Surface fluxes under shear-free convection

In this study, theoretical models of Schumann. Sykes er al., Beljaars, and Zilitinkevich er nl. are examined, compared with data, and evaluated with r egard to the calculation of the minimum friction velocity and the heat tran sfer coefficient. All data employed in earlier papers, namely those from me teorological 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 coeff icients are recalculated by fitting theoretical curves separately for field data and for data from LESs. Despite essential differences in the shapes o f the vertical profiles and the surface-layer height formulations applied i n 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 o ne hand, and LES data, on the other. It is argued that this difference resu lts from a contribution from uncounted mean-wind shear to the friction velo city in the held experiments. By this expedient, applicability of LESs to t he resistance and heat-mass transfer problem is confirmed. and empirical co efficients in the resistance and heat transfer formulations are refined.