Hindcasting the Directional Spectra of Hurricane-Generated Waves

One of the more critical factors affecting the design of an offshore structure is the estimate of the maximum wave condition that can be expected to affect the structure during its lifetime. In the Gulf of Mexico, the more severe wave conditions are associated with hurricanes. The maximum expected wave conditions are estimated using extreme-value statistical-analysis techniques. The wave data used in the extreme-value analyses must represent a statistically significant population of historical wave data. The use of population of historical wave data. The use of wave-hindcasting techniques in conjunction with the meteorological data that characterize historical storms is the most appropriate way of obtaining the required wave data.

The hindcast approach has been applied in the Gulf of Mexico with wave-hindcast models that use the significant-wave concept. These models characterize sea state only in terms of the significant wave height and period. Such a study recently described by Bea used period. Such a study recently described by Bea used the deep-water hindcast model developed by Wilson as calibrated by Patterson. That hindcast model used descriptions of the hurricane wind field provided by the empirical model developed by the Office of Hydrology of the National Oceanic and Atmospheric Admin. To produce such a description, the wind model required only a specification of radius-to-maximum wind, magnitude of -storm pressure anomaly, the forward speed of the storm, and the storm track. These parameters are available for all storms that have crossed the U.S. coast line since 1900.

The study reported here resulted in a state-of-the-art method for describing hurricane-generated winds and sea states. The development of the method dealt with both significant aspects of the problem-the hurricane wind-field model and the wave-hindcasting model. The wind-field model is based on a fundamental description of the interaction between the atmospheric pressure field and the wind field within a hurricane. However, even with this fundamental approach, the only input data required by the wind-field model is of the type available for historical hurricanes. The description of the sea state based on the concept of the directional wave spectrum. The hindcast model describes the growth, dissipation, propagation, and shallow-water modifications (refraction, shoaling, and bottom friction) of the waves in terms of the directional spectra.

The approaches taken in the development of these models were made possible by three important factors. First, a significant amount of wind and wave measurements in the hurricane environment has become available for model calibration and validation as a result of the Ocean Data Gathering Program (ODGP). Second, recent advances in dynamic hurricane modeling allowed the application of a numerical model of the hurricane frictional boundary layer to the wind-specification problem. Finally, the hindcast model could build upon the problem. Finally, the hindcast model could build upon the framework of contemporary deep-water directional spectral models that have been applied so successfully within the past decade to extratropical weather systems. past decade to extratropical weather systems. JPT

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