全球跨尺度天氣預報模式(MPAS)是由美國國家大氣研究中心(NCAR)所發展的新一代天氣預報模式，本研究使用MPAS經過兩次縮放的60-15-3公里可變解析度全球網格模型來模擬近年侵襲臺灣的三個颱風，包含蘇迪勒(2015)、梅姬(2016)、尼莎(2017)，並且比較與區域模式WRF在相似解析度配置與相同物理參數化方法下的模擬能力。模擬結果顯示出MPAS除了具有較佳的模擬結果之外，能夠模擬出尼莎颱風(2017)突然向北偏折的路徑。由於各國預報單位大部分未預報出此向北偏轉之路徑，因此在本研究中，將會利用MPAS模式的模擬結果進行風場結構與位渦收支診斷分析，了解尼莎颱風的演變與轉向過程。
本研究透過風場結構分析和位渦收支診斷分析方法，對尼莎颱風的演變及動力過程進行研究，並對路徑偏轉的機制進行解釋。位渦趨勢收支的不對稱量分解顯示了颱風移動與路徑偏折的重要動力機制。當垂直平流作用和潛熱加熱作用的影響達到平衡時，路徑偏轉主要是由wavenumber-1 PV水平平流項所主導，並導致了颱，，，風路徑向北偏轉的趨勢。
此外，本研究也針對幾項可能影響此機制的因素進行實驗，包括臺灣地形高度、南海熱帶低壓氣旋與20-60天季內週期系統。結果顯示，移除臺灣地形高度後，颱風結構更加對稱，向北的不對稱水平平流作用減少，導致較微弱的路徑偏折，熱帶氣旋海棠提供向北駛流的西南氣流，並與颱風外圍環流輻合，增強東半側風速，使向北移動的分量增加。另一方面，大尺度季內週期之系統除了MJO影響南海低壓的形成，也包含太平洋高壓的強度，對尼莎颱風的路徑亦有重要的影響。

The Model for Prediction Across Scales (MPAS) is the non-hydrostatic global atmosphere model which developed by the 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 typhoons impinging Taiwan, including Soudelor (2015), Megi (2016) and Nesat (2017). The MPAS performance is compared with a regional model (WRF) in similar configuration of resolution and physical parameterization methods. The MPAS not only simulates a better result, but also captures the northward deflection of Typhoon Nesat (2017) near landfall in Taiwan. Several international prediction operational agencies failed to capture the track defection.
In this study, the analysis of wind field structure and diagnostic potential vorticity budget analyses are used to investigate the essential dynamics of the evolving typhoon and explain the result of track deflection. Asymmetry decomposition of PV tendency budget highlights the relative importance of dynamic forcing terms in PV tendency in steering the typhoon movement and track deflection. The track deflection is mainly induced by wavenumber-1 PV horizontal advection with northward tendency when the effects of vertical advection and the effects of latent heating are balanced. The result also shows the relationship between typhoon motion and steering flow.
Several sensitivity experiments are used to test the influence of Taiwan terrain, low-pressure system and large-scale system. Without the Taiwan terrain, the structure of typhoon is more symmetric and northward tendency by asymmetric horizontal advection is reduced, thus leading to a less deflection near Taiwan. Additionally, the southwestly flow provided by the vortex in the South China Sea (Tropical Storm Haitang) converges with the circulating currents in the periphery of Nesat, and enhance the northward component of movement. On the other hand, the large-scale system, such as the MJO and the subtropical high pressure, also have the effect on the movement of Nesat impinging Taiwan.

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