...
namelist parameter: nu_top
Laplacian smoothing applied at the model top with coefficient nu_top*nu_top_scale(k)
preqx dycore (EAM v0,v1). Sponge layer modeled after that used by CAM-EUL
nu_top=2.5e5
nu_top_scale(1:nlev)=4,2,1,0,0,0….. (applied in top 3 model layers)
Laplacian term added to RHS of hyperviscosity operator
Theta dycore (EAM v2, SCREAM). Sponge layer modeled after CAM-FV ( Lauritzen et al., https://doi.org/10.1177/1094342011410088 )
nu_top = Resolution dependent. See https://acme-climate.atlassian.net/wiki/spaces/DOC/pages/1044644202/EAM+s+HOMME+dycore
Laplacian term applied independently from hyperviscosity, and with it’s own timestep controlled by hypervis_subcycle_tom. Default 1.
nu_top_scale(:) uses reference pressure based formula
Formula in terms of reference pressure computed from “eta” coordinate:
p_ratio = eta(1)/eta(k)
nu_top_scale = 16*( 1 ./ ( 1 + (p_ratio)^-2)).(mathematically equivalent to the 8*(1+ tanh(log(p_ratio))) formula used in CAM-FV)where (nu_top_scale>8) nu_top_scale=8
where (nu_top_scale<0.15) nu_top_scale=0
This formula creates a sponge layer that is active from [ptop/10,ptop]. For EAM’s 72L configuration, ptop=0.1 mb, meaning that the sponge layer starts at 1mb which might be too high. For SCREAM’s L128 configuration, ptop=2mb, so the sponge layer starts an 20mb, which might be too low. At NE256 and 72L, the model is unstable in the sponge layer.
Simplified option with control over sponge layer thickness:
To address the instabilities seen at high resolution, we propose to add namelist variable
tom_sponge_start, specified in mbAnd use a simplified version of the above formula to make it easier to control the thickness:
p_ratio = (tom_sponge_start/100) / eta(k)
nu_top_scale = .15 * p_ratio^2
Plots of the CAM-FV / EAMv2 sponge layer profile:
The plot shows the scaling factor, with a cutoff at 8.0. The cutoff at 0.15 is shown via the black line. The curves are shown extending beyond this in the plot, but in the model values below 0.15 are set to 0 and the sponge layer is not used. “NEW” shows the simplified option, with the sponge layer starting at 10mb.
...
Experiments
Some aqua planet experiments looking at zonal wind and EKE at model top: /wiki/spaces/NGDNA/pages/1060077843
At NE256 (and probably higher) with EAM’s 72L configuration, the model is unstable and the sponge layer needs to be larger (but not stronger):
...