Balanced Contributions to the Intensification of Hurricane Opal as Diagnosed from a GFDL Model Forecast

Abstract

Stationary and propagating asymmetric features of atmospheric or oceanic origin near a hurricane are known to have an impact on its evolution. Although theoretical and observational studies have investigated the influence of such features on hurricane intensification, the degree to which either environmental or near-core region asymmetries of heating, friction, or potential vorticity (PV), in contrast to symmetric processes, weaken or intensify a hurricane has not been established. The present study uses the symmetric balanced model formulation of Eliassen and its extension to asymmetric balance (AB) to evaluate the impact of heating and friction, as well as eddy fluxes, on the intensification of Hurricane Opal of 1995 in a Geophysical Fluid Dynamics Laboratory (GFDL) model forecast. The diagnostics are made in cylindrical coordinates, with the symmetric vortex as the basic state and asymmetries as the environment. The application of AB, which explicitly includes the effects of asymmetric heating and friction, uses PV inversion to isolate the balanced asymmetric wind and height fields associated with the asymmetric PV anomaly. Work by Molinari and coworkers has evaluated the contributions of eddy heat and momentum fluxes associated with environmental features to the intensification of tropical storms. Their studies give some insights into the asymmetric influences on the symmetric secondary circulation and thus the evolution of the symmetric cyclone. Due to data limitations, however, they were not able to evaluate the contributions due to potentially important convective forcing. The GFDL model output includes convective heating and so allows the explicit evaluation of its effects. Persing et al. evaluated symmetric and asymmetric contributions to the intensification of Hurricane Opal of 1995 in a GFDL forecast. Their study diagnosed the various mean and eddy forcings in the tangential momentum budget, and (following the present study) calculated the balanced (Eliassen) response to model-derived eddy vorticity fluxes. The present study diagnoses the same GFDL model forecast as used by Persing et al. to evaluate the contribution of eddy fluxes of heat and momentum, and asymmetric as well as symmetric heating and friction to the symmetric secondary circulation. The evaluation of the balanced contributions to Opal's evolution requires a modification of the symmetric vortex structure. The modification is accomplished by stabilizing the vortex in as local a region as possible. Results of the present study indicate that the symmetric tangential wind acceleration in the inner core of Hurricane Opal due to symmetric heating and friction is much greater than that from asymmetric eddy forcing. At the time of the analysis, during a period of rapid intensification, eddy forcing made a small contribution to Opal's lower-tropospheric near-core spinup. The diagnosis shows that the induced balanced symmetric secondary circulation can make a substantial contribution to the tangential momentum budget and should therefore be included in order to obtain a complete depiction of the factors responsible for the evolution of the vortex. The results imply that an unbalanced secondary circulation in the eyewall region counteracts the symmetric heating, thereby reducing its effective contribution to Opal's intensification by about one-half, and that gradient unbalanced regions of the vortex induce an unbalanced secondary circulation that counteracts effective momentum sinks, thereby intensifying the vortex in those regions. Moreover, asymmetric heating and friction tend to accelerate the inner core of the hurricane, opposing the deceleration induced by the asymmetric PV. The diagnostics also imply that only a fraction of the asymmetric heating and friction contributes effectively to the response. Implications of the results for the influence of an upper-level trough on Opal's intensification are discussed.