Tropical cyclone asymmetric eyewall evolution and intensification in a two-layer model

Here we use a simple two-layer model framework with a shallow-water model on top of a slab boundary layer (SBL) model to simulate a frictional boundary layer underneath the free atmosphere. Results from simulating a wavenumber 2 elliptical asymmetry suggest the VRW in the free atmosphere can organize the updrafts in the SBL, which is consistent with radar observations of enhanced reflectivity at the polygonal vertices.

Free atmospheric divergence in the shallow-water layer does not explain the coupling between vorticity and reflectivity. The coupling can be explained to first order by the one-way boundary layer response to the pressure gradient associated with the free atmospheric vorticity asymmetries, consistent with prior studies. Further simulations that allow two-way interaction between the layers show that the organization of the updrafts out of the SBL plays a critical role in the growth of a PV ring and intensification of the mean vortex.

In this framework, diabatic heating in the shallow-water layer parameterized by a mass sink driven by the free atmosphere–SBL interaction leads to rapid intensification of the vortex, thinning of the PV ring, and eventual barotropic instability and PV mixing. The simplified modeling framework with two-way interactions captures many of the essential dynamics of rapid intensification in the presence of evolving asymmetries, similar to those seen in the observations from Hurricane Michael (2018), which provides new insight into the complex interactions between dynamics and convection during hurricane intensification.