A station-based evaluation of south foehn forecasting with COSMO-1

This study investigates the skill of the COSMO model (v5.7) at 1.1 km horizontal resolution in simulating the near-surface foehn properties and evolution for five south foehn events and a 5-year-long analysis dataset. A significant cold bias, as well as a moist bias, are found in the major northern foehn valleys, including the Rhine Valley, during foehn hours in all the chosen cases and the 5-year climatological statistics. The model biases in different foehn types are inspected over the 5-year period. Among deep foehn types, the cold bias is larger (smaller) and the moist bias is smaller for the moister (drier) foehn events. Several possible causes of the cold bias are examined with sensitivity experiments for the five foehn cases. The sensitivity experiments include changes to the parameterization of the land-atmosphere interface (i.e. adoption of a skin temperature, a change of the heat resistance in the laminar sublayer, and a new formulation of the bare soil evaporation), to the 1D turbulence parameterization (including horizontal shear production of turbulence as a first step towards 3D effects), and to the horizontal grid spacing (1.1 km versus 550 m). While several of the sensitivity experiments impact the 2-m temperature indifferently during both foehn- and non-foehn hours, only a change in the horizontal grid spacing has a significant impact on the 2-m temperature during foehn hours. The 550-m run shows also an improvement in the simulated foehn duration and northward foehn extent. Possible reasons for the improvements and the remaining bias will be discussed.