台灣與東亞之夏季季風降雨變化

Summer Monsoon Rainfall Variation over Taiwan and East Asia

陳慶昌(Tsing-Chang Chen) ； 嚴明鉦(Ming-Cheng Yen) ； 王世宇(Shih-Yu Wang)

《大氣科學》 35卷4期 (2007/12) Pp. 305-352

摘要

台灣位處東亞大陸及西太平洋交界，東西向介於季風西風帶與貿易東風帶之間，南北向受中緯度與熱帶天氣系統雙重影響，是研究東亞夏季季風良好的基點。在東亞季風獨特的生命週期支配下，台灣的夏季降雨天氣型態比起其他季風區更複雜，降雨變化不但受制於不同天氣系統，本身亦包含日變化與季內變化，這些特性促成許多東亞降雨的研究。美國Iowa州立大學與國立中央大學自1999年以來密集合作，進行了多項與台灣降雨有關之研究，本文旨在系統性地回顧這些研究成果，並依兩大主題來探討。
一、大尺度季風環流，其中可細分五項議題：1.東亞季風之生命週期變化：其活躍－間歇－恢復三階段的生命週期充分反映在降雨季節變化上，這個降雨週期顯然與東亞常見的各種降雨天氣系統密切相關。
2.不同天氣系統對於降雨的氣候衝擊：在不同季風週期之中，降雨天氣系統可產生具有該週期環流場特性的降雨分布。
3.由主要季內模（30-60天與12-24天模）所支配的季風降雨變化：這二個季內模均勻地顯示在台灣平原與高山的測站資料中，並調節季風生命期。
4.梅雨季雨暴的動力過程：雨暴產生自中對流層的擾動，在傍晚發展於中南半島北部及中國西南部，或在凌晨發展於南海北部等地；位於中南半島及孟加拉灣上空之中對流層季節短波產生一道槽前噴流，是驅動雨暴擾動的主要動力來源。
二、地區性天氣系統，可分出下列三項議題；
1.台灣風場與降雨日變化：台灣島的海風於下午14時輻合最強，但對流降雨則是16∼17時才達極值，並在西部山坡地形成一道午後/傍晚對流雨帶。日降雨另具有一個發生於清晨的峰值，可能是由山脈下坡風破壞夜間穩定層所導致。
2.台北盆地顯著的午後雷雨：由淡水河及基隆河二道入口所引進的海風，於近午時分遭遇盆地東南側山坡地，並受到都市熱島效應激發午後對流降雨，因此最大降雨時間（下午三點）比起中南部提早近2小時。
3.西南部沿海地區的清晨降雨：當濕暖的西南季風遭遇到夜間較為乾冷的陸風時，容易在凌晨至清晨時分於台灣西南沿海產生一道氣流輻合帶，並引發局部陣雨。
降雨乃季風系統中最重要的元素，這些研究回顧表示，台灣的降雨變化可作為剖析各種尺度之東亞季風活動的良好指標。

關鍵字

東亞季風 ； 台灣季風 ； 季風環流 ； 降雨變化

並列摘要

Around Taiwan, the environmental flow in the east-west direction is formed by the monsoon southwesterlies of the continental thermal low to its west and the trade southeasterlies of the western North Pacific anticyclone to its east, and the intrusion of midlatitude weather disturbances and the northward propagation of tropical cyclones. Thus, the rainfall variations of Taiwan and its vicinity are affected by variations of the circulation elements of its environment. A series of systematic studies of these rainfall variations were made in the past decades from two different perspectives: rainfall variations caused by the large-scale monsoon circulation, and mesoscale regional weather systems. A review of these studies is presented in terms of these two perspectives:
I. Large-scale monsoon circulation – Five aspects of this area are covered by the first perspective:
1. The East-Asian monsoon life cycle (active-break-revival) is established by rainfall produced by different weather systems during different phases of this monsoon.
2. Rainfall contributions from different weather systems across Taiwan were estimated from each portion of the monsoon life cycle.
3. The rainfall variations caused by two intraseasonal (30-60 day and 12-24 day) modes can be detected both in the plains and the tallest mountains of Taiwan.
4. Meiyu rainstorms originate in the northern Vietnam-southwest China region during evening and in the northern part of the South China Sea during early morning. Geneses of these storms occur along the midtropospheric jet ahead of a climatological short-wave trough in southeast China and are modulated by the land-sea thermal contrast.
5. The diurnal variation of continental-scale rainfall over East Asia exhibits a clockwise rotation which is caused by the modulation of the westward-propagating global diurnal mode by the east-west land-sea thermal contrast and the north-south differential heating.
II. Mesoscale regional weather systems-Three issues were addressed from the second aspect:
1. The round-island diurnal variation of rainfall follows the round-island divergence/convergence of surface flow: the surface convergence reaches its maximum at 14 LST, but the maximum rainfall occurs at 16-17 LST along the western plains and mountain slopes. Another minor peak of rainfall at 05 LST appears along the western slopes and is likely generated by the convergence between the downslope surface flow and the monsoon southwesterlies.
2. The afternoon thunderstorm inside the Taipei basin is induced by the interaction between the sea breeze along the Tanshui and Keelung river valleys and the mountain slopes southeast of the basin. The convection generated by this interaction is enhanced by the urbanization of Taipei.
3. An early morning rainfall is produced in southwest Taiwan at 5 LST by the convection developed through the convergence between the cold downslope flow and the warm moist monsoon southwesterlies.
Because rainfall is the most important ingredient of a monsoon system, it becomes clear from this review that time variation of Taiwan rainfall can be used as a monsoon index to explore the multiple-timescale process of the East-Asian monsoon system.

並列關鍵字

East Asian monsoon ； Rainfall variation ； Taiwan monsoon ； Monsoon circulation