Modelling stable boundary layers with low winds over snow. Part I: A WRF model validation

In this paper we evaluate the Weather Research and Forecasting (WRF) mesoscale meteorological model for stable, low wind speed conditions over snow-covered surfaces, for three contrasting terrains (i.e. Cabauw, Sodankyla and Halley). For this task we use the full 3D model as well as the single column model of WRF. The SCM is driven by realistic forcings of the WRF 3D field. Several sets of SCM forcings are tested: A. no advection, B. varying geostrophic wind in time, C. wind advection on top of B, D. temperature and moisture advection on top of C, and E. force the SCM field to the 3D field above a threshold height. The WRF 3D produces overall surprisingly good results for wind speed, but the near-surface temperatures and specific humidity are overestimated for Cabauw and Sodankyla, and underestimated for Halley. Prescribing advection for momentum, temperature and moisture gives the best results for the WRF SCM, and simulations deviate strongly from reality without advection. Nudging the SCM field to the 3D field above a threshold height leads 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, improves the 2m temperature forecast. For all three sites, the forecasted temperature and moisture inversion was underestimated, though this improved when prescribing advection. Overall, the stable boundary layer over snow and ice can be forecasted to a good approximation if all processes are taken into account at high resolution, and if land surface properties are carefully prescribed.