梅雨鋒面北退與相關之南海低壓發展之個案診斷分析研究

Abstract

2012年6月13日至14日間，850 hPa天氣圖上顯示一低壓在南海北部發展並向東北方移動進入臺灣海峽，其低壓槽向東北延伸至臺灣，影響臺灣及其東方之梅雨鋒面。梅雨鋒面於13日1200 UTC 前向東南方移動，1200 UTC後轉而向北，亦即發生鋒面北退現象，且持續至 15日0000 UTC，鋒面已北退到達長江出口。鋒面北退期間，鋒上對流也隨鋒面北移而發展，於臺灣再次造成水災。本文即針對此梅雨鋒面北退個案進行診斷分析，探討鋒面北退之物理過程及南海低壓與鋒面北退之關係。 結果顯示，梅雨鋒面北退乃因13日1200 UTC以後，原來於鋒面北側盛行之東北風轉為東南風。利用渦度方程與鋒生方程進行診斷，顯示水平渦度平流(與水平相當位温平流)的作用方向向北，於鋒面北側產生顯著之正平流效應，讓梅雨鋒面伴隨之最大渦度(與相當位温梯度)向北平移，而使梅雨鋒面北退。利用片段位渦反演，診斷鋒面北側南風分量的來源，結果顯示該區於13日1200 UTC出現之東南風正是受南海低壓影響而產生。此說明鋒面北退是因為位於南海北部的低壓發展，建立氣旋式環流影響華南沿岸及臺灣地區，改變鋒面上水平平流作用的方向向北，讓鋒面北方的乾冷空氣北移，而由南方之暖濕空氣取代。其結果使最大相當位温梯度往北移動，如暖鋒結構造成鋒面北退的現象。 片段位渦反演及綜觀環境分析，亦顯示在南海北部發展的低壓強度與深對流之關係密切，低壓的垂直發展幾無傾斜，呈相當正壓之結構，顯示低壓乃透過第二類條件性不穩定(Conditional Instability of Second Kind，CISK)機制而發展增強。

During 13-14 June 2012, a low pressure system developed over northern South China Sea (SCS). It propagated northeastward and entered the Taiwan Strait with its trough extending through the northeast of Taiwan. Affected by this trough, the Mei-Yu front which originally travelled southeastward turned to move northward after 1200 UTC 13 June and until 0000 UTC 15 June. Meanwhile, organized convection developed along the retreating front and produced heavy rainfalls over Taiwan. The purpose of the present study is to better understand the dynamical mechanism of the retreat of the Mei-Yu front and the relationship between the low over northern SCS and the retrogression of the front. Synoptic evolution at 850 hPa revealed that the Mei-Yu front started its northward movement when southerly wind components appeared to the north of the front at 1200 UTC 13 June. Diagnosis on vorticity budget and frontogenetical function showed that the horizontal advection of positive vorticity and the horizontal gradient of equivalent potential temperature to the north of the Mei-Yu front was responsible for the northward movement of the front. In addition, results of piecewise potential vorticity (PV) inversion showed that the southerly wind component over the region north of the front was due to the low development over northern SCS. When cyclonic circulation associated with low affected the Mei-Yu front, winds north of the front veered from northeasterly to southeasterly, and caused the cold and dry air to retreat over the same area. Therefore, the area of maximum gradient of equivalent potential temperature associated with the Mei-Yu front was shifted northward, and the front retrogressed. Piecewise PV inversion technique also illustrated that the low system over northern SCS developed through conditional instability of the second kind (CISK) mechanism. The low developed vertically with a quasi-barotropic structure and its development was related to the latent heat release from the deep convection.