Hurricane landfall and the associated spin-down process that typically accompanies it is a complex problem. As a step towards understanding the hurricane spin-down problem, this work focuses on the spin-down dynamics over the ocean in a neutrally stratified atmosphere.
Recent work by Yang and Montgomery (henceforth YM) examined hurricane spin-down in a neutrally stratified atmosphere using an axisymmetric Navier-Stokes numerical model. YM's model simulations are qualitatively similar to the theory of Eliassen and Lystad (1977, hereafter EL), but a shorter spin-down time is observed in YM for vortices exceeding tropical storm strength.
In this work, we identify the cause for the enhanced spin-down rate observed by YM using a diagnostic analysis. This work also examines the dependence of the spin-down time on the fluid depth. In particular, EL's prediction of a linear dependence of the spin-down time with the fluid depth is tested with the Navier-Stokes numerical model using depths of 5, 10, and 15 km. The latter depths are more representative of a hurricane-type vortex than the 5 km used previously by YM and EL. The dependence of the spin-down time on the drag coefficient will also be investigated. Larger drag coefficients will assist in providing insight to the spin-down of hurricanes as they make landfall.