GABLS 3 SCM intercomparison and evaluation, what did we learn?

A weakly stable case is selected from the Cabauw long term archive. The case is used for an evaluation and intercomparison of nineteen SCM in the context of the GEWEX Atmospheric Boundary Layer studies. Care was taken to prescribe realistic geostrophic forcing and dynamic tendencies to the SCM’s. It is shown that in this way a direct evaluation of SCM runs with observations is feasible. The results are analyzed using a method which allows the interpretation of differences among models in terms of the dominating physical processes in the stable boundary layer, i.e. coupling to the soil, turbulent mixing and long wave radiation. Significant differences among models are found in the representation of these three processes. It is shown that on a single case basis, atmospheric forcings as obtained from a state of the art 3D atmospheric model are not accurate enough to drive the case if a direct comparison with observations is desired. Ensemble and composite approaches are applied on the basis of eight comparable nights, showing that deviations in the forcings between the model and the real-world are diminished to such extent that a very accurate comparison between models and observations becomes feasible. As a by-product, it is shown that systematic dynamical tendencies remain in the composite case, probably related to the presence of the sea.