Moist vortex resiliency in vertical shear flow

An examination of the resiliency dynamics of tropical cyclones in vertical shear flow is first motivated by a multi-case documentation of Doppler-derived vorticity and vertical motion asymmetry in the hurricane eyewall region. The baroclinic component of the azimuthal wavenumber one vorticity asymmetry, and its phase relationship to the low-wavenumber vertical velocity field, is presented for each case. This asymmetric structure, the observed symmetric vortex structure and estimates of vertical shear are used to initialize idealized linear simulations of hurricane-like vortices in vertical shear flow with a simple parameterization of asymmetric diabatic heating.

It is found that when the heating is concentrated within a narrow annulus well inside the radius of maximum tangential wind (RMW), the sheared vortex evolves in a manner consistent with the resonant damping paradigm. As the central radius of the annulus is moved beyond the RMW, and the annulus is broadened, the initial vorticity asymmetry generated by the vertical shearing evolves into spiral banded structures in the vortex core. In both cases the vortex is resilient, but the underlying mechanisms responsible for the resistance to vertical shearing appear distinct. To examine the impact of nonlinearity, a non-hydrostatic primitive equation model is also used. Implications of these results for realistic hurricanes are then discussed.