The Roles of Vortex Rossby Waves in Hurricane Secondary Eyewall Formation

A high-resolution, full-physics model initiated with an idealized tropical cyclone like vortex is used to simulate and investigate the secondary eyewall formation. The beta skirt axisymmetrization (BSA) hypothesis previously proposed is examined and the roles of axisymmetrizing vortex Rossby waves (VRWs) in the secondary eyewall formation are further investigated. During the formation period, convection outside the inner-core region is organized into an outer spiral rainband. The PV dipoles that are persistently generated by convective updrafts through tilting effect move along the rainband and inward toward inner-core region and are finally axisymmetrized in the preexisting beta skirt region. The formation of the secondary eyewall is preceded by a rapid intensification period, during which vortical hot towers, discrete VRWs, and sheared VRWs dominate the inner-core asymmetric structures. Sheared VRWs are repeatedly emanated from the outer edge of the eyewall and become more concentric when propagating outward, leading to the formation of a weak but nonnegligible secondary circulation near the VRWs' stagnant radius. The mean tangential flow is accelerated by the low-level convergence associated with the secondary circulation and also by the wave mean flow interaction mechanism, both of which are elucidated by absolute angular momentum budget calculation. The mean radial gradient of relative vorticity is enhanced across the stagnant radius, causing the extension of beta skirt to outer radii in the lower-tropospheric levels. Results from this study suggest that the stagnant radius mechanism and the BSA mechanism may work cooperatively in the sense that the former helps to establish an extensive beta skirt and the latter takes charge from then on.