Air-sea exchange of sensible heat over the Baltic Sea

The exchange of sensible heat at the sea surface has been studied with the aid of a comprehensive dataset from the marine site Ostergarnsholm in the Baltic Sea, with additional data from another site in the Baltic Sea, Nasskar. The measurements include turbulent fluxes at about 10 m above the water surface, profiles of temperature and wind data at several levels on towers, sea surface temperatures and wave data. The neutral Stanton number, C-HN, was found to follow predictions from surface-renewal theory quite well for unstable conditions up to a wind speed of about 10 m s(-1). For higher wind speeds the experimental data deviate to an increasing extent from the prediction based on surface-renewal theory, giving 20-40% higher values at 14 m s(-1). The CHN value at 14 m s(-1) and unstable stratification is about 1.5 x 10(-3); the corresponding value at the same wind speed but with slightly stable conditions is only 0.5 x 10-3. The interpretation is made that spray is the cause of the rapid increase of heat exchange with wind speed above about 10 m s(-1). It also explains the drop in CHN at neutral stability, the spray-mediated sensible-heat flux increasing the upward directed flux of sensible heat in unstable conditions and decreasing the flux in stable conditions. For stable conditions, CHN data are widely scattered, with a mean of about 0.75 x 10(-3). It appears that the data have an approximate upper bound given by surface-renewal theory. It is suggested that at least some of the strong suppression of the flux of sensible heat during stable conditions can be explained as a shear-sheltering effect caused by the presence of a low-level wind maximum.