Wind and turbulence evaluation of the ICON model (icon-2024.01-1) at sub-kilometer scales using Doppler lidar observations

Abstract. Regional numerical weather prediction models are increasingly run at sub-kilometer scale horizontal resolutions, where turbulence can no longer be considered as an entirely sub-grid scale process (the ”turbulent gray zone”). Existing turbulence evaluation methods often rely on high-resolution benchmark simulations. We present an alternative evaluation method based on Doppler lidar retrievals of winds and turbulent properties in the atmospheric boundary layer.

Two configurations of the ICOsahedral Nonhydrostatic (ICON) model are compared for horizontal mesh sizes ranging from 2.1 km to 78 m: the operationally used 1D TKE scheme "Turbdiff" which includes scale-adaptive features and accounts for some 3D turbulent terms, and a 3D Smagorinsky scheme, commonly used in large eddy simulations. Winds, turbulent kinetic energy (TKE), eddy diffusivity rate (EDR) and turbulent length scale from the Doppler lidar retrieval are used in the evaluation. Diagnostics for an equitable comparison of observed and simulated TKE are applied, accounting for grid-scale and sub-grid scale model contributions.

Results show that winds remain very similar across all resolutions and between configurations, while pronounced differences are found in TKE and EDR. The 3D Smagorinsky configuration performs poorly at coarse resolutions, overestimating both TKE and EDR, but shows comparable performance to Turbdiff at 78 m. Turbdiff more successfully re-partitions TKE between grid-scale and subgrid-scale contributions, roughly preserving the total TKE across mesh sizes. Both configurations show systematic errors in the stable nocturnal boundary layer, coinciding with poor representations of the turbulent length scale.

This study demonstrates the value of the Doppler lidar retrieval as an evaluation tool, shows that the scale-adaptive Turbdiff scheme suffers no loss of performance into the turbulent gray zone and highlights specific aspects of the scheme that limit performance for stable night-time conditions. We illustrate the relevance of the demonstrated model performance for two applications: the estimation of mixing layer height from EDR for dispersion modeling, and turbulence intensity derived from TKE for applications in the wind energy sector.