Extending turbulence generation methods for efficient mesoscale-to-LES coupling

Advances in computational power have enabled atmospheric simulations across a broad range of scales, including the coupling of mesoscale simulations to nested large-eddy simulations (LES), where the largest turbulent eddies in the atmospheric boundary layer are resolved. However, resolved turbulence does not instantaneously develop at the mesoscale-LES interface, necessitating turbulence generation methods to accelerate the transition from the smoother mesoscale inflow to resolved turbulence in the domain interior.
We evaluate two turbulence generation methods that both apply pseudo-random perturbations at the lateral inflow boundaries. These methods are the cell perturbation method (CPM), which introduces potential temperature perturbations, and the force CPM (FCPM), which applies vertical force perturbations at the lateral boundaries. Building on the previous suggestion for the FCPM, we derive an optimized scaling law for the perturbation amplitude in this turbulence generation method. The scaling law accounts for simulation setup and inflow characteristics, including wind speed, wind direction, and static stability, and is validated using LES of idealized boundary layers.
Additionally, we perform real-case atmospheric LES that reveal significant limitations of the CPM, such as spurious precipitation under specific atmospheric conditions. In contrast, the FCPM, particularly with our proposed extensions, demonstrates robust performance, producing minimal artefacts while effectively accelerating turbulence generation, making it the preferred method for turbulence generation.
We further discuss pathways to generalize the FCPM for larger domains and longer simulations, addressing challenges such as spatially and temporally varying boundary layer characteristics. These advancements represent a step towards a flow- and scale-aware turbulence generation method, facilitating efficient coupling between mesoscale simulations and LES.