Abstract
摘要:本研究目的在探討颱風通過台灣地形與通過前後,其路徑及通過時間長短所產生的變化,利用目前所擁有的傳統和非傳統觀測資料,同時針對颱風本身的條件因子及環境因素,整合過去侵台颱風現有資料,希望了解颱風在經過台灣地形時所產生的動力機制及其物理特性,並建立概念模式,作為改進並提供颱風預報作業人員在實際作業的參考。在前兩年度已經完成1977到2007年侵台颱風資料的整理與分析,並得到颱風通過台灣時路徑偏折與颱風移行速率的特徵,並且以數值實驗建立颱風侵台登陸前路徑偏折的概念模式。
前兩年研究結果顯示,颱風平均移速最大值出現在在陸期間比例最高,颱風登陸前均有加速的情形,颱風通過台灣前後主要呈現登陸後加速、離陸後減速的狀況。不同登陸區域的颱風移速分析顯示,在東北部登陸的颱風其最大平均移速有較大的機會發生在登陸前3小時,在陸平均移速為3組中最小的。東南部與西部登陸的颱風之最大平均移速則較容易發生在颱風在陸期間。另外,西邊登陸的個案在陸期間平均移速值均較同緯度分區的東海岸登陸颱風在陸期間平均移速值大,差值約大於5 km/hr。
在颱風登陸前一時段至颱風在陸期間,颱風登陸點前後的移向變化均為向左偏為主。以2階多項式客觀判斷48個統計結果顯示左偏個案有30,無明顯偏折個案(偏折角度≦5°或右偏)有18個個案,偏折發生在台灣外海約50~100km,在緯度23.5°以北大偏折角個案數較多,緯度23.5°以南較少。緯度23.5°以北偏折個案與偏折前移速相關係數達到0.66,緯度23.5°以南大偏折角個案多發生在緯度23°,移向偏北,與渦旋平流過山的時間尺度d/U有較好的關係。偏折角度與颱風最大風速則沒有明顯關係存在。本研究中也以淺水模式、台灣地形資料,單純探討有背景風場之渦旋與地形之交互作用。從實驗中我們發現與觀測有類似的統計特徵,颱風登陸前路徑左偏的主要控制因子為颱風移速與台灣南北不對稱分布之地形,在緯度23.5°以北偏折的颱風個案,偏折角度主要受颱風移速影響,偏折角度與移速呈現反比。在緯度23.5°以南偏折之颱風個案,由於台灣23度以南之地形大小高度皆較23度以北小,所以除了移速外尚須考慮地形的大小造成的偏折效應,以颱風被平流過山之時間尺度(d/U)來包含地形大小與移速之效應,發現偏折角度與d/U大致上呈現一正比的關係。從模式中我們還發現渦旋大小也是影響偏折角的一個重要因素,大渦旋造成之路徑偏折較小。
本年度將繼續以數值模式對於颱風侵台登陸前的路徑偏折與登陸前後的移速變化進行更深入的動力機制探討,並且根據前兩年的研究結果,建立颱風登陸前6小時內路徑偏折的概念模式,探討路徑偏折與動力上重要因子的迴歸關係,提供給預報員做參考的指標。除了路徑偏折方向的研究以外,我們也將接續上年度數值模式的研究方法,更深入的探討颱風遇到台灣地形時,登陸前及在陸期間發生的移行速率的變化,並選出幾個具代表性之颱風個案來進行探討,以增進颱風侵台期間移行速度變化的預報作業,進一步了解颱風經過台灣地形時之行為與原因。
Abstract: This proposal investigates the problem of typhoon landfall in Taiwan. Specifically, the weather characteristics in the period from 6 to 12 hour before the typhoon made landfall till typhoon leave Taiwan are to be studied. We will explore how the typhoon structure, size, strength, and the synoptic background may cause the variation in the wind and rainfall pattern and the length of duration period of typhoon landfall at Taiwan. We will use the meteorological data available to construct the climatology and use the shallow water model to explore the dynamics of track diffraction during the landfall. We hope to establish a conceptual model for the typhoon landfall forecast purpose.
48 typhoons which come from southeast to northwest in direction from 1977 to 2007 are analyzed. A polynomial curve is used to define the track deflection. There are 30 cases that have a cyclonic track and turn to the left hand side before landfall. The track deflections are often happened at about 50~100 km far from Taiwan coast. From the observations we found that there are more cases which have a large deflection angle at north of latitude 23.5 degrees then the cases at south of latitude 23.5 degrees. Cases with large deflection angle at south of latitude 23.5 degrees are often translate in a northerly trend. The regression analysis shows that the deflection angles of the cases at north of latitude 23.5 degrees are affected by the move speed of typhoon before deflection. The correlation coefficient is 0.66. The deflection angles of the cases at south of latitude 23.5 degrees are more likely to be affected by the time scale of vortex crossing through topography. We also found that there is no significant relationship between the maximum wind and the deflection angle. The similar results are also shown in our numerical experiments with a shallow water model. The vortex size is another important parameter to affect the deflection angle. It shows that bigger vortices tend to have smaller deflection angles. The size of typhoon may be the explanation of the variation of deflection angles.
Typhoon translation speeds increases as it approach Taiwan. If typhoon landfall at north east coast, the maximum mean translation speed appears at 3 hour before landfall. Otherwise, typhoon landfall at south east coast has a maximum mean translation speed on topography. The duration time and translation speed on topography are concerned with the accumulation rainfall. Our analysis shows that the accumulation rainfall has a positive correlation with the duration time and a negative correlation with the translation speed on topography.
This year we are going to establish a conceptual model of typhoon track deflection six hours before landfall and analyze the correlation among different dynamic parameters. We will also studying the variation of translation speed when typhoon passing through Taiwan with numerical simulations in order to have a more thorough understanding of the dynamic mechanism.
Keyword(s)
颱風移行速度
地形
颱風路徑偏折
typhoon translation speed
topography
typhoon track deflection