| dc.description.abstract | 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. | en_US |