Atmospheric turbulent exchange of heat and matter during stable stratification over the sea

Turbulent flux of sensible heat and matter between the atmosphere and the ocean is conveniently studied in terms of a height-constant flux and a series of resistances. Often it is assumed that the resistance to the flux over the air/water micro-layers has a dominant role. Assuming that the exchange across these layers is primarily achieved through molecular conduction, models based on the so-called surface renewal mechanism have been developed. In conditions when the stratification of the atmospheric boundary layer is stable, the resistance to the flux in the atmosphere can, however, be of equal magnitude. In a European field experiment called PEP in BALTEX (Pilot study of Precipitation and Evaporation in the Baltic Sea), evaporation was measured with the eddy-correlation technique at four sites in the Baltic Proprer, i.e. the part of the Baltic Sea South of the Åland islands. From a combination of data from these measurements and simulations with several meso-scale models, the evaporation over the Balic Proper could be derived for each month of the year. During March through June the surface of the Baltic is in general colder than the air, so that the stratification is predominantly stable. As a result, the monthly evaporation for these four months was found to range from 0 to 20 mm. These are remarkably small values, in particular considering that incoming radiation is strong and winds are often high. The mechanisms that are at work in the stable marine boundary layer are far from completely known. Detailed studies from Östergarnsholm and other sites in the Baltic show conclusively that the exchange coefficients for sensible heat and water vapour are strongly reduced as stability goes from unstable to stable stratification (a result noted already in 1982 by Large and Pond). The exchange coefficient data for stable stratification have big scatter. Predictions from surface renewal theory appears to give an upper limit for the data. One particular mechanism, called ‘shear sheltering' has been identified which causes considerable reduction of the turbulent exchange at the surface. It is related to the existence of strong shear in the atmospheric surface layer, which frequently occurs over the Baltic Sea in relation to low-level jets.