Workaround for z-star minimum number of vertical levels

[cbegeman]

For MPAS-O configurations where tracers are active, we want a minimum of 3 vertical levels. This was not easy to enforce for z-star (or z-level) coordinates. This is one possible work-around for z-star coordinates which relies on new min_vert_levels and min_layer_thickness config options AND requires that the config option bottom_depth be set to less than or equal to min(bottomDepth + ssh) (but if less than this value, it may be that min(maxLevelCell - minLevelCell + 1) > min_vert_levels).

Checklist

• [X] User's Guide has been updated
• [X] Documentation has been built locally and changes look as expected
• [X] Testing comment in the PR documents testing used to verify the changes

[cbegeman]

@xylar Have you thought at all about this situation? Because of drying_slope's bottom slope, when the config option bottom_depth is set to max(bottomDepth), -refBottomDepth[min_vert_levels - 1] < -min(bottomDepth).

This solution worked out alright for getting me to a drying_slope z-star configuration that I could try out before moving on to the z-star isomip_plus tests. I don't know what the best long-term implementation should look like though.

[cbegeman]

@xylar Putting this on your radar for when you get back. As noted above it's needed for drying_slope and I'm just about ready to open that PR.

[cbegeman]

Testing

This PR is BFB with main on chrys with intel, impi for all z-star tasks.

[xylar]

It might be good to add the new config options to the user's guide but it might be fine if they get added as they're used in individual tests.

[cbegeman]

It might be good to add the new config options to the user's guide but it might be fine if they get added as they're used in individual tests.

You're too fast to comment ;) I was just testing the docs build before pushing.

[xylar]

Great to have this in!