An air-sea interaction model which includes turbulent transport due to capillary waves (surface ripples) is developed. The model differs from others in that the physical premises are applicable to low wind-speeds (U10 < 5 m s-1), as well as higher wind-speeds. Another new feature of the model is an anisotropic roughness length, which allows a cross wind component of the stress to be modeled. The influence of the angle between the mean wind direction and the mean direction of wave propagation is included in the anisotropic roughness length.
Most models are not accurate at low wind-speeds, and tend to underestimate fluxes in low wind-speed regions, such as the tropical oceans. Improvements over previous models are incorporated in the momentum roughness-length parameterization. In addition, the dimensionless constant in the relationship between the capillary wave component of momentum roughness length and friction velocity is re-evaluated using both wave tank data and field data. The new value is found to be 0.06, a factor of three smaller than the original estimate of 0.18. Modeling the influence of capillary waves is shown to improve the accuracy of modeled surface fluxes and drag coefficients. Several sets of tropical observations are used to examine mean increases in modeled fluxes due to capillary waves. The changes in latent heat fluxes are compared to proposed increases due to convective overturning (sometimes called 'gustiness') and are found to be larger by a factor of four. For U10 < 7 m s-1, the mean estimates for tropical fluxes of momentum and latent heat are found to increase by 0.004 N m-2 and 6 W m-2.