Dependency of Mesoscale Organization on Grid Anisotropy in Large‐Eddy Simulations of Convective Boundary Layers at Gray Zone Resolutions

Abstract

A new generation of operational atmospheric models operating at horizontal resolutions in the range 200 m ∼ 2 km is becoming increasingly popular for operational use in numerical weather prediction and climate applications. Such grid spacings are becoming sufficiently fine to resolve a fraction of the turbulent transports. Here we analyze Large‐eddy simulation results of a convective boundary layer obtained by coarsening horizontal grid spacings up to 800 m. The aim is to explore the dependency of the mean state and turbulent fluxes on the grid resolution. Both isotropic and anisotropic eddy diffusion approaches are evaluated, where in the latter case the horizontal and vertical eddy diffusivities differ in accord with their horizontal and vertical grid spacings. For coarsening horizontal grid sizes entrainment at the top of the boundary layer tends to get slightly enhanced for isotropic diffusion, whereas for the anisotropic diffusion approach the vertically well‐mixed boundary‐layer structure becomes severely degraded. An analysis of the energy spectrum shows that anisotropic diffusion causes relatively more dissipation of variance at smaller length scales. This leads, in turn, to a shift of spectral energy toward larger length scales that also becomes apparent from a rather different kind of spatial organization of convection. The present study therefore suggests that details with regards to the representation of processes at small scales might impact the organization at length scales much larger than the smallest scales that can be resolved by the model.

Numerical Diffusion and Turbulent Mixing in Convective Self‐Aggregation

“…Many studies investigate the role of other important factors in the triggering of CSA besides the subgrid‐scale mixing, such as vertical resolution (Jenney et al., 2023), grid anisotropy (De Roode et al., 2022), microphysics (Shi & Fan, 2021), rotation (Carstens & Wing, 2022) and SST (Coppin & Bony, 2015; M. Khairoutdinov & Emanuel, 2013; Wing & Emanuel, 2014). Future work is needed to investigate whether the hypothesized pre‐condition on large‐scale horizontal variability in convection in the triggering of CSA would be valid for different values of the above cited factors.…”

“…Many studies investigate the role of other important factors in the triggering of CSA besides the subgrid-scale mixing, such as vertical resolution (Jenney et al, 2023), grid anisotropy (De Roode et al, 2022), microphysics (Shi & Fan, 2021), rotation (Carstens & Wing, 2022) and SST (Coppin & Bony, 2015;M. Khairoutdinov & Emanuel, 2013;.…”

The Time Scale of Shallow Convective Self‐Aggregation in Large‐Eddy Simulations Is Sensitive to Numerics

“…To investigate how the development of mesoscale fluctuations is sensitive to numerics, we vary the horizontal grid spacing $${\Delta }x={\Delta }y\in \left[50,100,200\right]$$ m. At their coarsest spacing, our grid cells attain rather high aspect ratios. Although such anisotropic grids are commonly used in large‐domain LES of shallow cumulus convection (e.g., Bretherton & Blossey, 2017; Janssens et al., 2022; Klinger et al., 2017; Vogel et al., 2016), the isotropic filter length scale λ consequently overestimates the vertical length scale required from the SFS model, and underestimates the horizontal length scale (de Roode et al., 2022). As will become clear in Section 5, we will be particularly concerned with this underestimation.…”

The time scale of shallow convective self-aggregation in large-eddy simulations is sensitive to numerics

“…To investigate how the development of mesoscale fluctuations is sensitive to numerics, we vary the horizontal grid spacing 𝐴𝐴 Δ𝑥𝑥 = Δ𝑦𝑦 ∈ [50,100,200] m. At their coarsest spacing, our grid cells attain rather high aspect ratios. Although such anisotropic grids are commonly used in large-domain LES of shallow cumulus convection (e.g., Bretherton & Blossey, 2017;Janssens et al, 2022;Klinger et al, 2017;Vogel et al, 2016), the isotropic filter length scale λ consequently overestimates the vertical length scale required from the SFS model, and underestimates the horizontal length scale (de Roode et al, 2022). As will become clear in Section 5, we will be particularly concerned with this underestimation.…”