Response of a Simulated Hurricane to Misalignment Forcing Compared to the Predictions of a Simple Theory

Abstract

his paper compares the tilt dynamics of a mature tropical cyclone simulated with a conventional cloud model to reduced modeling results and theoretical predictions. The primary experiment involves a tropical cyclone of hurricane strength on the f plane exposed to a finite period of idealized misalignment forcing. A complementary experiment shows how the vortex responds to the same forcing when moisture and symmetric secondary circulation (SSC) are removed from the initial condition. It is found that the applied forcing excites a much stronger tilt mode in the dry nonconvective vortex than in the moist convective hurricane. The evolution of tilt in both experiments agrees reasonably well with a simple linear response theory that neglects the SSC and assumes moisture merely reduces static stability in the vortex core. An additional experiment with suspended cloud water but no substantial SSC supports the theoretical notion that reduction of static stability is sufficient to inhibit the excitation of a tilt mode. However, there is some discrepancy between theory and details of asymmetric convection in the eyewall region of the simulated hurricane. Moreover, a final experiment without moisture but with an artificially maintained secondary circulation suggests that the SSC has a nonnegligible role in reducing tilt. Diagnosis of the primary hurricane simulation further illustrates how the SSC has discernible influence over misalignment at least in the eyewall. Sensitivity of tilt dynamics to the azimuthally averaged vortex structure is briefly addressed.