Influence of Atmospheric Asymmetries on the Intensification of GFDL Model Forecast Hurricanes

Abstract

Hurricanes Bertha of 1996 and Erin of 2001 both intensified rapidly during part of their time over the North Atlantic. A piecewise potential vorticity (PV) inversion is applied to model output from GFDL hurricane model forecasts to determine the contributions of atmospheric features in the hurricanes? environment to their intensification. The diagnosis indicates that Hurricane Bertha?s rapid intensification was directly augmented by an upper-level trough to the north. The significant positive impact of the trough provides quantitative confirmation of the inference of other authors. By contrast environmental interactions associated with troughs to the east and west of Hurricane Erin did not contribute directly to its rapid intensification. The implication of this result is that factors other than the troughs, including sea surface temperature, were sufficient to effect Hurricane Erin?s strengthening. Enhanced upper-level outflow concentrated northeast of the hurricane?s center that was associated with upper-level PV features to the north of Erin, including those ahead of the long-wave trough to its west, could have had some indirect contribution to its intensification. The present authors? previous piecewise inversion applied to a model forecast of Hurricane Opal of 1995 indicated that an approaching upper-level trough did not significantly contribute to the hurricane?s lower-tropospheric intensification. The conclusions of this paper demonstrate that this result is neither an exception nor the rule.