The Role of Environmental Asymmetries in Atlantic Hurricane Formation

Abstract

The radial circulation equation for a balanced vortex is solved with forcing functions obtained from numerical simulations of hurricane formation from prehurricane cloud clusters and depressions. The simulations were made with the Naval Research Laboratory hurricane model using initial conditions derived from the Colorado State University composite datasets. Separate solutions of the radial circulation equation using different forcing functions provide evidence that large-scale eddy fluxes of heat and momentum serve to trigger the formation of the model hurricanes. Such fluxes, found in the Colorado State University datasets, induce a radial circulation with surface inflow over a broad stretch of warm ocean. This inflow picks up water vapor and concentrates it in the region of the vortex where the resulting release of latent heat by cumulus convection serves as an additional driving force that further enhances the radial circulation. Mechanisms such as CISK and finite-amplitude instability, which depend upon Ekman pumping by a symmetric vortex, are less efficient, given the observed strength of the initial vortex. In the absence of eddy fluxes of heat and momentum, the radial circulation forced by heating and friction in the simulations from both cloud clusters and depressions decreases in intensity with time and the initial vortex weakens. In the presence of the observed eddy fluxes, the radial circulation intensifies and a hurricane forms in both simulations.