsoon onset. MCS mainly located in eastern and southeastern Taiwan in Autumn. Because of the dry and cold winter monsoon flow, the northerly wind component will decrease when MCS occurs in Autumn, causing the northeasterly wind turn to easterly or even southeasterly wind (especially after November). In addition, MCS events caused by the cold frontal system become more frequent in northern Taiwan after November. In winter, the environment is similar to November. However, because of the unfavorable convection condition, the frequency and convective strength are the lowest. |
