Hurricane Opal of 1995 developed rapidly over the Gulf of Mexico before weakening as it approached the Gulf coast. Although Opal is one of the most intensely studied hurricanes ever, the cause of the hurricane's rapid intensification is still a matter of controversy. During its intensification phase the hurricane interacted with an approaching upper-level atmospheric trough. While one study attributed Opal's rapid intensification to the influence of the upper-level trough, others infer only a small impact of the trough on the hurricane's strengthening. In particular, our recent study, which diagnosed a Geophysical Fluid Dynamics Laboratory (GFDL) model forecast, found that during a period of rapid intensification eddy fluxes due to asymmetric winds and heights made only a small contribution to the lower-tropospheric evolution of the model hurricane vortex near the core.

Thus, on face value, our result supports the conclusion of the other studies that found that the upper-level trough was not important to the intensification of Opal. As we noted, however, in order to evaluate the contribution of the trough itself, the technique of piecewise potential vorticity (PV) inversion is required. Given a balance condition and an appropriate basic state, this technique allows the association of a given PV anomaly with a unique wind and height distribution. In the present study, as in our previous study, the asymmetric balance (AB) formulation is applied and the basic state is the symmetric hurricane vortex. The PV anomaly is selected so as to isolate, among other asymmetric features, the contribution of the upper-level trough to the balanced asymmetric winds and heights. These winds and heights are then used (again as in our previous study) to drive the secondary circulation in a symmetric (Eliassen) balance formulation, so that the contribution of the PV anomaly itself to the intensification of the model Opal vortex can be evaluated. Results from the GFDL forecast of Hurricane Opal as well as other cases will be shown as available.