The dynamics of intensification in a Hurricane Weather Research and Forecasting simulation of Hurricane Earl

Abstract

We use a high-resolution numerical simulation of Atlantic Hurricane Earl (2010) to increase our understanding of Earl’s intensification in relatively strong vertical shear in the context of a recent paradigm for tropical cyclone intensification. The integrity of the simulation is judged by comparing analyses thereof with those of the unprecedented observational data gathered in Earl. Consistent with the classical view of spin-up, the amplification of the tangential wind field above the boundary layer is found to occur as the absolute angular momentum surfaces are drawn inwards by the aggregate heating of the rotating convective clouds in the interior of the vortex. In addition to this classical pathway, spin-up occurs within the inner-core boundary layer, where the maximum tangential winds occur. The latter is another element of the new paradigm. Despite the detrimental influence of the shear on the vortex alignment and in depressing the pseudo-equivalent potential temperature outside the developing eyewall, the combined eddy processes associated with the vortical plume structures in and around the developing eyewall region are shown to contribute to an enhanced overturning circulation and an intensifying storm. These eddy processes are distinctly agradient effects that are not features of the classical spin-up mechanism. It remains to be understood how the rotating convective updraughts combine to produce the diagnosed structures of the eddy terms themselves and how vortex Rossby waves and other eddies contribute to the alignment of the vortex during intensification.