LES results of the GABLS4 exercice : an intercomparison of models in extremely stable conditions over Antarctica

In polar regions and under stable stratification, models present large biases that are dependent upon the parameterizations used for the surface and boundary layers (Holtslag et al., 2013). However, no intercomparison study focuses on very stable stratification as the first three GABLS intercomparison cases dealt with moderately stable conditions. The goal of GABLS-4 is to study real-time diurnal cycles over the Antarctic Plateau, focusing on the boundary layer characteristics and the coupling with the surface.

The intercomparison uses observations collected at the Dome-Concordia (Dome-C) Research Station in Antarctica during the summer, and in particular the observations acquired on the 45-m tower. This site was chosen for its homogeneous surface with a low conductivity as snow and on a flat topography. Here, we focus on the intercomparison of large-eddy simulations that were run for 24 hours in a simplified setup. In total, 9 different LES participate to the intercomparison. The first analysis highlights a large variability in surface fluxes with variations around 10 W/m² during daytime and nighttime which is about 50-100 % of the ensemble mean value. Sensitivity to the roughness length, the subgrid turbulent scheme as well as sensitivity to the horizontal and vertical resolution were carried on to analyze further the causes of this variability. Eventually, LES ensembles are analyzed to determine the main mechanisms involved in the mixing occurring through the entire day.