本研究以哈吉貝颱風(2019)為例，探討眼牆置換中的動力機制。結果顯示，由內眼牆對流發展釋出的上對流層冰相粒子可能對眼牆置換有重要的貢獻。在本個案中，環境北風風切將大量冰相粒子集中至颱風南側，並在東南側的內眼牆與外圍雨帶之間產生明顯非絕熱冷卻。這些冷卻增強了徑向非絕熱加熱梯度，並在內眼牆與外圍雨帶之間引發逆向的二次環流，使得邊界層上從內眼牆非軸對稱的徑向外流與外圍雨帶外側的徑向內流產生強烈的水平輻合，並引發垂直運動。外圍雨帶外側較強的徑向內流同時也提供較大的角動量徑向平流，使邊界層內的切向風得以加強；邊界層上至低對流層的切向風增量則主要由軸對稱垂直平流產生。外眼牆的形成伴隨著梯度力的軸對稱化與超梯度力的發展，而哈吉貝颱風原有北側超梯度力、南側次梯度力，隨著外眼牆的軸對稱化逐漸消弭，並發展出超梯度力環繞在外眼牆內側。 綜合而言，非軸對稱擾動是外眼牆生成的肇始者，與外圍環流有關，用於啟動邊界層內不平衡動力，且增強邊界層內的切向風；軸對稱環流則在外眼牆發展後期以徑向平流增強邊界層內切向風，垂直平流增強邊界層上至低對流層切向風。同時，徑向非絕熱加熱梯度與外眼牆發展動力有關，在此個案中，徑向非絕熱加熱梯度最大的下風切左側即為主導外眼牆生成的重要區域。 ;This study takes the example of Typhoon Hagibis (2019) to investigate the dynamical mechanism of ERC. The result shows that the ice crystals in the upper troposphere diffused by strong inner eyewall convection could be a considerable contributor to ERC. In this case the northerly wind shear concentrates the ice crystals to the south of the cyclone and produces diabatic cooling in between of the inner eyewall and outer rainbands at the southeast quadrant evidently. The cooling effect strengthens the radial gradient of diabatic heating, which induces an inverse secondary circulation between the inner eyewall and the outer rainbands, causing intense horizontal convergence that forces vertical motion via asymmetrical radial outflow above the boundary layer and within the moat to converge with the radial inflow at the radial outward side of the outer rainbands. This radial inflow also provides positive radial advection of angular momentum, which intensifies the tangential wind in the boundary layer, wherase tangential wind increment above the boundary layer and in the lower troposphere is induced by axisymmetric vertical advection. The formation of the outer eyewall is accompanied by the axisymmetrization of gradient force and the development of supergradient force. Typhoon Hagibis performs supergradient force to the north and subgradient force to the south of the vortex center initially, but such agradient pattern is progressively reduced by axisymmetrization of the outer eyewall, with supergradient force building up at the inward side of the outer eyewall. Overall speaking, asymmetry (eddy) transport is the precursor of the secondary eyewall formation, caused by the outer rainbands, which initiates the unbalanced boundary layer dynamics and accelerates the tangential wind in the boundary layer, wherase axisymmetric processes spin up the tangential wind in the boundary layer at the later stage of the development of the outer eyewall via radial advection, and above the boundary layer and in the lower troposphere via vertical advection. Meanwhile, radial gradient of diabatic heating is related to the outer eyewall developing dynamics, favorable for downshear left quadrant where the strongest radial gradient of diabatic heating exists as the dominant for the formation of the outer eyewall in this case.
