東亞夏季季風與梅雨鋒（一）：雲型季風指標

The Asian Summer Monsoon and Mei-Yu Front (Part I): Cloud Patterns as a Monsoon Index

林博雄(Po-Hsiung Lin) ； 林和(Ho Lin)

《大氣科學》 25卷2期 (1997/06) Pp. 267-288

摘要

印度夏季西南季風肇始，索馬利噴流與印度雨季的來臨，是北半球春夏季節轉變完成的大氣環流特徵；然而東亞地區的春夏季節轉變過程，天氣型態特徵上是中緯大氣與熱帶大氣交互作用下的滯留鋒面，引發的連綿雨季現象為主。資料分析顯示，以地區性雨量突增或是風向氣候性改變的季風傳統定義，來界定東亞夏季季風肇始日期，是模糊難以判斷的；因此尋求有效的東亞夏季季風肇始指標，做為個案討論或是年際合成分析的依循是必要的。透過日本地球同步衛星近似黑體輻射溫度與數值模式網格資料的組合，本文設計了四種雲型，有組織性對流雲、一般性對流雲、有組織性對流雲石占以及有組織性中層層雲，用來討論1985年到1991年東亞夏季季風環流演變的階段性特徵。透過這些雲型數量的時間序列分析，可以清楚地界定日本、長江梅雨入梅的日期，以及孟加拉灣雲系與西南季風發展的過程，同時也有效地顯現華南前汛期雨帶之中的層雲與對流雲混合特徵。結果顯示，雲型分類可以做為一種有效的季風環流指標。因此經由南海地區的對流性雲型數量的突增現象，七年的東亞夏季季風瞬變期被標註出來，同一時期的大氣環流重要特徵，是南海海域上空風場，由準滯留的反氣旋封閉環流轉變為氣旋式環流，這一南海高壓消長的機制與細節有待進一步探討。

關鍵字

季風 ； 肇始 ； 地球同步氣象衛星 ； 梅兩鋒

並列摘要

The onset of Indian summer southwestern monsoon (ISWM), Somali jet and rainfall season arrival on India Peninsula are the main characteristics of spring-summer seasonal change in northern hemispheric circulation. In the meanwhile, shallow and transient frontogenesis induced by midlatitude-tropic interaction causes the prolonged raining period in eastern Asian area. Climate data shows that the onset of eastern Asian summer monsoon (EASM) is identified hardly by the general law of monsoon onset phenomenon which likes sudden increment of rainfall or sudden change of wind direction. A new and valid monsoon onset index is the base to study the case study or composite study of EASM dynamics. New cloud patterns were produced by combining radio sounding data and GMS temperature of black-like body data in this study. Four patterns, non-organized convective cloud, organized convective cloud, organized anvil cloud and organized stratus cloud, were used to study EASM circulation evolution from 1985 to 1991. The analysis of cloud patterns number time series guided not only the beginning day of Baiu front in Mainland and Japan, the onset phenomenon of deep convective cloud cells and southwestern flow in Bay of Bengal exactly, but also caught the feature of blend cloud patterns which embedded in the Mei-Yu fronts around Taiwan. The results show that cloud patterns would be a valid monsoon index. Through the convective cloud patterns increment around South China Sea, the transient periods of EASM setup in these seven years were also listed. It suggests that ”transient period” idea would be more suitable than ”onset” meanings for EASM circulation setup. Also the main weather feature in EASM transition period is the quasi-stationary anti-cycle circulation on the South China Sea disappearing dramatically. The physical mechanism of this phenomenon will be studied next step.

並列關鍵字

Monsoon ； Onset ； GMS ； Mei-Yu front