The GFDL hurricane prediction system became the official operational hurricane prediction model for the National Weather Service beginning in 1995, providing accurate track forecasts for forecasters at the Tropical Prediction Center. In regards to intensity prediction, the performance of the model has been disappointing as the model has exhibited a tendency to over predict weak systems and under predict strong systems. One of the likely causes for the model's large intensity errors include problems with the axi-symmetric assumption in the initialization. Analy- sis of the model output indicated that the environmental flow field in which the initial storm is embedded, had a large impact on the storm structure that evolved once the three dimensional model integration began. During the first 12 hours of the forecast, the storm structure underwent considerable change as it adjusted to the surrounding environment, with a storm structure evolv- ing that much more closely resembled the observed distribution, particularly regarding the storm's asymmetric structure. Guided by this evaluation, a modification to the storm initialization was for- mulated in which the forecasted asymmetries, obtained from the previous forecast, were added to the axi-symmetric bogus vortex at each of the various model levels. In the updated initialization, modification was also made to the symmetric component of the initial vortex so that the depth of the specified vortex was determined from the reported maximum surface wind speed. Test results indicated that the addition of both changes in the storm initialization resulted in improvements in both the intensity and track prediction from the GFDL hurricane system, particularly during the 12-24h forecast period. As a result, these changes were incorporated by the National Weather Ser- vice into the operational version of the GFDL hurricane prediction system, beginning with the 1998 hurricane season.
A further refinement to the above-mentioned scheme is currently being formulated at GFDL. Since the GFDL hurricane model is capable of simulating a realistic storm structure that is well adapted to its surrounding environment, in the new initializaton currently being developed, the entire forecasted vortex will be filtered from the forecasted field and used to specify the initial three dimensional storm structure. The basic framework of the new scheme as well as a summary of the test results will be presented. Comprisons with the old and new system will also be shown. It is anticipated that further improvements in intensity prediction will be possible with the new initialization. Also, as more accurate observations of the storm structure become available in real time, the structure obtained by this method should serve as a reasonable first guess field in future methods of real data assimilation.