Use of conventional stability parameters during swell


  • Anna Rutgersson,

  • Ann-Sofi Smedman,

  • Ulf Högström


The situation with swell is of climatological importance over the Baltic Sea since swell is present during as much as 40% of the time. In this study, two periods with unstable and two periods with stable stratification and wind following swell are investigated. Data are taken at a small flat island in the Baltic Sea. During unstable stratification the turbulent structure shows great resemblance to the free convective boundary layer and scales with the boundary layer height. Since surface heat flux is too small to support the high levels of turbulence present, inactive turbulence is probably the dominating source. For the stably stratified layer, there are smaller differences between data with and without swell. The turbulence is mainly transported upward into the atmosphere with the aid of pressure fluctuations induced by the waves. For most of the data with swell the gradients are smaller than for growing sea. During unstable conditions the wind gradients are negative, indicating the presence of a wave-driven wind. The gradients increase with increasing height above the surface. The drag coefficient is smaller than is usually found for both stable and unstable stratification and varies very little with wind and stratification. There are only small variations in the heat transfer coefficients with changing stratification, but they are significantly different for stable and unstable stratification.