Stratified boundary layers during swell

The air-sea interaction regime characterised by the dominating waves travelling faster than the wind (swell) is not fully understood. From a climatological point of view this situation is of interest, swell being present 40% of the time at one site in the Baltic Sea. Observations of swell situations with very low surface friction, and even momentum flux directed from the surface to the atmosphere have been found during several experiments over great lakes and the open ocean. The relative importance of mechanical and thermal forcing is described by Monin-Obukhov similarity theory. The stability parameter (z/L) relates these two mechanisms. For very low surface friction, as during swell, we get large values of the stability parameter, negative or positive depending on the sign of the heat fluxes. In this investigation four different periods, two stable and two unstable, with data from a small island in the Baltic Sea was used. A stability range from z/L=-8 to over 20 was covered. These stabilities are rarely measured over land. The question then arises, does the swell dominated, stratified boundary layer behave like the buoyancy dominated boundary layer?

It is shown, as was also seen in earlier investigations (Smedman et al, 1994) and (Smedman et al, 1999) that the unstable stratified swell dominated boundary layer shows great resemblance to the free convective boundary layer, but the dominating process is not buoyancy, but inactive turbulence (Högström, 1990).

For the stable stratified boundary layer, buoyancy forces are of larger importance, but the main source of turbulent kinetic energy is pressure transport. Turbulence is transported upwards from the waves, giving a TKE-budget changing with height. Due to the decoupling from the surface, mechanical production of turbulent kinetic energy is small for both the stable and unstable situations.

The relation between the gradients and fluxes is also investigated and it is shown that they are not in local balance. The heat transfer coefficient agrees relatively well with earlier obtained values for unstable conditions, but is significantly lower compared with earlier estimates during stable stratification.