葛拉絲颱風（1994）及其受到台灣地形影響之研究　第二部分：數值模擬

The Effect of Taiwan Terrain on Typhoon Gladys (1994) Part II: Numerical Simulation

吳俊傑(Chun-Chieh Wu) ； 卓瑜甄(Yu-Tseng Cho) ； 顏自雄(Tzu-Hsiung Yen)

《大氣科學》 27卷1期 (1999/03) Pp. 29-59

摘要

本研究選取1994年侵襲台灣之GLADYS颱風進行觀測分析與數值模擬。在第一部份（見吳等1999），主要是針對GLADYS颱風之觀測資料進行分析討論。第二部份（即本文）則使用GFDL颱風預報系統對GLADYS颱風進行預報及模擬。在預報結果方面，GFDL颱風模式大致已能掌握葛拉絲颱風的路徑運動。在路徑預報結果之分析中，透過深層大氣平均流場計算所得之駛流場和颱風相對位置的比較，我們發現颱風的運動和駛流場大致相當吻合，但當有明顯地形存在時，颱風運動和駛流場間會有明顯的交角存在，透過此分析過程我們發現以深層大氣平均流場來計算颱風駛流時，應考慮颱風發展的垂直高度，此與Dong與Neumann（1986）之結果一致。而地形的存在除會使颱風運動和駛流不吻合外，當颱風移經台灣時，颱風移速會有先減速再加速的現象，當颱風非常接近台灣時會開始明顯的北偏（因葛拉絲颱風的路徑較偏台灣北部），並呈一氣旋式的路徑經過台灣。此外，由預報結果也顯示出半徑較大的颱風其受地形的作用較明顯且所受β效應的作用也較大，會造成模擬結果中颱風路徑北偏的程度較大。在強度預報方面，GFDL颱風模式之預報結果中，對葛拉絲颱風自8月30日12 UTC至9月1日00 UTC其強度加強的趨勢無法很好的掌握，但對於颱風接近台灣地形及登陸時強度減弱的趨勢有不錯的預報。由位渦度收支診斷中我們亦得知地形造成颱風登陸後強度減弱的機制主要是透過摩擦作用直接或間接的效應和截斷水汽來源。由相對濕度及風場的分析，我們也可了解在颱風登陸地形前，颱風強度的減弱是由於地形會影響颱風風場和雨帶結構的對稱性而造成颱風強度的減弱。另外，我們亦模擬到經由焚風效應所形成位於花蓮附近之副低壓，並與本文第一部份由觀測分析所得的副低壓現象相當一致。透過此真實個案由GFDL颱風模式的預報結果和對照實驗之比較，我們已較了解地形對颱風以及環境流場的影響。藉由此研究，我們已能將模式遇到地形時颱風定位的誤差以及觀測資料的不足對颱風預報之影響等問題突顯出來。希望在未來我們能將二個問題加以改善，並提高模式的解析度以及將較準確且較多的觀測資料（如雷達資料或透過四維資料同化增進分析場的準確度）應用在颱風預報作業上，以改進颱風預報結果。只有當颱風預報結果和真實觀測現象十分一致時，我們才可定性且定量的探討地形對颱風和環境流場的影響，也較能明確地探討各現象的生成機制。

關鍵字

GFDL颱風模式 ； 副低壓 ； 颱風－地形之交互作用

並列摘要

Due to the complex terrain and elevated Central Mountain Range (CMR) in Taiwan, a typhoon often experiences great change in its movement and circulation when it moves over Taiwan. An observational study of Typhoon Gladys (1994) has been shown in Part I. In this paper (Part II) we also perform numerical integrations using Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model. To understand the effect of Taiwan topography on Gladys, four numerical experiments are performed. The first serves as the control experiment which makes a 72-hour forecast using the full GFDL hurricane prediction system with a bogused Gladys's vortex; the second excludes the Taiwan topography; the third removes the bogused vortex; the fourth contains a bogused vortex with doubled size and intensity.Results from the above numerical simulations indicate that the movement of Gladys mainly follows the environmental steering current. Existence of the Taiwan topography results in the deceleration of Gladys's translation speed when it approaches Taiwan, then the acceleration after it passes Taiwan. In general, a cyclonic track induced by the Taiwan topography is found. Although the GFDL hurricane model forecast underestimates Gladys's intensity, the model can capture the evolution of Gladys's intensity, especially the weakening of Gladys during landfall. Other meso-scale phenomenon, including the pattern of heavy precipitation and the formation of secondary lows, are well simulated from the model, though their locations are somewhat different from those in observations. These differences may be resulted from the inadequate resolution of the model topography. Note that small errors in predicting the large-scale flows can result in large errors in the meso-scale features above the high terrain.Results from the comparison of the model experiments indicate that two secondary lows to the west of CMR form due to the environmental easterly flow over CMR, but the secondary low near the southeast of Taiwan occurs due to the warming effect from the downslope current associated with Gladys's circulation. The experiment with a stronger and larger typhoon vortex shows that the motion of such vortex is affected by stronger beta effect, thus leading to different track. The evolution of Gladys is also studied from the potential vorticity perspective. It is shown that the Taiwan topography affects Gladys's intensity primarily through its cutoff of the water vapor supply, which is the essential energy source for storm maintenance.

並列關鍵字

GFDL hurricane model ； Secondary low ； CMR ； Bogus