The influence of large convective eddies on the surface-layer turbulence

Abstract

SUMMARYClose to the surface large coherent eddies consisting of plumes and downdraughts cause convergent winds blowing towards the plume axes, which in turn cause wind shears and generation of turbulence. This mechanism strongly enhances the convective heat/mass transfer at the surface and, in contrast to the classical formulation, implies an important role of the surface roughness. In this context we introduce the stability-dependence of the roughness length. The latter is important over very rough surfaces, when the height of the roughness elements becomes comparable with the large-eddy Monin-Obukhov length. A consistent theoretical model covering convective regimes over all types of natural surfaces, from the smooth still sea to the very rough city of Athens, is developed; it is also comprehensively validated against data from measurements at different sites and also through the convective boundary layer. Good correspondence between model results, field observations and large-eddy simulation is achieved over a wide range of surface roughness lengths and convective boundary-layer heights.

Influence of Canopy Seasonal Changes on Turbulence Parameterization within the Roughness Sublayer over an Orchard Canopy

“…However, we should note that the agreement of the RSL model with the observations becomes weaker under stronger instability (shear-free conditions). Under these conditions the effect of the larger-scale convective eddies on the turbulent transport in the RSL can be a relevant factor (Fitzjarrald et al 1988;Zilitinkevich et al 2006). We note that the RSL model does not physically account for these large convective eddies.…”

Online coupled regional meteorology chemistry models in Europe: current status and prospects

“…Turbulent fluxes at the surface are parametrized through different surface layer formulations (e.g. Louis, 1979;Zilitinkevich et al, 2006) and canopy (vegetation or urban) models (Hidalgo et al, 2008). Surface fluxes depend considerably on the land cover.…”

Towards improving the simulation of meteorological fields in urban areas through updated/advanced surface fluxes description

“…Therefore, in the suggested algorithm, the roughness for urban areas is characterised by, at least, two parameters: the roughness length and displacement height. Theoretical aspects of such an approach were discussed by Rotach (1994Rotach ( , 1998, Belcher and Coceal (2002), Belcher et al (2003), Zilitinkevich et al (2006b) and the COST-715 Action (Fisher et al, 2005).…”

“…This effect can be considerable especially for very rough surfaces, like the urban canopy. Therefore, Zilitinkevich et al (2006b) have derived an algorithm for recalculating the effective roughness as depending on a stabilitydependent parameterisation of the urban roughness length for momentum. The formulation for the effective roughness length for neutral and stable stratification regimes reads:…”