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

2017-01-16T01:35:33Z (GMT) by Nguyen, Chi Mai
Tropical cyclone intensity change by internal processes is studied using the Australian Bureau of Meteorology operational model, TCLAPS. An ensemble of high-resolution simulations of Hurricane Katrina (2005) reveal a robust feature, in which the majority of modelled vortices go through cycles of structure change, vacillating between a more symmetric and a more asymmetric phase during rapid intensification. • During the Symmetric phase the eye-wall has a high level of symmetry, consisting of relatively uniform elongated convective bands. Low-level vorticity and equivalent potential temperature exhibit a ring-like structure. The largest intensification rates occur near the radius of maximum tangential wind (RMW). • The Asymmetric phase is characterised by a highly asymmetric eyewall, having a polygonal form with vortical hot towers (VHTs) located at the vertices. Low level vorticity and equivalent potential temperature have a monopole structure with the maximum near the center. The largest intensification rates occur inside the RMW. Detailed analyses suggest the following transition mechanisms: • Symmetric to Asymmetric transitions are associated with the outbreak of VHTs in the eyewall, which result from a cooperative combination of barotropic and convective instability. These VHTs actively mix air between the eye and eyewall, thus, creating the monopole structure. • Asymmetric to Symmetric transitions occur as the VHTs weaken due to exhausted convective instability. They become horizontally strained convective bands that move radially outward as vortex rossby waves (VRWs). High intensification rates resume near the RMW as result of a) increased horizontal vorticity fluxes associated with redevelopment of convection in the reduced rapid filamentation zone outside of the weakened VHTs; and b) VRW-mean flow interactions. We hypothesise 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.