Vacillation cycles in WRF simulations of hurricane Katrina

The intensification of hurricane Katrina (2005) in non-hydrostatic simulations with the WRF model is compared to a hydrostatic simulation conducted in a previous study with the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TCLAPS). During the intensification both simulations exhibit inner-core vacillation cycles between highly asymmetric and more symmetric phases in the low-level potential vorticity (PV) and vertical velocity, although their evolution and structure are more complex in WRF. Although in WRF the low-level PV vacillates between symmetric and more asymmetric stages, similar to the hydrostatic TCLAPS simulation, no fully monopolar structure develops in asymmetric phases; the PV maintains its ring-like structure during the intensification. The asymmetric phases in WRF are characterised by isolated PV anomalies breaking from the eyewall and propagating towards the centre. While stirring high PV into the centre episodically, these isolated PV anomalies cause a stepwise drop in central pressure.