The GABLS Third Intercomparison Case for Boundary Layer Model Evaluation Part B: Results and Process Understanding

This study describes and analyzes the results of the third GEWEX Atmospheric Boundary Layer Studies (GABLS) intercomparison and evaluation study for singlecolumn models. The nineteen participating models were run with their own physical package including land surface, radiation and turbulent mixing schemes for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer like the onset of evening decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morningtime transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of dominant processes in the stable boundary layer. The diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of coupling to the soil, longwave radiation and turbulent mixing. Differences in longwave radiation schemes among models appears to be small. Variations in thermal coupling to the land surface explains most of the variations in 2-m temperature among models. Models with strong turbulent mixing overestimate boundary layer height and underestimate the wind speed at 200 m. Evening transition times spread 1.5 hour around the observed time of transition. Morning transition times spread 2 hours around observed transition time. The delay between the temperature increase at 200 m height and the surface morning transition is overestimated by the models.