Turbulence in the atmospheric boundary layer

In the atmospheric boundary layer, turbulence dominates the vertical exchange of all meteorologically relevant variables such as momentum, heat and moisture. To describe atmospheric turbulence we usually introduce dimensionless parameters. The ingredients of these parameters are, in the first place, the various production processes of turbulence, which are shear and buoyancy and, in combination with cloud processes, also condensation and evaporation. In the second place, the dimensionless parameters follow from the appropriate time and length scales of turbulence, which for instance may depend on boundary conditions, such as the presence of a wall. With the help of these dimensionless parameters we define so-called scaling regimes as those regions of the boundary layer which are dominated by the influence of a certain turbulent process or scale. As a result it is possible to describe meteorological variables in these scaling regimes in terms of only a limited set of parameters. The next step is to distinguish so-called prototypes of the atmospheric boundary layer which are a relevant combination of scaling regimes. These prototypes give us the framework in which to present in a systematic way the existing knowledge on the atmospheric boundary layer. The prototypes that we discuss in this paper are: the surface layer, the neutral boundary layer, the stable boundary layer, the convective boundary layer and, to illustrate the effects of clouds, the stratocumulus-topped boundary layer. Of these boundary-layer types, we shall consider the structure in terms of scaling relationships and their dynamics.