Hindcasting the Directional Spectra of Hurricane-Generated Waves

Abstract

A method for specifying hurricane-generated waves in the Gulf of Mexico in terms of the directional wave spectrum has been developed. The method includes a description of the surface-wind field from historical meteorological data through the application of a dynamic-hurricane frictional boundary-layer model. Introduction 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 P. 385

Hurricane Hindcast Methodology and Wave Statistics for Atlantic and Gulf Hurricanes from 1956 - 1975

“…Much hurricane data are lost because instruments cannot withstand the force of the storm, but some data are available; therefore comparisons can be made. Cardone, Pierson, and Ward (1976) and Cardone, Ross, and Ahrens (1977) conducted tests on evaluation of the constants to be used in their parameterization by using the wind data available from an oil rig directly in the path of Hurricane Camille. Their formulation was set up to recreate the hurricane wind field as accurately as possible.…”

Circulation in the Coastal Ocean

“…Extratropical storms have a typical diameter (half wavelength) of about 100 km, and exert a stress of the order of 1 Pa. Hurricanes are typically 3 times smaller in diameter, but their maximum winds are much higher, exerting a stress of order 3 Pa. Mooers et al (1976) discuss in some detail the average properties of extratropical storms over the Mid-Atlantic Bight, while Cardone et al (1976) describe a model hurricane.…”

A Parametric Wind Wave Model for Arbitrary Water Depths

“…The first generation of physical spectral wave models were mainly based on decoupled wave dynamics (Pierson et al 1966;Inoue, 1967; Barnett, 1968;Ewing, 1971;Isozaki and Uji, 1973;Cardone et al 1976). …”

“…Cardone et al (1976) developed this discrete spectral model specifically for forecasting wave spectra under extreme wind conditions such as in hurricanes. The model was calibrated to resolve adequately differences such as the response of the directional wave spectrum to erapidly turning winds (directional relaxation); the effect in representing the high-frequency equilibrium range with a f-4 or a f-5 dependence; the transition from a windsea to a fully developed sea; the proper choice of velocity in the growth terms (friction velocity u, versus 10 m or 20 m winds, Ua).…”