Impact of dominant waves on sea drag

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 which occurs when they break. A good comparison of the
model results for the sea drag with several data sets is reported.