梅雨鋒面之鋒生過程分析

A Study on the Frontogenetical Processes of Mei-Yu Front

林松錦(Song-Chin Lin) ； 王溫和(Wen-Ho Wang)

《大氣科學》 20卷1期 (1992/03) Pp. 63-79

摘要

在每年的初夏，當東亞地區正處於東北季風與西南季風的交替轉換季節，於天氣圖上從日本南方向西南延伸至華南地區經常可見有一近似滯留且略呈東西走向的鋒面。國內將這種鋒面稱之為梅雨鋒面，而日本則稱為“Baiu front　”。在此時期的降水性質，一般皆屬於連續性的，間或夾帶著雷陣雨或豪雨，每每造成相當的災害。本文利用1979年FGGE Level III的網格點資料，並以位溫、比濕及相當位溫等不同形式的鋒生方程式來探討何種物理量的鋒生函數較能顯現出梅雨鋒面的特徵。此外，也藉由資料的診斷分析來了解鋒生過程中究竟何種機制最為重要。分析結果發現，相當位溫的鋒生函數對於梅雨鋒面的鋒生過程有較理想的描述。因其在梅雨鋒面區不但有較大的正鋒生值存在，而且其鋒生區也有如鋒面般的帶狀結構。而這些帶狀結構的正鋒生值會隨著鋒面的移動而移動，與實際鋒面的發展情形相當配合。不考慮非絕熱效應時正鋒生值的大小，在1000 hPa以變形項與輻合項的貢獻為主，在850 hPa則以變形項最大，輻合項次之，扭轉項最小。分析結果亦顯示，伸張變形的作用大於切變變形，而且伸張變形在鋒生過程中幾乎都是正的貢獻。而切變變形的作用，在鋒面區裡有時為鋒生有時則為鋒消。這兩種變形作用在梅雨鋒面帶中，並未發現有偏重於鋒面的東段或西段的現象。

關鍵字

梅雨鋒面 ； 鋒生

並列摘要

During the early-summer season in the East Asia, when the prevailing wind changing from northeasterly to southwesterly, a quasi-stationary and nearly east-west oriented front usually can be observed extending from southern Japan southwestward to southern China. Such a front is called a Mei-Yu front in China and a Baiu front in Japan. The precipitation during this period is usually continuous and often characterized with hazardous rainfall. In this study by using FGGE Level III data, frontogenetical functions in terms of potential temperature, specific humidity and equivalent potential temperature are computed to understand the frontogenetical processes of Mei-Yu front. Results show that frontogenesis in terms of equivalent potential temperature as better description on the frontogenetical processes of Mei-Yu front. Positive values of frontogenetical function are found in the frontal area, the structure and movement of this positive frontogenetical area also consist with the development of the Mei-Yu front. When diabatic effect is not considered, deformation and divergence are the major contributions to the positive frontogenesis at the 1000 hPa level. While at the 850 hPa level, deformation plays the dominant role and the twisting term has the least contribution. As for the deformation contributions to the frontogenesis in the frontal area, the effect of stretching deformation is almost always frontogenetical and the effect of shearing deformation may sometimes be frontolytical. The effects of both types of deformation on the frontogenetical functions are similar in the eastern and the western section of a Mei-Yu front.

並列關鍵字

Mei-Yu front ； frontogenesis