Potential vorticity asymmetries and tropical cyclone evolution in a moist three-layer model

The role of potential vorticity (PV) asymmetries in the evolution of a trop ical cyclone is investigated using a three-layer model that includes bounda ry layer friction, surface moisture fluxes, and a convergence-based convect ive parameterization. In a benchmark experiment, a symmetric vortex is firs t spun up on an f plane for 24 h. The symmetric vortex has a realistic stru cture, including a local PV maximum inside its radius of maximum wind (RMW) . A weak azimuthal-wavenumber 2 PV asymmetry confined to the lower two laye rs of the model is then added to the vortex near the RMW. After an addition al 2 h (for a total 26-h simulation), the asymmetric PV anomaly produces ch anges in the symmetric vortex that have significant differences from those in dry experiments with the present model or previous barotropic studies. A diagnosis of the contributions to changes in the symmetric wind tendency d ue to the asymmetry confirm the dominance of horizontal eddy fluxes at earl y times. The barotropic eddy kick provided by the anomaly lasts similar to2 h, which is the damping timescale for the disturbance. Additional experiments with an imposed isolated double-PV anomaly are made. Contrary to expectation from the dry experiments or barotropic studies, ba sed on arguments involving "wave activity," moving the anomaly closer to th e center of the vortex or farther out does not change the overall evolution of the symmetric vortex. The physical mechanism responsible for the differ ences between the barotropic studies and those including moist physics as w ell as for the robustness of the response is established using a budget for the asymmetric vorticity. It is shown that the interactions between the as ymmetries and the symmetric hurricane vortex at early times depend on reali stic features of the model hurricane and not on interactions between the as ymmetries and the boundary layer, which possibly depend on the convective p arameterization. In particular, the changes in the symmetric wind tendency due to the asymmetry can be most simply explained by a combination of horiz ontal advection and damping of wave activity. In conjunction with horizonta l advection and damping, the reversal of the radial vorticity gradient asso ciated with the local PV maximum constrains the asymmetries to reduce the s ymmetric vorticity near the RMW. The location of the PV maximum controls th e response to the extent that moving the PV anomaly radially inward or outw ard has no qualitative effect on the results. The longer-term evolution of the vortex is more problematic and may depend on the convective parameteriz ation used.