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 (COSMO-1) in simulating the near-surface foehn evolution, with a focus on surface temperature, for a set of five south foehn events and a 5-year-long analysis dataset based on COSMO-1. A significant cold bias during foehn hours is found in the Rhine Valley as well as other northern Alpine valleys for all five cases and the 5-year climatology. Several possible causes of the cold bias are examined using 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 during 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.