Some aspects of turbulence and mixing in stably stratified layers

This paper presents a brief overview of recent work on turbulence and mixing in stably stratified flows. It is concluded (1) that the usual generalizations and 'scaling laws' of these flows need careful qualification, and (2) that the usual feature of unstratified turbulence of having a single length scale for all large-scale processes may not be present, especially when mean shear and buoyancy forcing have different length scales and when there is significant wave motion (i,e. the Ozmidov length scale is not the relevant macroscale), The second part of the paper reviews the modeling of motions and mixing in stable density interfaces interacting with contiguous layers of turbulence. Although the rates of mass and momentum transfer across stratified interfaces are much weaker than in unstratified turbulence, they play a crucial role in the heat and mass balances in the atmosphere and oceans. The significant point is that these transfer rates are sensitive to the internal structure of the interface as well as to the overall Richardson number determined by the buoyancy jump across the interface and external turbulence.