Geophysica (1997),33(2),45-50 Extending the Similarity Theory for Atmospheric Boundary Layers: Contribution from Background Stratification Sylvain M. Joffre Finnish Meteorological Institute P.O. Box 503 FIN-00101 Helsinki, Finland Abstract The structure of the atmospheric boundary layer has traditionally been described using only surface fluxes and the Coriolis parameter. We review here the proposal of Kitaigorodskii (1988) and Kitaigorodskii and Joffre (1988) that the background stratification of the atmosphere can influence the overall properties of the boundary layer through the process of entrainment. Theoretical predictions have been successfully compared to various data sets. Key words: Atmospheric boundary layer, similarity theory, turbulence, stratification effects 1 Introduction The atmospheric boundary layer (ABL) is the layer where interactions take place between the surface, which captures most of the solar energy and redistributes it under different forms, and the large-scald atmospheric flow which absorbs them in the end. This turbulent ABL is of great importance for transfer of momentum, heat and atmospheric constituents, dispersion of pollutants, for enhancing evaporation over the sea, in creating wind waves on water surface and producing drift currents in the upper layer of the water bodies. More recently, it has been recognized that transfers of many minor constituents (SO 2 , NO x , CO 2 , O 3 , C x H y ) which often enter the atmosphere or are deposited to terrestrial and aquatic ecosystems via the ABL is one of the main link in the global biogeochemical cycles. It is thus of primary importance to be able to understand, parametrize and simulate the structure and behaviour of the ABL. The classical way of describing its structure is through similarity theories (eg. Kazanski and Monin, 1960) where the only influencing agents are rotation and buoyancy. There are, however, other factors such as entrainment, the intensity of which is connected with background stratification. The height h of the ABL is the key parameter embedding all internal and external influences and having much importance for a host of applications is. Predictions of the ABL height has many theoretical and practical applications such as predictions of pollutant concentrations or of surface temperature. We will review in this paper new theoretical and empirical arguments proposed by Kitaigorodskii (1988) and Kitaigorodskii and Joffre (1988) for extending the similarity theory describing the height of the ABL. _______________________________________________________________________________________ Published by the Finnish Geophysical Society, Helsinki