Effects of Parameterized Diffusion on Simulated Hurricanes

The authors analyze and interpret the effects of parameterized diffusion on the nearly steady axisymmetric numerical simulations of hurricanes presented in a recent study. In that study it was concluded that horizontal diffusion was the most important control factor on the maximum simulated-hurricane intensity. Through budget analysis it is shown here that horizontal diffusion is a major contributor to the angular-momentum budget of the numerically simulated storms. Moreover a new scale analysis recognizing the anisotropic nature of the parametrized model diffusion shows why the horizontal diffusion plays such a dominant role. A simple analytical model is developed that captures the essence of the effect. The role of vertical diffusion in the boundary layer in the aforementioned numerical simulations is more closely examined. It is shown that the boundary layer in these simulations is consistent with known analytical solutions in that boundary-layer depth increases and the amount of "overshoot'' (maximum wind in excess of the gradient wind) decreases with increasing vertical diffusion. However the maximum wind itself depends mainly on horizontal diffusion and is relatively insensitive to vertical diffusion; the overshoot variation with vertical viscosity mainly comes from changes in the gradient wind with vertical viscosity. The present considerations of parameterized diffusion allow a new contribution to the dialog in the literature on the meaning and interpretation of the Emanuel Potential Intensity theory.