本研究利用WRF模式模擬柯羅莎颱風，探討西北向侵台颱風登陸前路徑打轉的機制。控制組實驗(CTR實驗)模擬結果和之前研究結果一致，颱風在靠近地形時，颱風西側風速有增強的情況，顯示出通道效應的影響。且發現在地形的東南側有渦度正值生成，且伴隨渦度變率極大值，導致颱風中心向南偏移。由擾動氣壓可知，渦度正值可能是由於此處的低壓所造成。除了通道效應，台灣東南側的相對低壓也是造成颱風路徑南偏的機制。根據空氣質點軌跡顯示，部份氣流過山後下沉增溫，造成此低壓之生成。根據渦度收支分析，輻散項以及渦度平流為此渦度變化的主要貢獻。 本研究另外使用中央大學中尺度模式進行理想個案的模擬，探討西北向颱風渦旋侵襲理想地形的路徑變化。模擬結果顯示，理想個案和上述真實個案有相似的機制，出現了通道效應以及在地形東南側渦度正值的生成，因此打轉機制不只是為柯羅莎颱風所特有。但颱風登陸位置若南移一些，即無正渦度值於地形東南側生成，颱風於登陸時並無向西南方偏移之現象。而在初始颱風較小實驗中，雖然較小颱風仍可伴隨渦度正值之生成，位於地形的東南側，但渦度變率最大值位於颱風內核圈之西側，主要貢獻為渦度平流項，造成小颱風和大颱風路徑偏移情形不太相同。 This research used the Weather Research and Forecasting Model to simulate Supertyphoon Krosa (2007) and examined the physical process responsible for the significant track deflection and the looping track. In the control experiment, the result is similar to previous studies. When the typhoon approaches Taiwan, the wind speed in the west of the typhoon increases more than the east. This means the channeling effect exists in this case. A positive vorticity field forms at the south-eastern side of Taiwan and results in a maximum of vorticity tendency. Because a low is at the same place as the positive vorticity field, it’s inferred the relative low leads to the positive vorticity. The maximum of vorticity tendency at the south-eastern side of Taiwan causes the looping track. And the backward trajectories show that the process leading the formation of the low is leeside subsidence warming. According to the vorticity budget, the divergence and advection terms contribute the change of vorticity. To test and verify the mechanism valid in real cases, this study also used the NCU-MM model to simulate north-westward typhoons attacking the idealized island. The result shows that the mechanism of the looping track in idealized cases is similar to real cases. The channeling effect and the positive vorticity form in idealized cases, so the mechanism occurs not only in the Krosa (2007) but also in other cases of the same conditions. However, no positive vorticity is found in the south-eastern side of the island for the south-approaching typhoons without, consequently, southward track deflection before landing. The sensitivity study of the typhoon size shows that the low also forms in the experiment of small typhoons but isn’t strong enough to effect the typhoon. The maximum of vorticity tendency dominated by the advection term in the western side of small typhoons is the main reason for track deflection different from big typhoons.
