Abstract

This study investigates a claim by Heng et al. (2017), and intimated soon after in Heng et al. (2018), that axisymmetric “balanced dynamics can well capture the secondary circulation in the full-physics model” during hurricane spin-up. Using output from a new, convection-permitting, three-dimensional, numerical simulation of an intensifying hurricane, azimuthally-averaged forcings of tangential momentum and heat are diagnosed to force an axisymmetric Eliassen balance model under strict balance conditions. The balance solutions are found, inter alia, to poorly represent the peak inflow velocity in the boundary layer and present a layer of relatively deep inflow extending well above the boundary layer in the high wind speed region of the vortex. Such a deep inflow layer, a hallmark of the classical spin-up mechanism for tropical cyclones comprising the radial convergence of absolute angular momentum above the boundary layer, is not found in the numerical simulation during the period of peak intensification. These deficiencies are traced to the inability of the balance model to represent the nonlinear boundary layer spin up mechanism. These results are contrasted with a pseudo-balance Eliassen formulation which improves the solution in some respects while sacrificing strict thermal wind balance. Overall, the quantitative results refute the Heng et al. claim and implicate the general necessity of the nonlinear boundary layer spin up mechanism to explain the spin up of a hurricane in realistic model configurations and in reality.

You do not currently have access to this content.