颱風璨樹(2021)往西北方移動靠近臺灣東南外海，接著出現明顯向右的轉折，使其沿著臺灣東部外海北上，至臺灣東北外海時，路徑則些微向左偏折。本研究使用全球模式 MPAS (the Model for Prediction Across Scales–Atmosphere)，分析璨樹行經臺灣附近的路徑演變，模式解析度為60-15-3及60-15-1公里。實驗結果顯示，增加解析度使璨樹的路徑預報更為準確。考慮大尺度環流的影響，濾除 MJO (Madden-Julian oscillation) 分量的實驗結果則指出颱風將會持續往西北方向移動。有、無地形的實驗證實這些偏折是來自於地形的影響——因地形而產生的繞流使得颱風路徑向右偏折，隨後颱風左側的環流受地形阻擋而減弱，較強的偏東南風使得其向左偏折。探討兩實驗駛流風場不對稱量的差異量，本研究亦發現一反氣旋式環流及一氣旋式環流分別位於颱風路徑的東側及西側，該對環流隨時間逆時鐘旋轉，影響著颱風的移動方向。位渦收支分析指出，向右的偏折由水平平流項主導，然而向左的偏折則是來自垂直平流和非絕熱加熱項的貢獻。由於真實個案模擬存在的不確定性，本研究亦進行數個理想化實驗，探討不同颱風中心位置及不同駛流風的情況下，颱風的路徑變化。位渦收支的結果可與MPAS實驗中的機制相互對應，進一步強調了受到地形影響的颱風不對稱結構對於路徑向左偏折的重要性。而這些渦旋的路徑偏折情形可以利用參數R/L_E (R為渦旋大小，L_E為有效地形長度) 及L_D (渦旋和地形的經向距離) 來決定。;The global model MPAS with multiple resolutions (60-15-1 km) is used to investigate the evolution of Typhoon Chanthu (2021) near Taiwan. Chanthu exhibited a rightward track deflection as it approached Taiwan from the southeast, and subsequently underwent a leftward deflection after moving offshore along the east coast towards northeastern Taiwan. Numerical experiments are conducted to identify the physical processes for the induced track changes of Chanthu. The rightward track deflection of the northward typhoon is primarily induced by the recirculating flow resulting from the effect of Taiwan topography, which is completely changed to a westward track in the absence of the large-scale MJO flow component. The wavenumber–one potential vorticity (PV) budget analysis indicates that horizontal PV advection dominates the earlier rightward deflection, while more affected by vertical PV advection and diabatic heating to move inland toward north Taiwan in the presence of Taiwan terrain. For such a northward typhoon, a pair of cyclonic and anticyclonic gyres in the wavenumber–one difference in the simulated flow with and without Taiwan terrain are rotating with time in the vicinity of the typhoon center, as found for different translational typhoons in other studies. The idealized WRF is also used to aid an interpretation of the track deflection under varying steering conditions. Idealized simulations confirm the track deflection mechanism in the real case with similar PV dynamics and further illustrate the sensitivity of the track deflection to the steering flow direction in this study. The magnitude of the rightward deflection is essentially determined by the ratio of R/L_E where R is the vortex size and L_E is the effective terrain length.
