Tropical cyclone intensification: Part II: The role of air-sea moisture fluxes

This talk examines further a non-axisymmetric paradigm for tropical cyclone intensification described in two recent papers with colleagues S. Nguyen and J. Persing. While surface moisture fluxes are required for intensification, the intensification does not require the 'evaporation-wind' feedback process that forms the basis of an earlier paradigm. In a three-dimensional context we find that the system-scale vortex intensifies via the generation of locally buoyant vortical hot towers and the near-surface convergence that the towers induce within the boundary layer. Whereas a persistent, but weak, vortex-scale subsidence into the boundary layer outside the core region is diagnosed in our numerical experiments, neither this process nor the convective downdrafts are strong enough to obstruct the slow, but persistent recovery of boundary layer moisture en route. Therefore, even with a capped wind speed in the surface fluxes, whose capping magnitude is chosen to give latent heat fluxes comparable to nominal trade-wind values, the boundary-layer recovery is still adequate to supply the vortical hot towers with sufficient local buoyancy relative to their local environment. As a result the vortex as a whole is able to amplify almost as rapidly as in the uncapped experiments.

It will be suggested that the above findings comprise a basic understanding of the intensification process in three dimensions and should prove useful in synthesizing recent field data collected during the intensification phases of western Pacific and Atlantic tropical cyclones.