Remove unsupported grids?

[gold2718]

Currently, NorESM has no plans to support several grids that are currently allowed. The workhorse grids for NorESM3 are labeled nexxnp4.pg3 which cause CAM to run physics on separate, approximately isotropic finite-volume grid with approximately the same resolution as the dycore. See https://doi.org/10.1175/MWR-D-18-0136.1 for more information.

Candidates for removal are any grid with the following atmosphere component grid:

• nexxnp4: examples ne30np4, ne5np4. These grids cause CAM to run physics using the spectral element Gauss Lobatto quadrature points which creates noise in the simulation output
• nexxnp4.pg4: These grids cause CAM to run physics on a higher resolution (16/9) than the dycore.

I suggest keeping the nexxnp4.pg2 grids. These grids cause CAM to run physics on a lower resolution (4/9) than the dycore. This can be useful if faster simulations are needed. See https://doi.org/10.1029/2019MS001684 for more information.

[mvertens]

At this point I think we should get rid of grids that don't work or won't be used.

[TomasTorsvik]

@gold2718 - Should we transfer this issue to a NorESM3 milestone?