山區雷雨系統之降水與運動場結構分析

The Precipitation and Kinematic Structures of a Mountainous Thunderstorm

周仲島(Ben Jong-Dao Jou) ； 張茂興(Mou-Hsiang Chang) ； 何台華(Tai-Hwa Hor)

《大氣科學》 25卷4期 (1997/12) Pp. 483-510

摘要

本文利用台灣地區中尺度實驗［TAMEX］期間NCAR CP4與NOAA TOGA都卜勒雷達高解析度體積掃瞄資料，分析山區雷雨系統的內部降水和運動場結構特徵，並討論地形對雷雨系統結構的影響。1987年6月20日（TAMEX IOP11）午後，在垂直風切微弱、對流不穩定的大氣條件下，於台灣中部山區形成一個由多個雷雨胞發展而成的組織性中尺度對流系統，其生命期長達約6小時。根據整體理查遜數的估計，該雷雨系統應歸類為非組織性多胞雷雨系統。本文主要是探討該組織性中尺度對流系統在發展前期的結構特徵與維持機制。雙都卜勒合成分析發現，該雷雨系統在斜坡上形成，發展初期具孤立雷雨胞特性，含單一上沖流結構。雷雨胞移動前緣為強盛上衝流，最大值在4公里高度，氣流場結構低層為輻合，高層為輻散，顯示動力過程為此上衝流產生主要機制，後側為較微弱下衝流，和大回波區分離，且近地層未出現明顯冷空氣池，顯示該下衝流並非單純由降水拖曳過程所造成，未飽和環境空氣逸入過程可能扮演重要角色。之後，該雷雨系統往較高山區移動，此時系統內部同時含多股強度不一的上衝流，表示係由多個不同生命期雷雨胞合併而成。其中較成熟之雷雨胞含顯著下衝流，但並未伴隨顯著降水，顯示該下衝流之形成與降水拖曳關係不大。進一步分析可知，本個案山區雷雨系統的維持，並非經由雷雨系統本身降水至地面發展外流邊界，與環境風場形成局部輻合，產生新雷雨胞予以維持；而是斜坡地形上不均勻加熱形成之上坡風提供必要舉升機制，使新雷雨胞在斜坡上發展，而後併入較高山區成熟之雷雨胞，形成較長生命期雷雨系統。此種過程的差異是山區雷雨系統產生不同行為的一個重要原因。另外，午後由沿海地區深入之海風環流所提供之水氣，在山區雷雨系統的增強上扮演重要角色。

關鍵字

山區雷雨系統 ； 降水與運動場結構 ； 雙都卜勒合成分析 ； 地形不均勻加熱 ； 海風環流 ； TAMEX個案

並列摘要

The precipitation and kinematic structures of a mountainous multicell thunderstorm were analyzed by using the high resolution Doppler data of NCAR CP4 and NOAA TOGA radars. The development and organization of the storm was analyzed and the possible effects of complex terrain and local circulation were discussed. In the early afternoon of 20 June 1987 (TAMEX IOP 11), several convective storms occurred at the sloping area in the central part of Taiwan. These storms merged from several individual thunderstorms and developed into an organized mesoscale convective system in the late afternoon. This mesoscale convective system lasted for more than six hours. The environment was with weak vertical wind shear and pronounced convective instability.The radar reflectivity showed new thunderstorm cell formed over the sloping area around 500 m altitude and moving toward higher terrain. From dual-Doppler analysis, in the early stage, the isolated thunderstorm cell contained a single updraft. The updraft was located at front of the storm and with its maximum intensity of 4 ms^(-1) at 4km height. Low level convergence and upper level divergence was shown in the flow structure. Weak downdraft was found at rear portion of the storm and separated from the major echo region. No cold pool was observed at low level. These results suggest that the downdraft was not caused by water loading. The entrained air from the unsaturated environment probably played the key role.The system was organized by merging isolated convective cells at different life stages. The cell had distinct downdraft in its mature stage, but no pronounced precipitation accompanied, so as no pronounced gust front at the surface. These results suggest that the development of the storm was not owing to the local convergence between the outflows produced by the precipitating thunderstorm and the impinging moist enviromental flow. It is suggested that the lifting mechanism for the new cell development is possibly due to the upslope motion caused by the uneven heating at the terrain slope. This is consistent with the observations that new cells developed first at the slope area.In this paper, it is also suggested that the late-afternoon development of organized thunderstorm complex in central Taiwan is closely related to the penetration of sea breeze circulation from coastal area into sloping area and provided the extra moist air for storm intensification.

並列關鍵字

Mountainous thunderstorm system ； Precipitation and kinematic structures ； Dual-Doppler analysis ； Upslope motion ； Sea breeze circulation ； TAMEX case study