| 摘要: | 在台灣地區的冬季天氣災害主要來自於大陸冷氣團南移造成寒潮爆發，且持續性的低溫為農漁業及民生帶來很大的災害。寒潮個案中，在冷空氣潰流發生前常有一割離低壓事件，並且在北方伴隨有高壓生成。割離低壓是發生在中/上對流層的綜觀尺度天氣系統，過去的研究多強調割離低壓伴隨強降水事件造成的災害。本文將藉由此個案討論割離低壓對於寒潮事件的影響。 利用日本氣象廳北半球500hPa印刷天氣圖(1996/04/01-2005/03/31)進行割離低壓的氣候特徵統計。結果顯示，在這十年期間，共有902個割離低壓事件；發生頻率較高的區域在歐洲大陸及北太平洋區域。四、五月份為發生較頻繁的月份；生命期小於三天的割離低壓占60%以上，生命期大於七天以上的個案占9.1%。割離低壓發生時，在其北方時有異常的高壓脊存在，進而在割離低壓北方形成一封閉的高壓區域；此現象發生的頻率為18.98%，尤其是發生在生命期九天以上的個案。 本研究選取2004年12月15日至29日發生在裏海北方的割離低壓個案做診斷分析，並討論它對東亞冷氣團發展與潰流的影響。發現在割離低壓個案期間有數個不同的噴流伴隨，因此，本研究利用噴流特性將割離低壓的生命期分為(Ⅰ)發展、(Ⅱ)成熟及(Ⅲ)消散。割離低壓形成前，200hPa的暖心結構已經存在， 60小時後，冷心結構出現於500hPa。受到溫度結構改變的影響，使得穩定度在中對流層增加，上對流層減少。發展期期間，位渦局部變化的增加有助於割離低壓的旋生。進入成熟期之後，受到割離低壓與其北方高壓滯留，使得地面冷氣團加強；從25-26日的位渦剖面可清楚看見位渦往地面冷氣團傳送的情形。進入消散期之後，在500hPa開始出現位渦垂直平流的正貢獻；且對流層漸漸回復。此時，冷空氣潰流，寒潮爆發。 In winter, migration of cold air from the north causes persistent low temperature in Taiwan, causing damage. In previous research, there were cut-off low formations before cold surge, and accompanied with a high pressure in the north. Some researches show that cut-off low often causes flash flood. The purpose of this paper is to evaluate the impact of cut-off low on cold surge. Daily Northern Hemisphere 500hPa weather chart from Japan Meteorological Agency Tokyo were need to find out cut-off low events. Results show that there were 902 events between 1996/04~2005/03. The maximum cut-off low frequency was observed over Europe and North Pacific. When cut-off low occurs, they are often accompanied with an abnormal ridge in the north, causing a high pressure formation. This kind of cut-off low usually lives longer than 9days. The case spans the period 15-29 December 2004, and occurred at northern Caspian Sea. During the life of cut-off low, there are several jets accompanying with cut-off low. According to the characteristic of jet, the whole lifetime divided into three stage: (i) develop, (ii) intense, and (iii) decay. Before cut-off low formation, warm core was at 200hPa. After 60 hours, a cold core happened on 500hPa. Due to the change of temperature structure, stability increased in mid-tropophere and decreased in upper-troposphere. During the intense stage, cut-off low and the high pressure are stationary, at the same time, the surface high intenes. On the profile of the potential vorticity, it observed that potential vorticity transport from upper-troposphere to surface. When cut-off low decayed, the tropopause ascends. At the same time, cold surge occurred in Taiwan. |
