Mean square slopes of the wind-disturbed water surface, their magnitude, directionality, and composition

Available laboratory and field data on fine structures of the wind-disturbed water surface are reviewed and analyzed to deduce the quantitative variation of directional mean square slopes with the wind condition. The mean square slope is associated with the wind-friction velocity, with its maximum component in the upwind-downwind direction and minimum component in the crosswind direction. Its angular distribution is suggested to follow an elliptical function. Spectral compositions of fine surface structures are estimated; in the field, gravity wave components contribute the most at low winds (U10 < 7 m s−1 or u* < 20 cm s−1) and capillary components at high winds; in laboratory tanks, gravity components are relatively much more important. These along with the other features are used to explain discrepancies between laboratory and field results as well as those within each group.

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