Surface Frictional Processes and Non-Local Heat/Mass Transfer in the Shear-Free Convective Boundary Layer

Abstract

During the last several decades the near-surface frictional processes in the convective boundary layer (CBL) were considered in the spirit of the Prandtl [49] theory of free convection, implying the concepts of (i) universal chaotic turbulence and (ii) local correspondence between turbulent fluxes and mean gradients. Accordingly the fluxes of heat and water vapour in the atmospheric surface layer are parameterized disregarding gross features of the CBL. Conventional practical tools are the Monin-Obukhov similarity theory and downgradient turbulence closure models. In strong convection, however, local-transport models break down. This is because of the buoyancy-driven semi-organised structures embracing the CBL, namely, comparatively narrow uprising plumes surrounded by wider downdraughts. The surface layer feeds the plumes through the CBL-scale convergence flow patterns. These can be treated as internal boundary layers of radial geometry strongly affected by the buoyancy forces. Generally, convergence flows superimpose on mean wind. In calm weather they yield their own velocity shears characterised by the “minimum friction velocity”. As a result, turbulent mixing and heat/mass transfer are facilitated depending on gross features of the CBL, such as the CBL depth and the surface roughness length. Only the first step has been taken in analysing the above mechanism. In the present paper an advanced theoretical model for the non-local frictional processes is proposed for the roughness length to the CBL depth ratios less than 0.0001. Theoretical predictions are compared with new experimental data. It is shown that data from large-eddy simulation (LES), which served as an “empirical” basis in early models, diverge drastically from field data. It is conceivable that LES, although efficient in other respects, has limited capability in treating surface roughness effects. The proposed model is in excellent agreement with atmospheric data.