cam6_4_016: Modify RRTMGP interface for MT configurations.

[brian-eaton]

Modify the RRTMGP interface for the special case when the minimum valid pressure for RRTMGP (1 Pa) falls in the top model layer (as it does in CAM's 93 level MT configuration). The modification is to use the "extra layer" code path, and add a very thin extra layer just below 1 Pa. The algorithm to calculate the midpoint pressure in the "extra layer" has changed from the original (which assumed a model top at 0 Pa). Hence the change affects answers for the low top model configurations (LT and cam6) as well as the MT configuration.

Note that this modification is still being tested for scientific validity in the MT configuration.

Issue #1097 - HEMCO reference in .gitmodules is a branch not a tag.

• Modify .gitmodules to resolve HEMCO reference in .gitmodules is a branch not a tag. #1097

Closes #1097
Closes #1063

[cacraigucar]

Did you make any changes beyond what @brianpm provided? If so, do we need a quick review from him?

[brian-eaton]

No. This PR currently has just what @brianpm provided.

[brian-eaton]

@brianpm, @PeterHjortLauritzen, @adamrher . Unfortunately these mods change answers for all RRTMGP runs. The calculation of the midpoint pressure in the extra layer is different for LT versions than it was before. The original midpoint pressure calculation in the extra layer ignored the fact that the top interface was placed at 1 Pa (effectively assumed it was 0), so the new calculation is more accurate, but different. Should we just go with this and say we expect all RRTMGP runs to have answer changes?

[brianpm]

Hmmmmm. I didn't think of that. I guess I'm leaning toward going ahead with the change. Interested to hear if @PeterHjortLauritzen or @adamrher have a different thought.

[PeterHjortLauritzen]

I concur with @brianpm

[brian-eaton]

Thanks! I agree that it's best to move ahead with this change. The new calculation for the midpoint pressure of the "extra layer" is more robust than the original. Also, this doesn't change answers when WACCM is run with RRTMGP. Only the MT and low top model configurations are affected.