A sea state parameterization for low winds and a non-arbitrary wave age.

Dr. Mark A. Bourassa, COAPS/FSU, Tallahassee FL 32306-3041
email: bourassa@coaps.fsu.edu
WWW: http://coaps.fsu.edu/~bourassa

with Dayton G. Vincent1 and W. L. Wood2

1 Department of Earth and Atmospheric Sciences, Purdue University, Indiana 47907, USA.

2 School of Civil Engineering, Purdue University, Indiana 47907, USA.

A model for significant wave height is coupled with a surface flux model to remove the (common) assumption of local equilibrium. The flux model includes the influence of surface tension (including capillary waves) on wave characteristics such as the dominant wave period and wave age. The effects of rising and falling seas are also examined.

Wave age (cp / u*) is a first order estimate of the sea state. Most models assume that the local-equilibrium value of the wave age is independent of wind speed. This reasoning is consistent with stress models (e.g. Charnock, 1955; Smith et al., 1992) that consider only gravity waves as a source of roughness. Models that also consider roughness due to other sources will have an equilibrium wave age that decreases as these source of roughness become more important. Smith's (1988) model considers roughness due to gravity waves and molecular viscosity. The BVW model considers stress due to gravity waves and capillary waves, with molecular viscosity contributing only when there are no waves present (U10 < ~2 m s-1).

The period of the dominant waves is often assumed to approach zero as the wind speed approaches zero (Pierson and Moskowitz, 1964). The BVW sea state model has a more realistic period: it approaches the 'period corresponding to the minimum phase speed' as the wind speed approaches the capillary cutoff. Many models of wave-wave interaction have terms that are proportional to Pierson and Moskowitz's period; for low wind speeds, this use of Pierson and Moskowitz's period could result in large overestimates.

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