Evaluation of ABL parameterization methods in ICON over the Swiss Plateau

Atmospheric Boundary Layer (ABL) processes directly influence the transport, diffusion and storage of passive tracers, such as greenhouse gases (GHGs). On a regional scale, local advection and mixing can modulate the vertical distribution of these trace gases. To test and optimise the representation of vertical mixing in numerical models, ICON hindcast simulations with a model grid spacing of 1 km have been performed over the Swiss Plateau for selected weather situations. These simulations are further evaluated against surface station and profiler observations and tower measurements at two the sites on Swiss Plateau namely Payerne and Beromüsnter, the latter is a tall tower (with a height of 212m) with measurements at five inlet heights. The simulations have been done with different configurations of the operational turbulence scheme in ICON, based on a 1D Turbulent Kinetic Energy equation (NWP turbulence), and with a newly developed turbulence scheme, based on prognostic equations for two energies turbulence (2TE+APDF). Results for the stable wintertime situation show that with the 2TE+APDF scheme, the fog becomes more persistent in the model and agrees better with the observations. The better skill of the 2TE+APDF scheme is partly attributed to an improved turbulence length scale formulation leading to a better representation of boundary layer top entrainment. This improved simulation of the stable boundary layer profiles will also be relevant for an improved simulation of periods with high air pollution and GHG concentrations. The results show that the 2TE+APDF scheme performs similar to or better than the combination of operational turbulence and shallow convection scheme in ICON for stable boundary layers.