Correlation between foam-bubble size and drag coefficient in hurricane conditions

A recently proposed model of foam impact on the air–sea drag coefficient C _d has been employed for the estimation of the effective foam-bubble radius R _b variation with wind speed U₁₀ in hurricane conditions. The model relates C _d (U₁₀) with the effective roughness length Z _eff (U₁₀) represented as a sum of aerodynamic roughness lengths of the foam-free and foam-covered sea surfaces Z _w (U₁₀) and Z _f (U₁₀) weighted with the foam coverage coefficient α _f (U₁₀). This relation is treated for known phenomenological distributions C _d (U₁₀), Z _w (U₁₀), and α _f (U₁₀) at strong wind speeds as an inverse problem for the effective roughness parameter of foam-covered sea surface Z _f (U₁₀). The present study is aimed at the estimation of the effective roughness of the sea surface assuming that the measurement data for the effective drag coefficient are known. The effective foam-bubble size is found as a function of the wind speed.