• tropical cyclone;
  • vortex Rossby waves;
  • barotropic instability;
  • vortical hot towers;
  • eyewall replacement cycles;
  • rapid filamentation zones;
  • vacillation cycles


A simulation of Hurricane Katrina (2005) using the Australian Bureau of Meteorology's operational model for tropical-cyclone prediction (TCLAPS) shows that the simulated vortex vacillates between almost symmetric and highly asymmetric phases. During the symmetric phase, the eyewall comprises elongated convective bands and both the low-level potential vorticity (PV) and pseudo-equivalent potential temperature θe fields exhibit a ring structure, with the maximum at some radius from the vortex centre. During this phase the mean flow intensifies comparatively rapidly, as the maximum acceleration of the mean tangential wind occurs near the radius of maximum mean tangential wind (RMW). In contrast, during the asymmetric phase the eyewall is more polygonal, with vortical hot towers (VHTs) located at the vertices. The low-level PV and θe fields have monopole structures with the maximum at the centre. The intensification rate is lower than during the symmetric phase because the mean tangential wind accelerates most rapidly well within the RMW.

The symmetric-to-asymmetric transition is accompanied by the development of VHTs within the eyewall. The VHTs are shown to be initiated by barotropic–convective instability associated with the ring-like structure of PV in the eyewall where the convective instability is large. During the reverse asymmetric-to-symmetric transition, the VHTs weaken as the local vertical wind shear increases and the convective available potential energy is consumed by convection. The weakened VHTs move outwards, similar to vortex Rossby waves, and are stretched by the angular shear of the mean vortex. Simultaneously, the rapid filamentation zone outside the RMW weakens, becoming more favourable for the development of convection. The next symmetric phase emerges as the convection reorganizes into a more symmetric eyewall. It is proposed that vacillation cycles occur in young tropical cyclones and are distinct from the eyewall replacement cycles that tend to occur in strong and mature tropical cyclones. Copyright © 2011 Royal Meteorological Society