How to design single-column model experiments for comparison with observed nocturnal low-level jets?

Single-column models (SCMs) are widely employed to evaluate boundary layer parameterizations under well controlled conditions. To compare SCM results to observations, these models must be driven by realistic forcings of the 3D atmospheric state. However, these forcings are inherently uncertain. The central research question is therefore: can observations be used to distinguish between different parameterization schemes in SCM simulations or is the spread due to uncertainties in the forcings too large? This study investigates this question for the nocturnal low-level jet (LLJ) at Cabauw. First, we analyse a single LLJ case that has been used for the third GEWEX Atmospheric Boundary Layer Studies (GABLS3) intercomparison study. To estimate the forcings, for this case a blend of local observations and 3D model output has been used. A sensitivity study to both the forcings and the turbulence formulation is performed by using the SCM version of the ECMWF model. The sensitivity to the turbulence parameterization is largest in the bulk of the stable boundary layer (SBL). The influence of the forcings manifests itself mainly in the upper part of the SBL and above. Second, an ensemble of eight comparable LLJ cases is considered. Using forcings derived from 3D model integrations, SCM results of the separate cases show significant deviations from the observations. However, the mean of the SCM simulations agrees well with the mean of the observations. Based on the eight selected LLJ cases, a composite SCM case is defined from the mean forcings of the 3D model. This improves the signal-to-noise ratio, which enables a better judgement of the quality of boundary layer parameterizations in a comparison with the observations.