An asymmetric hurricane wind model for storm surge and wave forecasting

With the increase in US coastal population, hurricanes become an increasing threat to the lives and properties of the residents living in the vulnerable coastal regions. Storm surge and inundation are the greatest threat to life and property along the immediate coast associated with a landfalling hurricane. Accurate forecast of the storm surge and inundation is critical to hurricane preparedness and evacuation plans. The accuracy of winds is one of the most important factors affecting accuracy of the forecasts of hurricane-caused storm surge, inundation and waves. Although in recent years full physics regional and mesoscale numerical models have been greatly improved, their performance in the hurricane intensity forecasts remains unsatisfactory. Therefore, simple axis-symmetric parametric hurricane vortex models, such as the Holland model, are still widely used to provide the wind forcing input to storm surge, wave and inundation models. These axis-symmetric models are not capable of describing the asymmetric structures in real-world hurricanes, which are rarely axis-symmetric.

To meet the needs of the storm surge and wave modeling community for an asymmetric hurricane wind model, a real-time hurricane wind forecast model is developed by incorporating an asymmetric effect into the Holland hurricane wind model. National Hurricane Center (NHC)'s hurricane forecast guidance is incorporated into the model for prognostic hurricane wind modeling. This model also assimilates the National Data Buoy Center (NDBC) real time buoy data into the model's initial wind field. The method is validated using all 2003 and 2004 Atlantic and Gulf of Mexico hurricanes. The results show that 6 h and 12 h forecast winds using the asymmetric hurricane wind model is statistically more accurate than using a symmetric wind model. Detailed case studies were conducted for four historical hurricanes, namely, Floyd (1999), Gordon (2000), Lily (2002) and Isabel (2003). Although the asymmetric model performed generally better than the symmetric model, the improvement in hurricane wind forecasts produced by the asymmetric model varied significantly for different storms. In some cases, optimizing the symmetric model using observations available at initial time and the forecast mean radius of maximum wind can produce comparable wind accuracy measured in terms of RMS error of wind speed. However, in order to describe the asymmetric structure of hurricane winds, an asymmetric model is needed.

This wind model can be applied in numerical simulations of storm surge and waves induced by hurricanes. An automated real time wind forecast system has been developed using this algorithm.