Impact of dominant waves on sea drag

Abstract

The impact of air-flow separation from breaking dominant waves is analyzed. This impact results from the correlation of the pressure drop with the forward slope of breaking waves. The pressure drop is parameterized via the square of the reference mean velocity. The slope of breaking waves is related to the statistical properties of the wave breaking fronts described in terms of the average total length of breaking fronts. Assuming that the dominant waves are narrow and that the length of breaking fronts is related to the length of the contour of the breaking zone it is shown that the separation stress supported by dominant waves is proportional to the breaking probability of dominant waves. The breaking probability of dominant waves, in turn, is defined by the dominant wave steepness. With the dominant wave steepness increasing, the breaking probability is increased and so does the separation stress. This mechanism explains wave age (younger waves being steeper) and finite depth (the spectrum is steeper in shallow water) dependence of the sea drag. It is shown that dominant waves support a significant fraction of total stress (sea drag) for young seas due to the air-flow separation that occurs when they break. A good comparison of the model results for the sea drag with several data sets is reported.