Influence of Atmospheric Asymmetries on the Intensification of Hurricane Opal: Piecewise PV Inversion Diagnosis of a GFDL Model Forecast

Abstract

Abstract Although Hurricane Opal of 1995 is one of the most intensely studied hurricanes ever, the cause of the hurricane's rapid intensification over the Gulf of Mexico is still a matter of controversy. While some authors have concluded that an approaching upper-level atmospheric trough had a significant impact on intensification, others have inferred only a small impact of the trough on the hurricane's strengthening. A recent study by the present authors diagnosed a Geophysical Fluid Dynamics Laboratory (GFDL) model forecast and found that eddy fluxes made only a small contribution to the lower-tropospheric evolution of the model hurricane vortex near the core. Thus, at face value, this previous study supported the conclusion that the upper-level trough was not important to the intensification of Opal. As noted in that study, however, in order to isolate the contribution of the trough by itself, the technique of piecewise potential vorticity (PV) inversion is required. The present study is the first to use this method in a diagnostic framework to determine the asymmetric features that contribute to tropical cyclone intensification. The present study uses the same GFDL hurricane model forecast as in the previous study to diagnose the balanced contribution of various pieces of the asymmetric PV anomaly to the intensification of the model Opal vortex. Though the upper-level trough is an outer-environmental feature, its influence is found to extend into Opal's inner-core region. The eddies associated with the trough induce an upper-level inner-core acceleration. An estimate of the impact of convective feedback on the influence of the upper-level trough on Opal's evolution is made. The results elucidate and modify the conclusions of other authors. There is no indication from the present diagnosis that the upper-level trough was a significant contributor to Opal's lower-tropospheric intensification.