Impact of Swell on the Marine Atmospheric Boundary Layer

A model that describes the impact of swell on the marine atmospheric boundary layer is proposed. The model is based on the two-layer approximation of the boundary layer: the near-surface inner region and the outer region above. The swell-induced momentum and energy fluxes are confined within the inner region. Swell loses energy to the atmosphere and enhances the turbulent kinetic energy in the inner region. The transfer of momentum results in acceleration or deceleration of the airflow near the surface. Following-wind swell accelerates the flow, which for a very low wind results in a swell-driven wind. Opposite-wind swell decelerates the airflow, which for a steep swell could cause the reverse airflow. The sea drag in the case of opposite-wind swell is considerably enhanced as compared with the following-wind swell case. Cross-wind swell causes the rotation of the wind velocity vector with height, which leads to the deviation of the turbulent stress vector from the wind velocity vector. The impact of swell becomes more pronounced when the wind speed decreases or when the swell phase velocity increases. In fact, it is the wave age of the swell that characterizes the swell impact because the dimensionless wave-induced fluxes of energy and momentum are proportional to the wave age parameter. Both fluxes are also proportional to the swell slope so that the swell impact is stronger for a steeper swell. The model reproduces qualitatively and quantitatively the main experimental findings for the ocean swell: the impact of swell on the sea drag is very pronounced for opposite-wind swell, is less pronounced for cross-wind swell, and is only at low wind speeds.