- 期刊
台灣梅雨季海峽北部與鄰近地區線狀對流之環境條件
Environmental Conditions for the Line Convection over the Northern Taiwan Strait and the Adjacent Areas in the Taiwan Mei-Yu Season
摘要
本文利用1988-1993年5-6月民航局中正機場都卜勒雷達非都卜勒狀態之最大強度回波觀測圖，選取59個線狀對流系統個案，依系統形成之回波型態分類，並依系統於成熟期時與地面梅雨鋒之相對位置分類，以探討線狀對流之一般特徵。然後利用ECMWF網格點資料與板橋探空站資料，以研究不同類型個案之合成環境條件。結果顯示，各類型線狀對流系統均發生於850 hPa梅雨鋒南側之西南氣流內，850-700 hPa之低層噴流提供低對流層強烈垂直風切，為線狀對流提供有利之形成與發展條件。中低對流層風場的水平輻合，舉升潛在不穩定大氣，配合水平水氣通量輻合，以維持線狀對流之發展，並使對流發展至200 hPa高度。同時發現，在鋒面類型中以鋒上型之平均生命期最長、移速最慢且系統長度最長，這些特徵似均與該型於成熟期時位於鋒面上，故低層水平輻合與水氣通量輻合最強且產生之高層輻散亦最強者一致。此外亦發現，鋒前型之低層Vn垂直風切較Vt垂直風切為小，但鋒上型與鋒後型者反是，此似與鋒前型平均生命期最短者一致。破線型與後造型環條件相近，但破線型低層噴流之下垂直風切Vn大於Vt，而後造型則Vn小於Vt。Vn之垂直風切以後造型較大，此似與該型生命期較長者一致。各類型之對流理查遜數顯示，除鋒前型環境有利於多胞颮線形成之外，其他各型環境均有形成超大胞系統之可能性。
並列摘要
Doppler radar echo data observed in May-June 1988-1993 at the CKS International Airport, the Civil Aeronautics Administration, were used to identify the mesoscale lines of convection. Fifty-nine lines identified were categorized into different types according to the echo patterns of line formation and the line positions with respect to the surface front. The general characteristics and the composite environmental conditions were then obtained by using ECMWF grid and Panchiao rawinsonde data.Results showed that all lines occurred in the southwesterly monsoonal flows in the south side of the 850 hPa Mei-Yu front. The strong low-level vertical wind shear below the LLJ at 850-700 hPa provided a favorable condition for line formation and development. Horizontal convergence coupled with the horizontal moisture flux convergence in the lower- and mid-troposphere ensured that the convection could reach 200 hPa level in the potentially unstable environment. Among all frontal lines, the ”on-front” type had the longest mean lifetime, the longest horizontal length scale, and the smallest propagation speed. These appeared to be consistent with the fact that the mature line of this type was located right on the front where the horizontal convergence and moisture flux convergence were the strongest and thus the upper-level divergence produced were also the strongest. The low-level vertical shear of Vn was smaller than that of Vt (∂Vn/∂z ＜ ∂Vt/∂z) for the ”ahead-of-front” type, and the reverse was true for the ”on-front” and ”behind-front” types. These seemed to be consistent with the shortest lifetime for the ”ahead-of-front” type. The broken-line type and the greater vertical shear in Vn than in Vt below the LLJ level, and the reverse was true for the back-building type. The greater vertical shear in Vn for back-building type as compared to the broken-line type was consistent with the longer lifetime of the back-building lines. The convective Richardson number suggested that the large-scale environment for the ”ahead-of-front” type was favorable for the formation of multi-cell squall lines and the formation of super-cell squall lines was possible in the large-scale environment of other types.