From near-neutral to strongly stratified: adequately modelling the clear-sky nocturnal boundary layer at Cabauw

In this study, the performance of an atmospheric single-column model (SCM) is studied systematically for stably stratified conditions. To this end, 11 years (2005-2015) of daily SCM simulations were compared to observations from the Cabauw observatory, The Netherlands. Each individual clear-sky night was classified in terms of the ambient
geostrophic wind speed with a 1 m s-1 bin-width. Nights with overcast conditions were filtered out by selecting only those nights with an average net radiation of less than -30 W m-2. A similar procedure was applied to the observational dataset. A comparison of observed and modelled ensemble-averaged profiles of wind and potential temperature
and time-series of turbulent fluxes showed that the model represents the dynamics of the nocturnal boundary layer (NBL) at Cabauw very well for a broad range of mechanical forcing conditions. No obvious differences in model performance were
found between near-neutral and strongly stratified conditions. Furthermore, observed NBL regime transitions are represented in a natural way. The reference model version performs much better than a model version which applies excessive vertical mixing as is done in several (global) operational models. Model sensitivity runs showed that for
weak-wind conditions the inversion strength depends much more on details of the land-atmosphere coupling than on the turbulent mixing. The presented results indicate that in principle the physics packages of large-scale models are sufficiently equipped for modelling stably stratified conditions for a wide range of forcing conditions.