TOGA COARE密集觀測期間寒潮個案分析與模擬

Cases Study on the Analysis and Simulation of the Cold Surges during the Intensive Observation Period of the TOGA COARE

張庭槐(Ting-Huai Chang) ； 柯文雄(Wen-Shung Kau)

《大氣科學》 23卷2期 (1995/06) Pp. 145-177

摘要

寒潮和梅雨、颱風及乾旱並列台灣四大災變天氣，爆發時伴隨的強烈天氣突變，常造成農漁業在經濟上的慘重損失。本文使用ECMWF之網格資料分析TOGA COARE密集觀測期間侵台之寒潮個案。結果顯示，在寒潮爆發前，500mb烏拉山脊的建立是寒潮爆發的重要指標。寒潮爆發時由300k等位溫面的位渦分析顯示，冷空氣南侵的路徑可歸納成兩大類，其一自西歐高空東行越過烏拉山脊後，沿青康藏高原東側往下往南侵襲台灣及南海，其二為西亞高空東走之氣流，受阻於青康藏高原而分裂，繞行其南側之分支在華北以反氣旋式環流，迴流南侵台灣。此二類路徑間的過渡期南侵的路徑亦自西歐東行，越過烏拉山後沿30°N附近之緯度東走，在西太平洋上轉向西南迴流南侵台灣。1993年（個案D、E）之高層（200mb）緯向強風區的位置比1992年（個案A、B）偏南，其位置變化的時間和路徑變化的時間有相當的一致性。透過資料之分析，並無法確立寒潮爆發後，其和熱帶對流增強之相關性。我們利用數值模擬顯示寒潮南侵會增強南海附近低層輻合，當附近原有對流擾動存在時，則會加強對流，但當附近並無較明顯之擾動時，此低層之輻合亦無法維持。若冷空氣侵台的路徑是來自西太平洋上空之迴流，亦會引發熱帶西太平洋上低層輻合之加強，此時若有配置良好之低頻運動系統時，對流可明顯加強，否則輻合加強並無法使對流加強。

關鍵字

寒潮 ； 等位溫面位渦 ； 數值模擬

並列摘要

Cold surges, Mei-yu, typhoons and droughts are four main natural disasters in Taiwan area. Upon cold surges outbreak, weather patterns will change abruptly and severely, usually accompanied by enormous losses on economics of agriculture and fishery.The data from the ECMWF grid points are applied in this paper to analyze cold surges cases invading the Taiwan area during the intensive observation period of the TOGA COARE. Results indicate that the important pre-cold-surge phenomena direct to a 500mb ridge field built around the Ural mountains. Right upon its outbreak, from the analysis of the potential vorticity on 300K isentropic surface, the southbound invasion path of the cold air can be classified into two categories; one is moving eastward from the high levels of the western Europe, after crossing over the Ural mountain, it flows along the eastern brink of the Himalayas, downward and southward, toward the Taiwan area and the South China Sea, the other is streaming eastward in the upper levels of the western Asia, obstructed by the Himalayas and split, the south-brink curving branch forms as an anticyclonic circulation in the northern mainland China with its circulation invading the Taiwan area. The southward invasion path during the transitional phase within this period is also eastward-bound from the western Europe, upon crossing the Ural mountains, the air is eastward moving, following the air-stream around 30°N and turning into southwesterly circulation on the western the Pacific Ocean and further invading the Taiwan area. The position of the zonal strong-wind zone of the upper level (200mb) in 1993 (case D, E) is situated more southward than that in 1992 (case A, B), the time of changing is consistant with path changing.Through data analysis, the correlation between the onset of the cold surges and the enhancement of the tropical convections can not be vividly certified. By adopting the numerical simulations, it indicates that the southward invasion of the cold surges will enhance the low-level convergence around the South China Sea. Suppose there already exist convective disturbances, the convection will be intensified; but, when there appear no obvious convections around, the low-level convergence can not sustain itself. If the invasion path of the cold air is from the circulation of the upper level on the Pacific Ocean, it will also ignite the enhancement of the tropical low-level convergence on the western Pacific Ocean, if superposed upon with suitable low-frequency motion systems, the convection will clearly be enhanced; otherwise convergence enhancement can not force convection to intensity.

並列關鍵字

Cold surge ； Potential vorticity on isentropic surface ； Numerical simulation