臺灣梅雨季弓形回波(Bow Echo)之結構與演變

Structure and Evolution of a Bow Echo in the Taiwan Meiyu Season

陳泰然(George Tai-Jen Chen) ； 周鴻祺(Hon-Chi Chou) ； 黃心怡(Shin-Yi Huang) ； 楊進賢(Jinn-Shang Yang)

《大氣科學》 39卷2期 (2011/06) Pp. 177-200

摘要

2000年6月6日午後在台灣海峽北部形成一條呈東北－西南走向之線狀回波，以13ms^(1)之速度向東南快速移動，並於1430-1800LST間呈現弓形回波，整個弓形回波的生命史約為3.5小時。在1623 LST通過桃園國際機場時，產生16.5ms^(1)西北風瞬間最大陣風。本文即針對此弓形回波個案，進行綜觀環境、中尺度環流及都卜勒雷達回波與徑向風場分析，以了解其形成與演變過程，並探討其伴隨之中尺度現象。
結果顯示，6月6日0300 UTC時，對流可用位能高達3047平方公尺s^(-2)，中對流層並有明顯乾空氣存在，中低對流層(1000-500 hPa)具有適中的垂直風速風切(256°, 14.5ms^(1))，這些皆為有利弓形回波發展的環境條件。弓形回波長約120公里，最大回波強度達58dBZ，其後方之強烈後方內流噴流(rear inflow jet, RIJ)主軸與弓形凸出主軸緊密結合。顯然，大量對流可用位能和強垂直風切有利於高架式後方內流噴流的發展，並促使下衝流增強，造成線狀回波變成弓形，後緣的回波凹洞不僅透過前述過程形成，且透過環境中對流層乾空氣的內流而產生並增強，最後也使凸出部份之回波減弱。在弓形回波成熟期，於其南（北）側1~3公里高度，分別出現中尺度反氣旋式（氣旋式）渦旋。當這對渦旋偶（即書夾式渦旋）形成時，後方內流噴流的強度明顯增強，伴隨強後方內流噴流之强下衝流對環境垂直風切的扭轉效應，為此對書夾式渦旋產生和維持的機制。

關鍵字

弓形回波 ； 書夾式渦旋 ； 環境條件 ； 中尺度現象

並列摘要

A NE-SW oriented line echo formed over the northern Taiwan Strait in the afternoon of 6 June 2000 and moved rapidly southeastward with a speed of 13 ms^(1). It evolved into a bow echo in the period of 1430-1800 LST with a life span of 3.5 h. As it passed the Taoyuan International Airport at 1623 LST, a gust wind from northwest direction with a speed of 16.5 ms^(1) was observed. The purpose of this paper is to investigate the formation and evolution processes as well as the associated mesoscale features of this bow echo by analyzing environmental conditions, mesoscale circulations, and Doppler radar echo and radial winds.
Results showed that the convective available potential energy (CAPE) reached a high value of 3047 m^2 s^(-2) at 0300 UTC (1100 LST) on 6 June. Meanwhile, dry air occurred in the mid-troposphere and the proper vertical wind shear (256°, 14.5 ms^(-1)) existed in the lower to middle troposphere (1000-500 hPa). The environmental conditions were favorable for the formation of a bow echo. The length of the bow echo was about 120 km with a maximum intensity of 58 dBZ. A strong rear-inflow jet was observed to collocate with the bulged axis of the bow echo. Apparently, the high CAPE and the strong vertical shear were favorable for the formation of the elevated rear-inflow jet. Downdraft was also enhanced to turn a line echo into a bow echo. This process was responsible not only for the formation of a reflectivity notch in the rear side, but also for the intensification of the notch by the inflow of mid-tropospheric dry air. Finally, it caused the decaying of the bulged echo. It was also found that a mesoscale anticyclonic (cyclonic) vortex formed at the height of 1-3 km to the south (north) side of the bow echo. As the vortex couplet (i.e. bookend vortices) formed, the rear-inflow jet was apparently intensified. The twisting effect of the downdraft on vertical shear was responsible for the formation and maintenance of the bookend vortices.

並列關鍵字

bow echo ； bookend vortices ； environmental conditions ； mesoscale feature