颱風運動之合成分析

A Composite Analysis of Typhoon Motion

李清勝(Cheng-Shang Lee)

《大氣科學》 22卷2期 (1994/06) Pp. 247-276

摘要

本文利用探空合成分析方法，探討影響颱風轉向之重要物理過程。分析時，選取左轉、直行和右轉三類颱風，每類颱風再取3個時段，其中階段0為轉向前12～24小時，階段1為轉向前0～12小時，而階段2為開始轉向後0～12小時（或轉向中）。結果顯示，在結構特徵上，轉向前左、右轉颱風分別在運動方向的左、右方有範圍較廣的中、高層暖區。而500 hPa太平洋高壓脊的強弱，西伸或東退／分裂，與颱風運動方向的改變（左、右轉）關係密切。在風場特徵上，500 hPa在半徑4～5°以內，左轉颱風移向右方之切向風速較左方為強，右轉颱風則在左方較右方為強，直行颱風左、右方之切向風速分佈大致對稱。結果亦顯示，右轉颱風有由左向右通過颱風中心之流場，左轉颱風則相反。平行颱風運動之環境風場，三類颱風之差異主要出現於600 hPa以上，且左、右轉之風速皆較直行者為小。取200 hPa和900 hPa平均之垂直於颱風運動方向之風場，計算颱風中心在移向前、後半徑5～11°之風切，左轉颱風為氣旋式，右轉颱風為反旋式（颱風轉向前12～24小時）。500 hPa渦度收支分析結果顯示，颱風在左（或右）轉前，颱風中心之左（或右）前方半徑6～10°處，有正的A+D項（A+D項定義為水平渦度平流和輻散項之和），此種對比特徵在颱風轉向中，即不再顯著；而且在此區域，渦度平流項和輻散項兩者值之大小相當，平流項並非絕對最重要。綜合分析結果顯示，導致颱風轉向之因素，主要是環境駛流場的改變或是颱風移至一環流特徵不同環境所導致。而此環境流場特徵之差異主要反應在中、高層，且以颱風運動方向前方較顯著。

關鍵字

颱風 ； 颱風運動 ； 合成分析

並列摘要

This paper discusses the important physical processes relating to the turning motion of typhoon using radiosonde composite analysis technique. In the analysis, three categories of typhoon were selected and composited, namely the left-turning typhoon, the straight-moving typhoon and the right-turning typhoon. Each category contained three time stages. Stage 0 was 12-24 hours before the turning time period, stage 1, 0-12 hours before and stage 2, 0-12 hours after the turning motion. Results showed that the mid-upper level warm region was broader in the front-right (or front-left) direction before the typhoon turned right (or left). The behavior of the 500 hPa subtropical ridge played an important role on the typhoon motion change. The 500 hPa tangential wind within 4 degree radius was stronger in the right (or left) side for the left-turning (right-turning) typhoon composites. No apparent left v.s. right side difference presented for the straight moving typhoon composites. Results also showed that the wind field perpendicular to the motion direction was from right to left for the left-turning typhoon and left to right for the right-turning typhoon. The differences in the wind field parallel to the motion direction appeared to be pronounced above 600 hPa for three composites. The straight-moving typhoon had stronger wind speed than the turning typhoons. The 200 hPa and 900 hPa averaged, rear to front wind shear at 5-11 degree radius was cyclonic for the left-turning typhoon and anticyclonic for the right-turning typhoon at 12-24 hours before the turning time period. The vorticity budget analysis at 500 hPa showed significant contrast between the two turning motion typhoon composites. The A+D (defined as advection term + divergence term) was positive at 6-10 degree radii in the front-right (or front-left) direction before typhoon turned right (or left). Such contrast disappeared during the turning period. In addition, the magnitudes of advection term and divergence term were of the same order at these radii. In sum, our analysis had demonstrated that the most important factor leading to the turning motion of typhoon was the changes in the environmental steering flow that the typhoon was embedded in. Such change in the environmental steering flow could be due to the changes of the large scale circulation pattern or to that typhoon had moved to an environment with different flow regime. Such different flow feature appeared primarily at mid-upper levels and in front of the typhoon center.

並列關鍵字

typhoon ； typhoon motion ； composite analysis