A station-based evaluation of near-surface south foehn evolution in COSMO-1

This study investigates the skill of the Consortium for Small-scale Modeling (COSMO) model (v5.7) at 1.1 km horizontal grid size in simulating the near-surface foehn properties and evolution for five south foehn events and a five-year-long climatology. A significant near-surface cold bias is found during foehn, with an average bias of -3$$ -3 $$ K in the Rhine Valley in the five foehn cases and -1.8$$ -1.8 $$ K in the major northern foehn valleys in the five-year foehn climatology. The cold bias tends to be larger in the stronger and moister deep foehn events. Sensitivity experiments are carried out to examine the possible causes of the cold bias, including changes to the parameterization of the land-atmosphere interaction, 1D turbulence parameterization, and horizontal grid spacing. Most sensitivity experiments have only a very minor impact on the cold bias, except for the model run with a horizontal grid spacing of 550 m. The 550-m COSMO run shows a reduced cold bias during foehn hours and also an improvement in the simulated foehn duration and northward foehn extent. By inspecting the vertical dimension, we found that the near-surface cold bias downstream might partly originate upstream. A further contribution to the downstream cold bias is likely due to insufficient vertical mixing in the foehn flow. The latter is possibly enhanced in the 550-m model run, leading to a less stably stratified atmosphere in the lower few hundred meters of the atmosphere and a reduction of the reported model cold bias. Five foehn episodes and a five-year long climatology are investigated to assess the performance of the COSMO model in forecasting south foehn in the major northern valleys of the Swiss Alps. A consistent cold bias is found during the south foehn events (orange boxes) in all five northern valleys, likely caused by an upstream cold bias and a misrepresentation of the boundary layer in the foehn valleys.image