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 Östergarnsholm in the Baltic Sea, with additional data from another site in the Baltic Sea, Nässkär. 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 C HN value at 14 m s − 1 and unstable stratification is about 1.5×10 −3 ; the corresponding value at the same wind speed but with slightly stable conditions is only 0.5×10 −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 C HN 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, C HN data are widely scattered, with a mean of about 0.75×10 −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. Copyright © 2004 Royal Meteorological Society