Near-surface turbulent fluxes in stable stratification: calculation techniques for use in general-circulation models

Practically oriented flux-calculation techniques based on correction functions to the neutral drag and heat/mass transfer coefficients are further developed. In the traditional formulation, the correction functions depend only on the bulk Richardson number. However, data from measurements of turbulent fluxes and mean profiles in stable stratification over different sites exhibit too strong variability in this type of dependencies. Indirect evidence from climate and weather prediction modelling also shows that the traditional flux-calculation technique is not sufficiently advanced. It is conceivable that other mechanisms besides the surface-layer stratification and, therefore, other arguments besides the bulk Richardson number must be considered. The proposed technique includes a newly discovered effect of the static stability in the free atmosphere on the surface-layer scaling and accounts for the general essential difference between the roughness lengths for momentum and scalars. Besides bulk Richardson number, recommended correction functions depend on one more stability parameter, involving the Brunt–Visl frequency in the free atmosphere, and on the roughness lengths

Surface fluxes under shear-free convenction

In this study, theoretical models of Schumann, Sykes et al., Beljaars, and Zilitinkevich et al. are examined, compared with data, and evaluated with regard to the calculation of the minimum friction velocity and the heat transfer coefficient. All data employed in earlier papers, namely those from meteorological campaigns SCOPE, TOGA COARE and BOREX-95, and the Schmidt and Schumann and Sykes and Henn large-eddy simulations (LESs), are considered. To achieve objective comparison between different formulae, empirical coefficients are recalculated by fitting theoretical curves separately for field data and for data from LESs. Despite essential differences in the shapes of the vertical profiles and the surface-layer height formulations applied in different models, practically all of them perform rather similarly and in fairly good correspondence with the chosen data set. However, a remarkable systematic difference is observed between data from measurements, on the one hand, and LES data, on the other. It is argued that this difference results from a contribution from uncounted mean-wind shear to the friction velocity in the field experiments. By this expedient, applicability of LESs to the resistance and heat-mass transfer problem is confirmed, and empirical coefficients in the resistance and heat transfer formulations are refined

Turbulent entrainment in convective shear flows

SIGLEAvailable from TIB Hannover: RO 1728(68) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

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

Close 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. © Royal Meteorological Society, 2006