第6卷‧第4期,
197804
, pp. 378-387
颱風路徑之數值預報
- 作者:
蔡清產
- 作者服務機構:
國立臺灣大學大氣科學系
- 中文摘要:
木文乃以相當正壓模式預報垂直平均流線函數的方法,預報颱風侵襲臺灣的路徑。本文先討論預報公式、數值方法、綜觀尺度及颱風資料的處理,以及初始流線函數的計算步驟等。然後討論以1975年的妮娜(Nina),貝蒂(Betty),艾爾西(Elsie)及1976年的畢莉(Billie)等颱風為對象的預報結果。我們特別著重Helmholtz term對副熱帶高壓位移的影響,以及兩種數值方法的長短處的討論。對於Helmholtz term,我們以嘗試錯誤的方法求取最恰當的數值以表示之。數值方法方面,我們採用Arakawa Jacobian及Shuman and Vanderman Jacobian等方法。在這兩種方法的預報圖中,我們發現颱風分裂成幾個中心,因而嚴重的影響預報路徑的不合理現象。這種預報誤差是由於颱風水平尺度較小,使得數值方法產生計算能量分散(computationalenergy dispersion)的結果。為了消除此種計算能量分散的誤差及改進颱風路徑預報,我們建議採用quasi-Lagrangian的數值方法預報颱風路徑。
- 英文摘要:
Numerical Prediction of typhoon tracks inthe area of Taiwan and its vicinity by an equi-valent barotrophic model has been studied. Anarea of 38 by 28 grids with 240 km grid distanceis selected for the study. The Helmholtz termin the vertically integrated vorticity equationis empirically determined by the best forecatedmovement of the subtrapical high pressure systemin the Pacific. We selected tracks of typhoonsNina, Betty, and Elsie in 1975 and Billie in1976 to study the effects of Helmholtz term andnumerical methods. The model with implicit calculations ofthe interactions between a typhoon vortex andthe basic flow is emphasized. In other words,typhoon tracks are predicted mainly by themodel without the separation of the typhoonvortex from the basic flow. The verticallyaveraged stream function is computed by solvingthe Poison's equation from the vorticity fieldof the basic flow and the typhoon vortex. Thevorticity field of the basic flow is computedfrom the averaged wind data of 700, 500, and300 mb. Over the area with sparse data e.g.oceans and Tibeta Plateau, bogus winds are addedusing geostrophic relationship. In the vicintyof typhoon, the moving velocity of a typhoonin the past six hours is used to place the observ-ed wind data. On the other hand, the vorti-city field of a typhoon is computed from anempirical formula. In the computation of thevorticity advection term, both Arakawa's andShuman and Vanderman's Jacobian have beenused, predictions by both space differencingschemes show the problem of computationalenergy dispersion. As a result, typhoons areunrealistically split into several centers and theirmovements are seriously affected. A quasi-Lagra-nigan computational scheme is suggested to eli-minate the problem of computational energydispersion.
- 中文關鍵字:
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- 英文關鍵字:
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