Intensity change for Hurricane Opal (1995) in the GFDL hurricane model

The operational forecast fields from the Geophysical Fluid Dynamics Laboratory Hurricane Prediction System (GFDL model) for Hurricane Opal (1995) are used to diagnose the rapid intensification of a tropical cyclone. The environmental wind is computed in an averaging annulus. An episode when the component of the environmental wind perpendicular to the direction of storm motion vanishes is coincident with the most rapid intensification of the storm. Using a mean tangential wind budget, positive eddy vorticity fluxes aloft are identified in the vicinity of Opal (~600 km), but this signature is as equally present during the intensifying stage as in the subsequent weakening stage of the storm. A detailed examination of each of the terms of the budget (mean and eddy vorticity flux, mean and eddy vertical advection, and friction) shows a greater vertical extension of total forcing for mean tangential winds near the center of the storm, particularly from the mean and eddy vertical advection terms. Upper-level divergence exhibits significant vertical structure, such that single level or layer average analysis techniques will not capture the divergence signature aloft. Divergence aloft is also significantly influenced by far-field convective events that are, perhaps, only indirectly influenced by the hurricane. The environmental vorticity distribution is examined for asymmetries (particularly in relation with an upper level trough to the northwest) and a mechanism for their removal is presented as a prelude to an attribution study of the specific dynamical effects of environmental features on a hurricane.