The finite-amplitude nature of tropical cyclogenesis

Abstract

We have constructed a simple, balanced, axisymmetric model as a means of understanding the existence of the threshold amplitude for tropical cyclogenesis. In the present model the cumulus updraft mass flux depends simply and directly on the buoyancy of lifted subcloud-layer air and is not explicitly constrained by moisture convergence. The downdraft mass flux is equal to the updraft flux multiplied by (1 - ε), where ε is the precipitation efficiency. The existence of low-precipitation-efficiency clouds is crucial to the model hurricane development. When a weak vortex is placed in contact with the sea surface, the enhanced surface fluxes together with adiabatic cooling induced by Ekman pumping destabilize the atmosphere. Once amplification begins, these clouds continue to dominate the convection outside the eyewall, keeping the boundary layer θe relatively low. Without low-precipitation-efficiency clouds, large heating occurs in the outer region and the vortex expands and weakens. -from Author