Foam input into the drag coefficient in hurricane conditions

A semi-empirical model is proposed for the estimation of the foam impact on the variation of the effective drag coefficient, C_d, with the reference wind speed U₁₀ in stormy and hurricane conditions. The proposed model treats the efficient air-sea aerodynamic roughness length as a sum of two weighted aerodynamic roughness lengths for the foam-free and foam-covered conditions. On the basis of available optical and radiometric measurements of the fractional foam coverage and partitioning of the ocean surface into foam-covered and foam-free areas, the present model yields the resulting dependence of C_d vs. U₁₀ within the range from low to hurricane wind speeds. This dependence is in fair agreement with those obtained from both open-ocean and laboratory measurements of the vertical variation of the mean wind speed. The velocity value, at which the fractional foam coverage is saturated, is found to be responsible for the difference of C_d behavior in the laboratory and open-ocean conditions.