Atmospheric boundary layers influenced by convection and surface waves

Both small- and large-scale flow processes such as turbulence, vortical structures, buoyant plumes and internal waves are phenomena of importance in the atmospheric boundary layer. In the marine boundary layer an additional factor is introduced due to the presence of moving surface waves compared to a boundary layer over land. These processes influence the vertical transport of both momentum and scalars. Most atmospheric models are dependent on the turbulent exchange of momentum and scalars at the air-sea interface, but surface wave processes and their coupling to the marine boundary layer are poorly represented or neglected in weather and climate models. From large eddy simulations (LES) certain aspects of the dynamics of the marine boundary layer can be explored toward a better understanding and representation of the governing processes.

In this study a LES model, with the capability to resolve a moving sinusoidal wave at its lower boundary, have been used to study the effects of swell on the structure of the marine atmospheric boundary layer, under neutral and slightly unstable stratification. Previous numerical simulations and measurements have shown that, in certain circumstances, wind following fast-moving swell can induce momentum transport from the ocean to the atmosphere. This is accompanied by the formation of a wave-driven wind jet near the surface, and a modified boundary layer structure compared to conventional shear-driven boundary layers.

Spectral analysis along with visualization of LES data show similarities in the turbulence structure of the marine boundary layer during swell and free convection boundary layers. These results are also in qualitative agreement with atmospheric measurements collected at the Östergarnsholm site in the Baltic Sea.