梅雨季台灣南部近海準線狀對流系統的特性分析

Case Study on the Characteristics of Quasi-linear Convective System off the Southern Coast of Taiwan in the 2003 Mei-yu Season

魏志憲(Chih-Hsien Wei) ； 何台華(Tai-Hwa Hor) ； 張茂興(Mou-Hsiang Chang) ； 李文兆(Wen-Chau Lee)

《大氣科學》 34卷2期 (2006/06) Pp. 157-176

摘要

2003年6月7日自華南和南海北部移入台灣南部近海之中尺度對流系統，其前緣強降水回波區呈現準線狀對流(quasi-linear convective system)的型態，其中並伴隨彎曲狀的對流雨帶，外型與發生在美國中西部地區的弓狀回波相似。本研究利用中央氣象局墾丁與七股都卜勒氣象雷達觀測資料進行分析，以瞭解此海上準線狀對流系統之結構特徵，並與美國地區之陸上弓狀回波系統進行比較，以期探究兩者之間結構特徵的異同。
初步研究發現，此副熱帶地區準線狀對流系統生命期達七小時以上，從地面到700hPa之環境垂直風切約爲10ms^(-1)，與熱帶海洋地區研究個案(~13ms^(-1))相似。從雷達回波發現此系統前導線位置出現彎曲的波狀類型，前導線後方存在著後方噴流(rear-inflow jet)、後方入流凹區(rear-inflow notch)及氣旋式的中尺度渦旋等中尺度特徵，其中噴流強度約在25至30m/s之間，渦度強度則小於10^(-3)S^(-1)數量級，都比中緯度地區所觀測的弓狀回波個案爲弱，而且此後方入流凹區的空氣不如中緯度地區乾燥，氣旋式的中尺度渦旋只在此降水系統北邊出現，而反氣旋式渦旋結構並沒有發生。

關鍵字

準線狀對流系統 ； 後方噴流 ； 後方入流凹區 ； 中尺度渦旋

並列摘要

On 7 June 2003, an eastward propagating mesoscale convective systems (MCSs) was captured by the Taiwan Doppler weather radar network after initiating in the northern part of the South China Sea and exhibited the features of quasi-linear convective system (QLCS) that several bowing segments embedded in it. Therefore, it's applicable to compare the characteristics of QLCS in the subtropical ocean area with those of bow echoes in the mid-latitude continental area during spring season.
Preliminary studies show that the vertical wind shear (10 ms^(-1) between surface and 700 hPa) had a similar magnitude as that in tropical area (~13 ms^(-1)). The radar reflectivity collected by Chiku and Kenting Doppler radars characterized its bend-shaped convective line in the leading edge and widespread stratiform precipitation in the rear area. The system lasted more than 7 hours. Moreover, between the convective rainband and the stratiform cloud, there was a region of weak echo. The maximum Doppler velocity in the rear of convective line went up to 25 and 30 m/s as a rear inflow jet embedded in the stratiform area with features of rear inflow notch. A mesovortex in the cyclonic flow was observed at the northern tip of the convective system and its intensity in vorticity was less than 10^(-3)s^(-1), which was weaker than the mid-latitude continental case. However, the mesovortex in the anti-cyclonic circulation wasn't well identified in the southern tip of the system.

並列關鍵字

Quasi-linear convective system ； Rear inflow jet ； Rear inflow notch ； Mesovortex