The Response of a Simulated Tropical Cyclone to Moderate Misalignment Forcing

The internal mechanisms by which tropical cyclones can limit tilt and retain strength under misalignment forcing, such as moderate environmental shear, are not fully understood. To gain further insight, the present study uses a standard cloud model to examine the response of a simulated hurricane to a period of idealized forcing designed to tilt the vortex while leaving the final background flow nearly quiescent. It is found that the perturbation generated by this forcing is relatively weak and does not seem to resemble a coherent, precessing tilt mode (quasi-discrete vortex Rossby wave). By contrast, the same forcing more clearly excites a robust tilt mode in a reduced version of the simulated hurricane that has approximately the same baroclinic structure but no moisture and no mean secondary circulation. Two additional numerical experiments are conducted to gain insight into what factors contribute significantly to the differences between the dry and moist-convective simulations. The first experiment demonstrates that adding a sufficient amount of suspended cloud water to the dry vortex (thereby decreasing static stability) can inhibit the excitation of a coherent tilt mode, in basic agreement with an earlier theoretical result that will be reviewed. The second experiment removes moisture from the simulated hurricane but maintains the mean secondary circulation with a distributed heat source. Here too the tilt created during the forcing period is noticeably reduced, appearing to support intuitive expectations that mean secondary circulation helps maintain vertical alignment. Sensitivity studies are underway to test the robustness of the preceding results to various details of the computational setup. A complementary analysis of tendency terms has been carried out for the azimuthal wavenumber-1 component of the disturbance to help quantify the importance of mean secondary circulation and nonlinearity on wavenumber-1 dynamics in the core of the simulated hurricane. The results and limitations of this analysis will be discussed. This work is supported by NSF grant AGS-1101713.