2008年「西南季風實驗」(SoWMEX)第八次密集觀測 (IOP-8) 期間,台灣地區先後發生兩波豪雨,其綜觀環境由同一道梅雨鋒面所主宰。第一波豪雨為 6月 14 至 15 日,在大陸東南沿岸與台灣海峽上有組織良好的中尺度對流系統(颮線) 發展,並由西向東往台灣地區移行。第二波於 6 月16至17 日,則是在台灣西南方海面上形成一局地對流系統,兩波對流皆造成台灣中南部地區的豪大雨。
地面天氣圖與分析資料顯示,梅雨鋒面於 14 日時最接近台灣地區,其個案期間鋒面皆無通過台灣。第一波降水之綜觀條件於中層有明顯的槽脊系統,並於槽前的正渦度平流區伴隨著低層噴流(LLJ)之發展。此LLJ一方面補充水氣支持對流的持續發展;另一方面則提供低層垂直風切,組織對流使其向東移行。第二波降水之綜觀條件較不顯著,LLJ位置隨時間由東向西退,與第一波相比則較不利於降水系統之移行。
本研究使用日本名古屋大學之非靜力雲解析風暴模式(CReSS),模擬 2008 年 6 月 12 日至 18 日台灣地區之暖季降水個案。進一步透過唯有綜觀環境變化(全海洋表面)、及唯有局部環流日夜變化等(全、無地形)敏感度測試實驗進行比對。結果發現,於無綜觀有利條件下(唯有日夜變化),將不利於降水系統之移行,顯示此個案期間主要由綜觀環境在控制大部分降水的時間與位置,但局部環流對於降水移行仍有一定的調節作用。
此外,實驗結果亦顯示,低層盛行氣流因受到台灣地形的阻擋產生繞流現象,有利降水系統於台灣海峽上輻合增強,使降水集中於台灣中部平原。透過平坦地形及全海洋表面實驗,降水系統於海峽上增強現象則較不明顯,降水易發生於台灣偏北處、甚至移至東北部。且因地形阻擋所造成之冷池現象,對於第二波之後造型降水系統發展與維持亦扮演著重要的角色。
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