Testing the Nonlocal Three-dimensional Transilient Turbulence (NLT3D) scheme in the ICON model

In numerical weather prediction (NWP) models atmospheric processes are usually split into grid scale and subgrid scale ones with the latter to be parameterized. Traditionally, all subgrid scale dynamics is classified as turbulence which is mainly active in mixing the planetary boundary layer and the coupling to the surface. Modeled turbulence is essential for the proper representation of the fluxes of tracers, energy and momentum in the lower atmosphere and, therefore, for the quality of weather forecasts.

However, in modern numerical weather prediction models grid sizes keep on decreasing as affordable with increasing computational power. Thus, originally subgrid scale physical processes such as turbulence have become partially resolved on the model grid. After the classical K approaches, this first lead to the introduction of vertically non-local turbulence parameterization schemes to explicitly represent the vertical extent of the largest eddies in the PBL.

Meanwhile also horizontal grid sizes in the order of 1 km have become small enough to also resolve at least the larger eddies extending over the whole PBL on the model grid. This is accounted for in the NLT_3D scheme. It employs a transilient matrix formulation which has proven to be a flexible way to extent the non-local representation of
turbulent fluxes also to the horizontal direction. NLT_3D has already been tested in the framework of the WRF model in several short term simulations over the North American prairie and Appalachian mountain range. It has now been implemented into the ICON model of Deutscher Wetterdienst to be analyzed in longer term simulations over Europe. Additionally, the impact of different grid size of the hosting model is analyzed over more inhomogeneous terrain.

We will present results from ICON simulations including NLT_3D and discuss comparisons with the results from the classical ICON model setup including its operational turbulence scheme. For validation also comparisons with observational data from field campaigns and with data of the operational station network of DWD will be shown.