Towards a fundamentally new understanding of the marine atmospheric boundary layer

Accurate parameterization of the turbulent exchange of momentum, sensible and latent heat at the surface of the ocean is of fundamental importance for numerical climate prediction and weather forecasting. Based on data from ships and near-shore platforms schemes have been developed for parameterization of the fluxes in terms of so called ‘bulk parameters’, i.e. wind speed near the
surface of the ocean, surface temperature and temperature and humidity at a near-surface reference height. DeCosmo et al. (1996) report results from the HEXOS experiment, at a North Sea platform; Fairall et al. (1996) from the tropical Pacific (the TOGA COARE experiment); Mahrt et al (1996) from Danish coastal waters (Vindeby) etc. A close look at the results of these studies reveal large data scatter and even apparent inconsistencies in results. It is obvious from other recent studies, which specifically include measurements of the wave field, Donelan et al. (1997), Drennan et al. (1999), Rieder and Smith (1998) and our own studies (see below), that effects from waves are of fundamental importance for the turbulent exchange processes in the marine boundary layer and should be included in parameterizations in models. This contribution summarizes results from measurements during an eight-year period (May, 1995 – present) at the air-sea interaction station Östergarnsholm in the Baltic Sea. It illustrates vividly that the ‘classical’ concept of the sea surface as an analogue to a solid surface with moving roughness elements is valid only for the much studied case of growing waves and that understanding the role of relatively long waves, which travel faster than the wind, is crucial for a correct treatment of the air-sea exchange processes.