The Influence of Sea Sprays on Drag Coefficient at High Wind Speed

Field and laboratory observations indicate that the variation of drag coefficient with wind speed at high winds is different from that under low-to-moderate winds. By taking the effects of wave development and sea spray into account, a new parameterization of drag coefficient applicable from low to extreme winds is proposed. It is shown that, under low-to-moderate wind conditions so that the sea spray effects could be neglected, the nondimensional aerodynamic roughness first increases and then decreases with the increasing wave age; whereas under high wind conditions, the drag coefficient decreases with the increasing wind speed due to the modification of the logarithmic wind profile by the effect of sea spray droplets produced by bursting bubbles or wind tearing breaking wave crests. The drag coefficients and sea surface aerodynamic roughnesses reach their maximum values vary under different wave developments. Correspondingly, the reduction of drag coefficient under high winds reduces the increasing rate of friction velocity with increasing wind speed.