5C.1
Symmetric and asymmetric contributions to the intensification of Hurricane Opal in a GFDL model forecast
J. Dominique Möller, University of Munich, Munich, Germany; and L. J. Shapiro
Although theoretical and observational studies have investigated the influence of asymmetric atmospheric 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 same used by Persing et al. The application of AB uses PV inversion to isolate the balanced asymmetric wind and height fields associated with the asymmetric PV anomaly.
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.
Session 5C, Tropical Cyclone Intensity Change III (Parallel with Sessions 5A, 5B, & 5D)
Tuesday, 30 April 2002, 11:00 AM-12:30 PM
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