The influence of ambient atmospheric features on the intensification of a hurricane is a subject of continuing interest. The degree to which these features, including upper-level atmospheric troughs, affect a hurricane's development is not clear. While some studies have indicated that upper-level troughs can have a positive impact on a hurricane's intensification, others conclude that the development of a hurricane is controlled by sea-surface and outflow temperature, with a trough possibly having only a negative impact due to vertical shear.

The approach of our continuing diagnostic analysis of hurricane intensity change involves the use of potential vorticity (PV) to evaluate the impact of ambient atmospheric features on the intensification of hurricanes in real-data numerical forecasts of the Geophysical Fluid Dynamics Laboratory (GFDL). PV is the natural context in which to understand three-dimensional hurricane dynamics. Given a distribution of PV, a balance condition between winds and pressure, a basic state background flow, and appropriate boundary conditions, an inversion can be made to diagnose the complete three-dimensional wind and temperature field. A particularly powerful application of this techinique is that of piecewise PV inversion, which allows the evaluation of the wind and temperature fields associated with a given atmospheric feature, such as an upper-level trough. The study of Shapiro and Moeller (2003) was the first to use the method of piecewise PV inversion to diagnose the features asymmetric with respect to a hurricane's center that contribute to tropical cyclone intensification. The foundation of the analysis was the balanced symmetric formulation of Eliassen for a slowly evolving vortex and its generalization to asymmetric balance (AB). The hurricane in question was Hurricane Opal of 1995. During its intensification phase the hurricane interacted with an approaching upper-level trough. The results of Shapiro and Moeller (2003) gave no indication, however, that the upper-level trough was a significant contributor to Opal's lower-tropospheric intensification.

Our continuing research diagnoses GFDL model forecasts for other hurricanes in order to determine if the results for Opal were an anomaly or the rule. In particular, Hurricane Bertha of 1996 and Hurricane Erin of 2001 are being studied. Hurricane Bertha intensified rapidly as it approached the east coast of the United States. Some authors have indicated that the intensification may have been associated with the influence of an approaching mid-latitude atmospheric trough. Warm sea surface temperatures just off the U.S. coast may also have contributed to intensification. The GFDL model closely approximated Bertha's intensification rate at this time. A similar piecewise PV inversion to that for Hurricane Opal was used to diagnose the reason for the intensification of Hurricane Bertha in the GFDL model. The result of the piecewise PV inversion for Hurricane Bertha demonstrate diagnostically for the first time that an appoaching mid-latitude upper-level trough had a significant positive impact on a hurricane's intensification. Results of this analysis, and those for Hurricane Erin as available, will be presented at the Conference.