Impact of swell on wind and turbulence in the wave boundary layer

Wind waves resist wind through drag by providing roughness to the ocean surface. As wind waves are generated by the local wind, the properties of waves and wind are closely related. Swell, on the other hand, is generated elsewhere and is therefore effectively uncoupled from the local wind forcing. Typically having a phase speed and direction of propagation different than the local wind, swell can impact the marine atmospheric boundary layer through the modulation of atmospheric turbulence and surface roughness. To gain a better understanding of the influence of swell on wind and turbulence, high resolution measurements of both the waves and wind are required, a tough requirement to meet in a marine environment.

In this study we present rare observations of wind and waves obtained more than 100 km offshore at a water depth of 124 m during mild and extreme weather conditions. This includes wind anemometer measurements within the (atmospheric) wave-boundary layer and directional wave spectra obtained from four laser gauges positioned on a flare bridge of an offshore platform. Results show that swell distinctly influences the vertical turbulence velocity component of the wind in the marine atmospheric boundary layer during both mild and extreme weather. Pre-multiplied co-spectra of the horizontal and vertical turbulent velocity components show imprints of swell at its corresponding frequencies. Our results substantiate the importance of swell on the momentum exchange between wind and waves.