Forecasts of tropical cyclone (TC) track and intensity remain significant operational challenges. Track forecasts have improved gradually over the last five decades, largely in response to improved observation and modeling of the environmental steering currents that control TC motion. By contrast, progress on intensity has been slow and statistical techniques still provide the best forecast guidance. The fundamental process in TC intensification is conversion of latent heat extracted from the sea into available potential energy and kinetic energy of the wind. The research to be undertaken under this award has two aspects 1) analysis of the TC vortex response to imposed heat sources and 2) theoretical studies of linear and nonlinear motion of a two-layer baroclinic TC-like vortex. The analytical axially symmetric, baroclinic mean-vortex structures used in these studies will be realistic representations of actual TCs derived from a large sample of in situ observations.
The Principal Investigator will explore TC intensification processes by studying the responses to imposed heat sources with these sources being represented as truncated Fourier series in both time and azimuth. Only the axially symmetric, temporally constant component of the heat source supplies net heating. Its effect is readily computed from the classical Sawyer-Eliassen equation. The next level of complexity uses an axially symmetric heat source that varies sinusoidally in time. A single governing equation is derived from the Navier-Stokes equations linearized on a mean vortex in hydrostatic and gradient balance. The perturbation flow may be nonhydrostatic and sub- or super-gradient. If the frequency with which the heat source changes is low enough, this model calculation should resemble the results from steady forcing. At higher frequencies, effects such as changes in the effective local deformation radius due to time variation or projection onto gravity-wave modes can become factors. The next level of complexity considers a similarly derived equation for asymmetric heating with a given azimuthal wavenumber and frequency. Phenomena to be studied include changes in vortex response as a function of forcing frequency, the roles buoyancy and dynamic pressure, diabatically induced super- and sub-gradient winds, evolution of the mean vortex as a result of eddy fluxes of heat or momentum due to the perturbations, and hydrodynamic stability of mean-flow vortices constructed to model real hurricanes.
The vortex-motion studies will use a two-layer semi spectral, vortex-tracking model to study the resiliency and realignment. Questions to be addressed include re-examination of resonant-damping theory, assessment of the roles of nonlinearity and evolution of the symmetric vortex, study of environmental potential vorticity and planetary vorticity gradients, and the effects of realistic initial mean-vortex structures.
Broader impacts of this study include: fundamental new physical understanding essential to accurate track and especially intensity forecasts and support of Florida International University's (FIU) nascent meteorology program.
