東亞梅雨鋒面之綜觀氣候特徵研究

On the Synoptic-Climatological Characteristics of the East Asian Mei-Yu Front

陳泰然(George Tai-Jen Chen)

《大氣科學》 16卷4期 (1988/12) Pp. 435-446

摘要

本文針對東亞梅雨鋒面進行觀測資料之診斷分析研究，利用1975~1984年5~7月綜觀天氣圖與NCAR網格點資料，以分析梅雨鋒面之綜觀氣候特徵與其成長之大氣平均環境，並探討梅雨系統與中緯度環流之交互作用。梅雨鋒面之綜觀氣候分析結果顯示，鋒生頻率隨季節減少，生命期在6月上半月之後亦隨季節變短，鋒生區則隨季節向西北偏移，鋒面出現頻率東西向最大主軸亦隨季節北移。台灣與長江流域梅雨期鋒面頻率最大區所在位置，顯然與低緯度大尺度環流息息相關。梅雨鋒與極鋒之成長環境大異其趣，前者在微弱地轉變形場且幾無緯向溫度梯度之環境形成與演變，平均南來地轉風使鋒面平均向北退移；後者在最強緯向溫度梯度區之斜壓大氣裡形成與演變，平均北來地轉風使其快速向東且向南移動。此外，發現梅雨鋒南側對流之高層輻散外流強盛，此局部哈德里胞高層之南來輻散氣流之向東柯氏加速，似為梅雨期東亞中緯度高層噴流加強與維持之一可能機制。

關鍵字

鋒生頻率 ； 鋒面頻率 ； 梅雨季 ； 梅雨鋒環境 ； 中低緯度交互作用

並列摘要

A diagnostic study of Mei-Yu frontal system and large-scale circulation during the East Asian Mei-Yu season was carried out. Synoptic charts and NCAR grid-point data in May-July 1975-1984 were used to analyze the synoptic-climatological feature and the mean atmospheric environment of the Mei-Yu front. The interaction between the Mei-Yu system and the midlatitude circulations was also studied.Results of the frontal study showed that the frontogenesis frequency decreased as season proceeded and the life time of front decreased after mid-June. The area of maximum frontogenesis as well as the axis of maximum frontal frequency shifted northward with time. Locations of the maximum frontal frequency in Mei-Yu season over Taiwan and over the Yangtze River Basin were primarily controlled by the large-scale circulations in the low latitudes.The mean atmospheric environment was quite different for the Mei-Yu front from that of the polar front. The Mei-Yu front formed and evolved in a weak geostrophic deformation field where the zonal temperature gradient was a minimum. The polar front, on the other hand, formed and evolved in a strong baroclinic atmosphere where the zonal temperature gradient was a maximum. The Mei-Yu front retrograded northward under the mean geostrophic southerlies, whereas the polar front moved southward due to the mean geostrophic northerlies.Finally, the upper-level divergent outflows due to the deep convections to the south of the Mei-Yu front were strong. The southerly branch of this local Hadley cell in the upper troposphere appeared to be a possible mechanism for strengthening and maintaining the East Asian jet due to eastward Coriolis acceleration of this southerly divergent wind.

並列關鍵字

frontogenesis frequency ； frontal frequency ； Mei-Yu season ； Mei-Yu frontal environment ； low-Iatitude and mid-latitude interaction