本研究使用MPAS (Model for Prediction Across Scalses-Atmosphere)全球預報模式進行模擬，解析度為60-15-3公里可變解析度全球網格，在有興趣之區域解析度為3公里，此全球網格配置好處在於無邊界條件問題。本篇研究主要探討利奇馬颱風在2019年8月8日至9日在臺灣北部近海出現之路徑向北偏折現象。前人研究中有使用中央氣象局版的全球模式FV3GFS成功模擬出此颱風路徑偏折之現象，但由於初始場具有不確定性，因此一組誤差小的模擬結果有可能為隨機偶然事件。因此本篇研究將初始場進行渦漩初始化，改善初始場颱風強度後，再進一步產生20組系集擾動初始場，並進行120小時之長預報。預報結果顯示整體系集預報在颱風接近臺灣地形時路徑變得相當發散(有登陸臺灣再進入閩南一帶，也有在琉球一帶就北轉)，可知此時路徑預報不確定性相當高，而本篇選用系集預報中與最佳路徑誤差較小的幾組系集成員(分別為M03、M06、M09)和控制組進行分析討論，其中M03路徑誤差雖然小，但路徑偏折角度比最佳路徑小，而M06、M09誤差小且還能大致掌握路徑偏折。而在強度的部分，各系集皆反應出利奇馬颱風在接近台灣時顯著增強的情況，因此可判斷出此時強度不確定性較低。 進一步分析颱風環流結構，發現控制組和三組系集成員皆有接近台灣產生繞流，由颱風中心南側流入颱風東側內核區，使颱風東側風場增強，加大風場不對稱，使其路徑相較於移除地形時的路徑有偏北的情況。由有、無地形時的波數一風場之差異可以發現，主要為繞行渦漩中心的一對迴流(gyre)，而其原本位於颱風東西側，再透過颱風風場平流後移至颱風南北側，造成颱風移動先往北再往西。接著透過角動量及動能收支探討當颱風靠近地形逐漸增強時，各項之變化，由角動量結果可推斷出水平平流項和颱風強度最具關連性，其原因為當低層入流增強時，將會使低層水平平流項正貢獻增強，有利於颱風增強。而在動能收支中，則為平均徑項壓力梯度力功率項與強度較為相關。最後，我們再經過位渦收支分析，了解路徑偏折情況發生時，位渦極大值方向，乃由西北至北北西，轉為西北西至西北，此與移動方向變化一致。此利奇馬颱風模擬結果與其路徑偏折機制，與前人使用CWB FV3GFS 模擬比較相當一致，顯示利奇馬颱風鄰近台灣時路徑偏折，係受到台灣地形作用的影響所致。 ;The Model for Prediction Across Scales (MPAS) is the nonhydrostatic global atmosphere model developed by National Center for Atmospheric Research (NCAR). This study uses a variable-resolution mesh of 60-15-3 km with 3-km resolution targeted on Taiwan’s vicinity to simulate Typhoon Lekima. When approaching Taiwan from the southeast during 8th to 9th August 2019, the typhoon began to deflect northward. We have used FV3GFS to successfully capture the observed northward track deflection, but the deflection may be randomly induced because of the predictability with the initial uncertainty. This study uses dynamical vortex initialization (DVI) to enhance the initial typhoon intensity and then adds perturbations to the initial 20 ensemble members for 120-h forecast. The forecast results show the spreading of ensemble tracks when Typhoon Lekima approaches northern Taiwan. Three members with smaller track errors are chosen for dynamic analysis, two of wich can further capture the observed track deflection. The movement of Lekima is dominated by a pair of gyres around the typhoon center in the asymmetric wavenumber-one flow difference between the simulated flow fields with and without Taiwan terrain, which tends to counterclockwise rotate the vortex motion vector, first northward and then westward. Potential vorticity budget is analyzed to investigate the dynamics of the evolving typhoon and help explain the track deflection. Asymmetric decomposition of PV tendency budget highlights the relative importance of dynamic forcing terms in the PV tendency in steering the typhoon movement with induced track deflection. The track deflection is mainly dictated by wavenumber-1 PV horizontal advection with northward tendency, while vertical advection and differential diabatic heating only have minor impacts. The track deflection mechanism of Lekima as explored by MPAS in this study is in agreement with that obtained from the global FV3GFS simulations in our earlier study, as a manifest result of the topographic effects of Taiwan terrain.
