ON THE DEPENDENCE OF SEA-SURFACE ROUGHNESS ON WAVE DEVELOPMENT

The aerodynamic roughness of the sea surface, z0, is investigated usin g data from Lake Ontario, from the North Sea near the Dutch coast, and from an exposed site in the Atlantic Ocean off the coast of Nova Scot ia. Scaling z0 by rms wave height gives consistent results for all thr ee datasets, except where wave heights in the Atlantic Ocean are domin ated by swell. The normalized roughness depends strongly on wave age: younger waves (traveling slower than the wind) are rougher than mature waves. Alternatively, the roughness may be normalized using the frict ion velocity, u, of the wind stress. Again, young waves are rougher t han mature waves. This contradicts some recent deductions in the liter ature, but the contradiction arises from attempts to describe z0 in la boratory tanks and in the field with a single simple parameterization. Here, it is demonstrated that laboratory waves are inappropriate for direct comparison with field data, being much smoother than their fiel d equivalents. In the open ocean there is usually a mixture of swell a nd wind-driven sea, and more work is needed before the scaling of surf ace roughness in these complex conditions can be understood.