A new look at hurricane intensity: perspectives from a dry and moist hurricane frameworks

We analyze the dynamic and thermodynamic structure of axisymmetric hurricanes in the framework of a high resolution numerical simulation, and evaluate the underlying assumptions of Maximum Potential Intensity (MPI) of Emanuel (1986). Our simulations produce super-intense winds, i.e. wind speed greater than the theoretical predictions of the MPI theory. Our analysis indicates that the superintensity can be attributed to two separate effects: the presence of super-gradient wind resulting from the overshoot of the inflow, and the enhancement of the entropy gradients near the radius of maximum wind due to turbulent mixing in the boundary layer. The frictional dissipation and energy production in the inflowing boundary layer air is described in terms of surface fluxes of heat and momentum. However, the surface flux closure underestimates the theoretical strength of Emanuel's hurricane.

An axisymmetric, nonhydrostatic version of the ZetaC model, with the GFDL radiation package on a 10 degrees domain, has been used to produce a large number of simulations covering a vast range of environmental conditions (surface temperature, tropopause temperature, latitude, surface heat flux and other parameters were varied). In the moist simulations we use the Lin microphysics. The horizontal resolution of the model is 1 km or 500m thus convection is fully resolved. Simulations are run for about 25 days, producing a steady state.

Our analysis of the numerical simulations suggests that the entropy distribution in the boundary layer cannot be determined solely from the surface fluxes. In particular, turbulent mixing across entropy and angular momentum surfaces is associated with a strengthening of the entropy gradient near the radius of maximum wind. In our simulations, this thermodynamic effect did result in a 5 to 10 percent increase in the maximum wind speed. Numerical simulations show that the maximum tangential winds are stronger than theory predicts, which means that the MPI theory is not really an upper bound for intensity. The superintensity can be explained by the presence of ageostrophic winds within the eyewall inflow region.