Environmental Interactions in the GFDL Hurricane Model for Hurricane Opal

Hurricane Opal (1995) crossed the Gulf of Mexico rapidly intensifying to a 130-kt storm, then fortunately weakening before landfall on the Florida panhandle. This intensification was underforecast by the National Hurricane Center. Forecast fields from the 1997 version of the Geophysical Fluid Dynamics Laboratory Hurricane Prediction System (GFDL model) for Hurricane Opal are used to diagnose the rapid intensification of the tropical cyclone. While falling short of the realized peak intensity, the simulation did capture the phase of intensification. This study presents the first step toward diagnosing the mechanisms for intensification within a moderate resolution (;15 km) hydrostatic model and testing the extant hypotheses in the literature. Using a mean tangential wind budget, and the Eliassen balanced vortex model, positive eddy vorticity fluxes aloft are identified in the vicinity (;600 km) of Opal, but are not found to aid intensification. A detailed examination of each of the terms of the budget (mean and eddy vorticity flux, mean and eddy vertical advection, and ‘‘friction’’) shows for the most rapidly intensifying episodes a greater forcing for mean tangential winds near the center of the storm, particularly from the mean vertical advection and mean vorticity flux terms. Variations in these mean terms can be primarily attributed to variations in the heating rate. Upper-level divergence exhibits significant vertical structure, such that single-level or layer-average analysis techniques do not capture the divergence signature aloft. Far from the storm ($400 km), divergence features near 200 mb are significantly influenced by convective events over land that are, perhaps, only indirectly influenced by the hurricane. While there is a trough interaction simulated within the model, we suggest that the hurricane develops strongly without an important interaction with the trough. A synthetic removal of specific potential vorticity features attributed to the trough is proposed to test this hypothesis. Imposed shear is proposed to weaken the storm at later times, which is at odds with other recent ‘‘nontrough’’ theories for the behavior of Opal.