Inner-core vacillation cycles during the rapid intensification of Hurricane Katrina

High-resolution simulations of Hurricane Katrina (2005) using TCLAPS show that the modeled vortex vacillates between symmetric and asymmetric phases during periods of rapid intensification. During the symmetric phase the eye-wall comprises relatively uniform elongated convective bands and the low-level vorticity and θe fields exhibit a ring-like structure. Further, the largest intensification rates occur near the radius of maximum tangential wind (RMW). In contrast, the asymmetric phase is characterized by a highly asymmetric eyewall, having a polygonal form with vortical hot towers (VHTs) located at the vertices. The low-level vorticity and θe fields have monopole structures with their maxima near the center. In this phase, the largest intensification rates occur inside the RMW. These two phases are very similar to Regimes 1 and 2 respectively found by Kossin and Eastin (2001) using aircraft observations.

The transition from the symmetric to the asymmetric phase is associated with the development of VHTs within the eyewall, resulting from a combination of barotropic and convective instability. Baroclinic conversion is shown to be maximized during the asymmetric phase. In contrast, the reverse transition occurs as the potential energy available to the convection is consumed and the VHTs weaken. During this transition, the VHTs become strained in the horizontal and move radially outwards resembling vortex Rossby waves (VRWs).

We hypothesize that these cycles are an alternative mode of hurricane intensification during rapid intensification of less mature storms as opposed to Eyewall Replacement Cycles that are observed primarily in strong hurricanes with a mature structure.