- 期刊
台灣地形對理想型颱風環流變化之影響
The Influence of Taiwan Topography on the Variation of Idealized Typhoon Circulation
摘要
本文利用中尺度數值模式，探討移動性颱風入侵中尺度地形期間環流演化過程，模式中包含高階邊界層參數化、雲物理及颱風模擬之初始化過程。理想化模擬個案結果指出，颱風路徑變化會因渦旋大小、移動速度和登陸地點不同而不同。由東入侵對稱地形時，移速快的渦旋，無論大小渦旋登陸前都會向南偏；其中較小的渦旋，登陸前會南偏繞山後北偏出海，而較大的渦旋則在登陸後北偏過山。移動慢的渦旋，小渦旋登陸前會南偏繞山，大渦處在登陸前則偏向不明顯，登陸後即北偏過山。由東入侵真實台灣地形時則都會先向南偏，接近台灣北部者，登陸前會略向北偏，繞過台灣北部而行；接近中部的渦旋則會先向南偏，登陸前3小時會北偏；接近台灣南部的渦旋則會向南偏向，繞台灣南側西行，但氣壓中心呈現不連續的北跳現象。敏感度測試指出除去潛熱作用會導致渦旋南偏程度減小，又非靜力作用對水平20公里解析度個案模擬的路徑及環流變化並不重要。累積降水量主要分佈在地形東側和西南山區迎風面上之斜坡；降水量多寡和颱風登陸地點有關，當颱風接近地形的南端時降水量最大，登陸點越靠近北部則降水量會減少。移速較慢且小型的颱風個案其動量收支分析結果顯示，颱風向西接近台灣時，會因受到地形的阻擋而減速，但是在登陸前的偏向期間則會向西加速移動，主要是平流的作用，而擾動氣壓梯度力則扮演阻擋的角色；而南北動量收支指出，渦旋在偏向初期受到擾動氣壓梯度力的作用向南加速，使渦旋向南偏轉，當渦旋更接近地形時，擾動氣壓梯度力轉向北加速，渦援開始減速轉而向北，所以渦旋在登陸前的路徑偏向與擾動氣壓梯度力有相當大的關係。渦度收支的分析結果顯示，颱風渦旋在登陸前會向南偏轉，造成其渦度變化的主要因素為平流作用項和輻合輻散的作用項（即抽拉項）。水平及垂直擴散作用項的重要性主要是在山區，而其貢獻量則相反。在本模式中，適當的加入積雲參數化效應可以產生強烈颱風，初步結果指出颱風渦旋的發展與次網格尺度的積雲對流過程有相當大的關連。
並列摘要
A mesoscale numerical model is used to investigate the evolution of typhoon circulation interacting with mesoscale topography. The numerical model consists of higher-order planetary boundary layer parameterization, explicit cloud microphysics and an initialization scheme for typhoon simulations. Idealized case simulation results show that typhoon tracks are dependent on initial vortex sizes, moving speeds and landfall positions. For a westward vortex impinging at a faster speed on symmetric topography, it is deflected to the south before landfall. Smaller vortices pass around the southern part of the idealized topography and then rebound to their initial latitude, and larger vortices are deflected northward just after landfall. For westward typhoons impinging on real topography of Taiwan, they are also deflected southward before landfall. The vortex approaching northern (southern) Taiwan are deflected northward (southward) before landfall. The typhoon approaching southern Taiwan tends to exhibit a discontinuity on the track of pressure centers that appear to jump toward the north. Results of sensitivity tests indicate that the elimination of latent heating results in less southward deflection, and nonhydrostatic effects are not significant for typhoon track and circulation change resolved by a horizontal grid size of twenty kilometers. The accumulated precipitation is mostly on the upstream eastern and southwestern slopes of the terrain and its intensity is in great relation to landfall positions. The total precipitation is more (less) for a typhoon impinging southern (northern) Taiwan.Analyses of momentum budget for a slower and smaller typhoon indicate that a westbound approaching typhoon is decelerated by terrain blocking effect. The vortex is accelerated westward during its southward deflection before landfall primarily due to advection effect despite that the gradient force of perturbation pressure acts to decelerate it. The momentum budget also shows that as the vortex begins to deflect, it is accelerated southward due to the perturbation pressure gradient force. As the vortex is closer to the terrain, perturbation pressure gradient force is reverse toward the north and results in a northward movement of the track. Vorticity budgets also indicate that the typhoon will turn southward before landfall, and they are mainly contributed from both advection and divergence effects. Horizontal diffusion and vertical diffusion are important only on the orographic region. The inclusion of proper cumulus parameterization would enable the model to generate an intense typhoon, indicating that subgrid convective clouds play a significant role on the development of an evolving vortex.