Wind stress variation over the “spots” of horizontally variable roughness on ocean surface: theory and experiment

The areas of suppressed (as over the oil spots and over “slicks” over strong internal waves) or enhanced (as over the converging currents) surface waves are ubiquitous in the ocean. Localized changes of the surface roughness result in horizontal variations of wind profile in the near-water boundary layer. Here, two new nonlinear models of wind profile variation over the water due to horizontal variation of surface roughness (short wind waves) are suggested. One is an extension of the known 2-layer model of turbulent boundary layer to three layers, which allows a smoother matching of flows in the layers. Another is the direct numerical integration of Reynolds equations with a simple closure hypothesis. We found that within their applicability range, both models produce similar results. We further present the results of laboratory experiments performed in a closed wind-wave tank. Wind-driven surface roughness was changed either by the surfactant oil to suppress the roughness or by adding a paddle-generated surface wave which increases the roughness. Surface roughness spectra were measured by a wire gauge. Wind speed profiles above the surface were measured by a hotwire anemometer calibrated with a Pitot tube. Comparison between theoretical and experimental results reveals their quantitative agreement.