Momentum transfer and foam production via breaking waves in hurricane conditions

Generated under hurricane conditions, a slip layer composed of foam, spray, bubble emulsion, etc. determines the behavior of surface drag with wind speed. This study estimates foam's contribution to this behavior. A logarithmic parametrization of surface drag is introduced, where the effective roughness length of the underlying surface is decomposed into three fractional roughness lengths. These correspond to the foam-free area (as determined by open-ocean data at low wind speeds and laboratory data at high wind speeds), which includes the effects of spray, bubble emulsion, etc., and ocean areas covered by whitecaps and streaks, each weighted by their respective coverage coefficients. A key concept of this approach is the use of well-established experimental bubble-size spectra produced by breaking surface waves to obtain the foam-produced effective roughness length. This method provides a fair correlation of the logarithmic parametrization of surface drag against wind speed with a wide class of experimental data. Additionally, this approach estimates the hurricane's potential intensity, demonstrating reasonable agreement with experimental findings. •We model variation of the effective drag coefficient in stormy and hurricane conditions.•The foam roughness is represented as a splitting of two sub-fractional roughness lengths.•The roughness lengths are determined using the bubble-size distribution functions.