Recent ideas on roughness, inhomogeneities and mixing in the atmospheric boundary layer

The Monin-Obukhov similarity theory forms the basis of the methodology used to calculate surface fluxes of momentum, and sensible and latent heat in numerical models of the atmosphere. However, observed surface properties are often not homogeneous over the subgrid length scales used in models as assumed in the theory. Conditions necessary for homogeneity are discussed. Several simple models are then presented to generalise the concept of roughness to apply in non-homogeneous conditions because of variations in terrain, vegetation, or the presence of buildings or other obstacles. These models are preliminary and indicate the need for a comprehensive theory for heterogeneous flow. Recent results from the HAPEX-MOBILHY, FIFE, BLX83 and other experiments are used to illustrate spatial variability and the problem of averaging over regional scales. Turbulent mixing within the atmospheric boundary layer (ABL) is another important element of ABL parametrisation. Several different approaches including first-order closure, higher-order (second-moment) closure, transilient and similarity theory parametrisations are discussed and their performances are compared in homogeneous and non-homogeneous situations.