Effect of Wave Age on Wind Gust Spectra Over Wind Waves

This paper considers the effect of wave age on wind gust spectra over wind waves. Here, two wind gust spectra based on maritime data are used to serve the purpose of demonstrating the effect of wave age; the spectra according to Wills (1986 "Offshore Mean Wind Profile," Department of Energy, Offshore Technical Report No. OTH86226) and Ochi and Shin (1988, "Wind Turbulent Spectra for Design Considerations of Offshore Structures," Proceedings of the 20th Offshore Technology Conference, Houston, TX, Paper No. 5736, pp. 461-467). The effect of wave age is taken into account by using the Volkov (2001, "The Dependence on Wave Age," in Wind Stress Over the Ocean, Cambridge University Press, Cambridge, UK, pp. 206-217) sea surface roughness formula. The wave age independent Charnock (1955, "Wind Stress on a Water Surface," Q. J. R. Meteorol. Soc., 81, pp. 639-640) sea surface roughness is used as a reference. The results are valid for wind waves in local equilibrium with steady wind in laterally homogeneous flow. An example of results demonstrates a clear effect of wave age on the wind gust spectra; also, distinct differences between the two spectra for a given wave age are demonstrated. Finally, an example of application is provided, demonstrating clear effects of the two spectra and the sea surface roughness on slowly varying surge motion of a marine structure due to wind gust.