西南氣流影響下台灣東南部局部環流個案分析

A Study of an Orography-induced Local Circulation at Southeast Taiwan under Southerly Flow

潘大綱(Peter Da-Gang Pan) ； 戴志輝(Jyh-Huei Tai) ； 蔡晉東(Ching-Tung Tsay) ； 顏自雄(Tzu-Hsiung Yen)

《大氣科學》 32卷1期 (2004/03) Pp. 73-93

摘要

台灣在熱帶系統所引進的西南氣流影響下，西南部常出現劇烈的地形降雨；然而2002年八月五至六日，台灣及其週邊海域受輕度颱風卡莫里(KAMMURI)外圍西南氣流影響，除西南部陸地出現較為集中的降雨之外，東南部沿海的成功鎮，在八月五日單日就累積了近六百毫米的雨量，且在東南部沿海豪雨期間，中央氣象局成功、台東測站及空軍台東機場均出現異常之強陣風。由於本個案豪雨所出現的位置十分特殊，其成因值得進一步探討；本文藉由各種觀測資料，分析個案期間的綜觀與局部環境、氣流場、降雨及強陣風等分布之時空關係，並利用數值模擬結果，來說明地形所引起的局部環流特徵，在東南部局部豪雨及強陣風過程中的重要性。
觀測分析顯示，全島氣象雷達在東部海域所觀測到的回波，應區分為東部外海及東南部沿海兩個不同系統，二者的演化過程不同，而個案中局部豪雨與強陣風的出現，與東南部沿海回波的發展關係密切。模擬結果顯示，地形的分流效應，於東部外海出現垂直發展有限的輻合帶，在條件性不穩定的環境下，成為東部外海組織性對流發展的重要條件；而本研究的重點，則是在中央山脈影響下，隨著西南氣流垂直增強、增厚，在東西向的二維結構上，上游中層西風動量過山後向低層傳送，於背風面至沿海所發展出的下坡風環流特徵，此下坡風激發了垂直運動，形成東南部沿海回波的細長線狀外觀，此過程除了可說明東南部沿海局部雨量與風速的異常，同時也說明了過山氣流所導致的局部環流，對劇烈天氣之發展扮演重要的角色。因此台灣在西南氣流環境下，除了地形舉升所可能引起的迎風面降雨現象之外，還應當注意肇因於其他地形因素的局部環流過程，也有激發劇烈天氣現象的可能性。

關鍵字

劇烈地形降雨 ； 強陣風 ； 數值模擬 ； 局部環流 ； 下坡風

並列摘要

Previous research results have indicated that windward precipitation tend to be more significant at southwest Taiwan as southwesterly was enhanced by tropical systems. Common precept further suggests that windward coastal area, through terrain lifting, generally experiences higher rain amount than other regions.
Under the influences of Typhoon Kammuri during 4-6 August 2002, Taiwan and its vicinity were prevailed by southerly flow. Organized convective systems both offshore and inland were noticed. Chenggon, a small town 40 km north northeast of Taitung, received 600 mm daily rain amount. Strong wind was recorded both at Chenggon and Taitung. Torrential rain and strong wind occurred at southeastern region was out of operational meteorologists expectation. With the aids of numerical model simulation, traditional observations, radar and satellite data, we intend to reveal the possible causes for the southeastern abnormal weather event.
Our research results indicate convective activities east of Central Mountain range could be categorized into north and south potions. Northern convection was formed by low-level convergence due to topographiy-induced diffluent flow and environmental flow. The conditional unstable atmosphere further maintained its convective development. The south part mainly resides inland at all times. It caused torrential rain and strong wind at Chenggon and Taitung. Numerical simulation results indicate that southern convection was resulted from the deepening and intensifying environment southerly. It possesses obvious downslope wind circulation both mesoscale circulation and environmental low level convergence contribute to the initiation and evolution of southern convective activities. While the convergence associate with northern portion has limited vertical extend, strong and narrow convection activity accompanied with southern one. The existence and maintain of the southern convection is closely related to the orography-induced mesoscale local circulation. This study strongly suggests that under the southwesterly flow pattern forecasters should focus their attention not only at windward precipitation but also the possible occurrence of any other orography-induced local circulations.

並列關鍵字

Significant windward precipitation ； Strong gust ； Numerical simulation ； Local circulation ； Downslope wind