Dependency of mesoscale organization on grid anisotropy in large-eddy
simulations of convective boundary layers at Gray Zone resolutions.
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 LES 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. 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
towards larger length scales. This can also be clearly seen from the
different kinds of spatial organization. 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.