Axisymmetric Spindown Dynamics of Hurricane-like Vortices

Abstract

This work examines the spindown problem of hurricane-like vortices subject to a quadratic drag law in the surface layer. Since intense hurricanes over the open ocean are approximately axisymmetric due in part to Rossby elasticity and axisymmetrization processes that tend to keep the vortex erect and circular, the axisymmetric spindown problem serves as a useful benchmark. As a basis for the numerical experiments presented, the essential results of Eliassen and Lystad’s balanced spindown‐spinup theory are reviewed first. The theory is then tested with an axisymmetric Navier‐Stokes numerical model. The numerical experiments broadly confirm the theoretical predictions for a range of vortex heights, maximum tangential wind speeds, constant and variable drag coefficients, and vortex sizes considered relevant for tropical storm and hurricane strength vortices. But unlike the monotonic decay of the swirling flow predicted by theory, the numerical simulations reveal a temporary spinup of the tangential winds in the boundary layer before the demise of the vortex. The theory is shown to furnish a consistent description of the weakening phase of two hurricanes observed by research aircraft. Despite the idealizations employed to yield a tractable model, the theory appears useful in elucidating weakening episodes of hurricanes not associated with strong asymmetries.