Axisymmetric spindown dynamics of hurricane-like vortices

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 elasti city 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 o f Eliassen and Lystad's balanced spindown-spinup theory are reviewed first. The theory is then tested with an axisymmetric Navier-Stokes numerical mod el. The numerical experiments broadly confirm the theoretical predictions f or a range of vortex heights, maximum tangential wind speeds, constant and variable drag coefficients, and vortex sizes considered relevant for tropic al 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 d emise of the vortex. The theory is shown to furnish a consistent description of the weakening ph ase of two hurricanes observed by research aircraft. Despite the idealizati ons employed to yield a tractable model, the theory appears useful in eluci dating weakening episodes of hurricanes not associated with strong asymmetr ies.