RAMS sensitivity to grid spacing and grid aspect ratio in Large-Eddy Simulations of the dry neutral Atmospheric Boundary Layer

Large Eddy Simulation (LES) is being established as a commonplace modeling tool in many areas of research interested in reproducing Atmospheric Boundary Layer (ABL) turbulence. LES results can however be highly dependent on the combination of the numerical schemes, subgrid scale model and computational grid, and their impact on simulations should be examined. The present work focuses on assessing the impact of grid spacing on LES of an idealized neutral ABL realized with the Regional Atmospheric Modeling System (RAMS), a commonly employed mesoscale model. To this aim, nine simulations with varying grid resolutions and otherwise identical setups have been performed. The grids are obtained combining three different horizontal (64, 32, 16 m) and vertical (16, 8, 4 m) spacings, covering a domain of 4096 × 4096 × 1024 m. Results are post-processed in terms of mean profiles of momentum flux, horizontal velocity and velocity variances, as well as velocity spectra and instantaneous snapshots of velocity fields. The analysis reveals that the turbulent flow can be simulated satisfactorily by employing a computational grid with a sufficiently fine horizontal spacing and an aspect ratio that alleviates potential adverse effects of the combination of RAMS numerics and subgrid model on the solution. Based on the present results, a horizontal spacing smaller than around 30 m is suggested for the examined regime, and an aspect ratio of 4 is recommended, while both larger and smaller values should be avoided. When using different aspect ratios RAMS LESs of the neutral ABL were found to be affected by an excessive dissipation of turbulence kinetic energy.