Diffusion in the stable boundary layer (Invited Presentation)

Scalar quantities in the stable nocturnal boundary layer are diffused by both turbulence and mesoscale motions. Mesoscale motions include gravity waves, pulsating drainage flows and a variety of motions vaguely referred to as "meandering". The mesoscale motions are not adequately described by existing similarity theory or spectral models. The reasons for this failure, and limited solutions, are discussed.

Turbulence diffusion must be studied in terms of different stability regimes of the stable boundary layer. Traditional vertical structure is examined in terms of the roughness sublayer, surface layer, local similarity, z-less stratification and the region near the boundary-layer top. In the very stable case, the strongest turbulence may be detached from the surface and generated by shear associated with a low level jet, gravity waves or meandering motions. In this case, similarity theory and the traditional concept of a boundary layer break down. The elevated turbulence may intermittently recouple to the surface. These cases correspond to minimum dispersion of pollutants. Possible modeling approaches are outlined.