H. A. M. Sterk

In this article, we evaluate the Weather Research and Forecasting (WRF) mesoscale meteorological model for stable conditions in clear skies with low wind speeds. Three contrasting terrains with snow‐covered surfaces are considered, namely Cabauw (Netherlands, snow over grass), Sodankylä (Finland, snow over a needle‐leaf forest) and Halley (Antarctica, snow over an ice shelf). We used the full three‐dimensional (3D) model and the single‐column versions of the WRF model. The single‐column model (SCM) was driven by realistic forcings of the WRF–3D field. Several sets of SCM forcings were tested: A, no advection; B, varying geostrophic wind in time; C, momentum advection in addition to B; D, temperature and moisture advection in addition to C; E, forcing the SCM field to the 3D field above a threshold height.The WRF–3D model produced good results overall for wind speed, but the near‐surface temperatures and specific humidity were overestimated for Cabauw and Sodankylä and underestimated for Halley. Prescribing advection for momentum, temperature and moisture gave the best results for the WRF–SCM and simulations deviated strongly from reality without advection. Nudging the SCM field to the 3D field above a threshold height led to an unrealistic behaviour of the variables below this height and is not recommended. Detailed prescription of the surface characteristics, e.g. adjusting the snow cover and vegetation fraction, improved the 2 m temperature simulation. For all three sites, the simulated temperature and moisture inversion were underestimated, though this improved when prescribing advection.Overall, in clear‐sky conditions, the stable boundary layer over snow and ice can be modelled to a good approximation if all processes are taken into account at high resolution and if land surface properties are carefully prescribed.

To enhance the understanding of the impact of small‐scale processes in the polar climate, this study focuses on the relative role of snow‐surface coupling, radiation and turbulent mixing in an Arctic stable boundary layer. We extend the GABLS1 (GEWEX Atmospheric Boundary‐Layer Study 1) model intercomparison for turbulent mixing with the other relevant physical processes in the stable boundary layer over sea ice. We use the Single Column Model (SCM) version of the Weather Research and Forecasting (WRF) mesoscale meteorological model and run different combinations of boundary layer and radiation schemes, using a state‐of‐the art land surface scheme. With this intercomparison of schemes, we confirm a wide variety in the state of the atmosphere and the surface variables for the selected parameterization schemes. To understand this variety, a sensitivity analysis for one particular combination of parameterization schemes is performed, using a novel analysis method of process diagrams. The variation between the sensitivity runs indicates a relative orientation of model sensitivities to variations in each of the governing processes and these can explain the variety of model results obtained in the intercomparison of different parameterization schemes. Moreover, we apply the same method for several geostrophic wind speeds to represent a large range of synoptic conditions. Results indicate a shift in process significance for different wind regimes. For low wind regimes, the model sensitivity is larger for surface coupling and radiation, while for high wind speeds, the largest sensitivity is found for the turbulent mixing process. An interesting non‐linear feature was found for turbulent mixing for frequently occurring wind speeds and low wind speed cases, where the 2 m temperature increases for decreased amounts of mixing.