Hurricane Eyewall Evolution in a Forced Shallow Water Model

A forced shallow water model is used to understand the role of diabatic and frictional effects in the generation, maintenance, and break down of the hurricane eyewall potential vorticity (PV) ring, and the relationship to vortex intensity variability. Diabatic heating is parameterized as an annular mass sink of variable width and magnitude, and the nonlinear evolution of tropical storm-like vortices is examined under this forcing. Diabatic heating produces strengthening and thinning PV ring in time due to the combined effects of the mass sink and radial PV advection by the induced divergent circulation. If the forcing makes the ring thin enough it can become dynamically unstable and break down into polygonal asymmetries or mesovortices. Friction is shown to help stabilize the PV ring slightly by reducing the eyewall PV and the unstable mode barotropic growth rate. Unlike previous unforced studies, this forced study demonstrates how a vortex can intensify by both the maximum sustained wind and minimum central pressure during barotropic instability, consistent with observations. The location and structure of the heating is shown to be critical for the intensity variability. While it is well known that it is critical to heat in the region of high vorticity to efficiently spin-up the hurricane vortex, these results demonstrate the additional importance of having the heating as close as possible to the center of the storm, partially explaining why tropical cyclones with very small eyes can rapidly intensify to high peak intensities.