華南氣壓下降之機制及其與臺灣2012年6月10～12日連續超大豪雨之關係

Abstract

1993～2020年臺灣梅雨季發生連續3天單日累積雨量高達350 mm的事件共有3個，分別為2005年6月12～15日、2006年6月8～10日及2012年6月10～12日。2005年與2006年事件為梅雨鋒面接近臺灣地區後，才造成劇烈降雨。2012年事件為唯一在梅雨鋒面未影響臺灣地區前，在臺灣中部與南部地區便已發生500 mm以上的劇烈降雨事件。因此，本文目的在探討這次獨特事件的天氣條件及其演變，以了解梅雨鋒面未影響臺灣地區前，臺灣南部與中部地區發生劇烈降雨的成因。 分析結果顯示，2012年6月10～12日臺灣地區劇烈降雨是由強烈且持續的LLJ從臺灣西南方外海朝臺灣地區傳送不穩定的暖濕空氣，與地形交互作用導致劇烈持續的降雨。LLJ的形成是由於6月8～10日華南地區低層氣壓顯著下降(7.8 hPa)，同時位於臺灣東南側的太平洋副熱帶高壓仍維持其強度，使得南海海域的水平氣壓(或重力位)梯度增強形成LLJ。 由氣壓趨勢方程診斷華南地區氣壓下降的成因發現，華南地區整個對流層受暖平流和動力效應(輻散合項加垂直速度項)影響導致華南地區發生大範圍氣壓下降。暖平流的產生是由於華北至華南地區存在大規模的合流配置，為華南地區提供西風與西南風環流，將較暖、密度較小的空氣從低緯度地區傳送到該地區。在動力效應方面，華南地區位於南亞高層反氣旋東北象限的分流區之下，在6月8～10日期間在華南地區高層輻散超過低層輻合產生淨質量流出，並進一步抵消上升運動的影響，形成有利於華南地區氣壓下降的情況。整體而言，暖平流仍是造成2012年6月8～10日華南地區氣壓下降的主要驅動因素。

Three incidents with daily rainfall achieved 350 mm for three consecutive days during the Mei-yu season in Taiwan between 1993 and 2020. They occurred from 12 to 15 June 2005, 8 to10 June 2006, and 10 to12 June 2012. The events that brought heavy rain from 2005 and 2006 occurred after the Mei-yu front approached Taiwan. However, the event in 2012 was the only one that surpassed 500 mm of accumulated rainfall in Taiwan before the arrival of the Mei-yu front. Hence, this study aims to discuss the synoptic conditions and their evolution leading to this unique event. The analysis results showed that the severe rainfall in Taiwan from 10 to 12 June 2012, was caused by the strong and persistent low-level jet (LLJ) that transported warm, moist, and unstable air from the upstream of Taiwan and then impinged on the island. The LLJ developed due to the enhanced horizontal pressure gradient when the pressure at low-levels fell significantly (by ~8 hPa) in South China during 8–10 June, but the subtropical high to the southeast maintained its strength. Using the pressure tendency equation to diagnose the cause of the pressure drop in South China, it is established that the entire troposphere in the Southern China is influenced by warm advection and dynamic effects (column divergence and transport of mass by vertical motion) resulting in a large-scale pressure fall in the area. The generation of warm advection is due to a large-scale confluent pattern in China, which provides westerlies and southwester circulation for the area while bringing warmer, less dense air from lower latitudes. Regarding dynamic effects, the Southern China is located under the diffluent zone of the northeast quadrant of the South Asian upper-level anticyclone. Between 8 and 10 June, the upper-level divergence exceeded the low-level convergence, causing a net mass outflow and further canceling out the impact of the upward transport of mass. The phenomenon was suitable for cultivating a pressure drop in the area. The warm advection was still the main driving factor for the pressure drop in the Southern China from 8 to 10 June 2012.