THE SEA-SURFACE IS AERODYNAMICALLY ROUGH EVEN UNDER LIGHT WINDS

The sea surface is generally considered to be aerodynamically rough at high winds (U > 7 m/s), where the roughness length increases with win d velocity: below this velocity, the atmospheric surface layer enters a transition region and then becomes aerodynamically smooth as the win d velocity further decreases. The sea surface is shown. however, to re ach its smoothest condition at a wind velocity of about 5 m/s. and the n become rough again at lower velocities, In the latter case, the roug hness length increases as the wind velocity decreases in accordance wi th the surface-tension relation governing wind-wave interactions.