Abstract
We revisit the longstanding problem of grid sensitivity, i.e., the lack of grid convergence in large-eddy simulations (LES) of the stable boundary layer. We use a comprehensive set of LES of the well-known Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study 1 (GABLS1) case with varying grid spacings between 12.5 m and 1 m to investigate several physical processes and numerical features that are possible causes of grid sensitivity. Our results demonstrate that there are two resolution regimes in which grid sensitivity manifests differently. We find that changing the numerical advection schemes and the subgrid-scale models alters the simulation results, but the options tested do not fully address the grid-sensitivity issue. Moreover, sensitivity runs suggest that the surface boundary condition and the interaction of the surface with the near-surface flow, as well as the mixing with the free atmosphere, are unlikely to be the causes of the observed grid sensitivity. One interesting finding is that the grid sensitivity in the fine grid-spacing regime (grid spacings le 2,mathrm{m}) is closely related to the reduction in the energy content of large-scale turbulence, leading to less turbulence kinetic energy and hence lower boundary-layer heights. The present work demonstrates that there is still an urgent need to address this grid-sensitivity issue in order to perform reliable LES of the stable boundary layer.
Highlights
The last two to three decades have witnessed a surge of studies using large-eddy simulations (LES) to investigate the atmospheric boundary layer and other high-Reynolds-number turbulent flows
While we see distinct differences in the mean profiles compared to the ILES runs, the grid sensitivity is not affected
Maronga et al (2020b) proposed a correction for the interactive calculation of the surface fluxes of heat and momentum via MOST, hypothesizing that the incorrect treatment of the surface boundary condition might be responsible for the grid-convergence issue
Summary
The last two to three decades have witnessed a surge of studies using large-eddy simulations (LES) to investigate the atmospheric boundary layer and other high-Reynolds-number turbulent flows. Where κ = 0.4 is the von Kármán constant and κz represents the wall effect, results in much better grid convergence, it can lead to l > This behaviour might be put into question by the reasoning that the size of the SGS eddies (l can be seen as a measure for their diameter) cannot be larger than the grid spacing. Basu and Porte-Agel (2006), for example, used a scaledependent dynamic SGS model and reported no significant differences in bulk boundary-layer parameters based on LES runs with relatively coarse grid spacings between 12.5 m and 5 m.
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