Towards a fundamentally new understanding of the marine atmospheric boundary layer

Analysis of a data set of 11.000 hours from the Östergarnsholm air-sea interaction site in the Baltic Sea has led to fundamentally new understanding of the marine atmospheric boundary ( MABL). The turbulence structure is very much influenced by the state of the sea. For growing sea (young waves) travelling slower than the wind, the MABL resembles the boundary layer over land and thus results valid for over-land conditions can be used also over the sea. One exception is the roughness length z0, which is a function of wave state (wave age). But as soon as some waves are travelling faster than the wind, mature sea or mixed sea, the MABL starts to deviate from the boundary layer over land. The momentum flux is decreased, leading to a failure of Monin-Obukhov similarity theory. During those conditions the stability parameter z/L is a measure of the combined effect of stability and the influence of waves. When the long waves, travelling faster than the wind, dominate, the MABL is quite different from the BL over land. As the momentum flux is strongly reduced due to upward transport of momentum by the long waves, the MABL shows strong similarity with the free- convection boundary layer over land (u*=0) but instead of being driven from below (surface heat flux) the source of turbulence is situated at the top of the MABL and brought down by pressure fluctuations.