Influences of Potential Vorticity Asymmetries on a Tropical Cyclone in a Moist Three-Layer Model

In a recently completed study (Shapiro 1999), the role of potential vorticity (PV) asymmetries in the evolution of a tropical cyclone was investigated using a three-layer model that includes boundary layer friction, surface moisture fluxes, and a convergence-based convective parameterization. In that study weak azimuthal-wavenumber two and double-cluster PV asymmetries confined to the lower two layers of the model were added to a symmetric vortex spun up to hurricane strength on an f-plane near its radius of maximum wind. After 2 h the asymmetric PV anomaly produced changes in the symmetric vortex that have significant differences from those in dry experiments or previous barotropic studies. The physical mechanism responsible for these differences as well as for the robustness of the response was established. It was shown that the interactions between the asymmetries and the symmetric hurricane vortex at early times depend on realistic features of the model hurricane, and not on interactions between the asymmetries and the boundary layer that possibly depend on the convective parameterization. The longer-term evolution of the vortex is more problematic, and may depend on the convective parameterization used.

Further studies are being made to determine the dependence of the results on the amplitude, radial and tangential scale of the asymmetric PV disturbance as well as its vertical structure. Contributions from the symmetric and asymmetric parts of the PV anomaly will be separated. In the course of the analysis a method has been developed for determining the vertical structure of a PV anomaly that is consistent with a diabatic source of arbitrary amplitude. An upper-level negative anomaly is induced above a lower-level positive one; the relative strengths of these anomalies depends on the thermal structure of the cyclone vortex. The upper-level anomaly tends to reduce the effect of the lower-level one on the evolution of the vortex. The cumulative effect of multiple convective "pulses" on the evolution of the vortex is being studied, as in Montgomery and Enagonio (1998) and Möller and Montgomery (1999), and the conditions under which the pulses are effective in intensifying the cyclone vortex are being evaluated.

As time permits, experiments will also be made with an imposed azimuthal-wavenumber one PV anomaly to establish the presence of an algebraically growing disturbance as elucidated by Nolan and Montgomery (1999). For these experiments minimal horizontal diffusion and maximum horizontal resolution will be used.

Experiments designed to evaluate these aspects of the problem are in progress and will be reported upon as available.