Surface fluxes under shear-free convenction

Abstract

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.