暖季弱綜觀強迫下中北台灣午後對流的氣候特徵

Study on the Warm Season Afternoon Convection over Northern and Central Taiwan

陳泰然(George T. J. Chen) ； 周鴻祺(H. C. Chou) ； 廖珮娟(P. C. Liao) ； 楊進賢(J. S. Yang)

《大氣科學》 37卷2期 (2009/12) Pp. 155-194

摘要

本文利用民航局桃園國際機場C波段都卜勒氣象雷達回波資料、中央氣象局板橋探空站資料以及ECMWF網格資料，分析1999-2006（缺2003）年暖季（5-8月）弱綜觀環境條件下台灣中北部176個午後對流個案，依5月、6月、7月及8月各月，以及梅雨季（5-6月）與夏季（7-8月）兩季，探討對流活動與對流條件之氣候特徵。
結果顯示，梅雨季北台灣對流有向東北移動及／或發展之勢，而中台灣對流沿地形向東移動及／或發展之勢較向西向平地移動及／或發展之勢更為顯著。夏季北台灣對流有明顯向台北盆地與向西向緩坡及／或平地移動及／或發展之勢，而中台灣對流有先向東向地形山坡，後向西向平地沿海移動及／或發展之勢。
中北台灣對流日初生期梅雨季與夏季之對流發生頻率在1100 LST均少，1200 LST梅雨季較夏季增加更多，兩季均在1300 LST急速增大為最大值，且夏季較梅雨季為大，之後逐漸減少，於1500 LST急速減少。成熟期，夏季與梅雨季之對流發生頻率在1400 LST均小，兩者均在1500 LST急速增大為最大值，且梅雨季較夏季為大，之後夏季頻率逐漸減少，但梅雨季頻率則遲至1700 LST才快速減少。梅雨季與夏季初生期對流活躍度，各月北台灣均較中台灣活躍，中台灣5月對流最不活躍，北台灣對流最不活躍月份則發生在7月。成熟期之對流活躍度，中台灣以5月最不活躍，而6月最活躍，北台灣則以6月最活躍，而7月最不活躍。
梅雨季與夏季午後對流之綜觀環境條件有明顯差異，梅雨季對流日相較於前一日而言，有較強暖濕西南氣流，較大的CAPE，較有利對流發展。夏季對流日相較於前一日而言，暖濕西南氣流稍弱，但有較大的CAPE，此較大的CAPE與午後對流顯係由局部環流之發展而來。

關鍵字

弱綜觀環境強迫 ； 午後對流 ； 環境條件 ； 對流可用位能

並列摘要

The C-band Doppler radar reflectivity data at Taoyuan International Airport of CAA, the rawinsonde data at Panchiao station of the Central Weather Bureau, and the grid data of European Centre for Medium-Range Weather Forecasts (ECMWF) were used to identify the afternoon convection during May-August from 1999 to 2006 under weak synoptic forcings over northern and central Taiwan. The climatological characteristics of convective activities and environmental conditions of the selected 176 cases were investigated, classified into every single month, the Mei-Yu season (May-June), and the summer season (July-August).
Results showed that in the Mei-Yu season, the convection over northern Taiwan tended to move and/or to develop northeastward whereas over central Taiwan, the tendency for the convection to move and/or to develop eastward along mountain slope is greater than that of westward movement towards the plain area. In the summer season, the convection over northern Taiwan tended to move and/or to develop towards the Taipei Basin and westward towards the slope and/or the plain area, while the convection over central Taiwan tended to move and/or to develop eastward towards the slope first and then westward towards the plain and the sea.
Over northern and central Taiwan, the convection frequency at initial stage was few at 1100 LST both in the Mei-Yu season and in the summer season and increased rapidly to its maximum at 1300 LST. At mature stage, the convection frequency was few at 1400 LST for both seasons and increased rapidly to its maximum at 1500 LST. At initial stage, the convection was more active over northern Taiwan than over central Taiwan for each month. The convection was most inactive in May over central Taiwan and in July over northern Taiwan. At mature stage, the convection was most inactive in May and most active in June over central Taiwan. Whereas over northern Taiwan, it was most active in June and most inactive in July.
Results also showed that synoptic situations and environmental conditions were quite different in the Mei-Yu season and the summer season. In the Mei-Yu season, the stronger warm moist southwesterlies and larger CAPE occurred on the convection day. On the other hand, the weaker warm moist southwesterlies and larger CAPE occurred on the convection day in the summer season, suggesting the key role of local circulation in the afternoon convection development.

並列關鍵字

Weak synoptic forcing ； afternoon convection ； environmental condition ； convective available potential energy