PUMAS update for vapor deposition onto snow

[Katetc]

Issue Type

Externals Update

Issue Description

@adamrher and @andrewgettelman want to add a single-line code update to PUMAS that fixes a problem with vapor deposition onto snow. This will be an answer-changing update for all composts that include PUMAS, so we will put it in its own CAM PR.

Will this change answers?

Yes

Will you be implementing this yourself?

Yes

[andrewgettelman]

Correction: this is NOT a conservation problem. It is an adjustment because @adamrher indicated he did not like the impact of vapor deposition onto snow.

There is not a conservation problem.

This modification reduces vapor deposition onto snow in PUMAS in a way consistent with the original vapor deposition treatment.

[Katetc]

My apologies, I was looking at the conservation code surrounding it and thought it was related to that. I agree that there is NOT a conservation problem in PUMAS.

[adamrher]

Here's my email summarizing answer changes for this issue, for the ChangeLog:

The CLDICE and SNOW don't change much, but the reduction in vapor sink in the 6-9 km range is picked up by clubb's saturation adjustment, as you can see by the greater drying in RVMTEND_CLUBB and heating in STEND_CLUBB. This results in a slight increase in CLDLIQ in the mean, which brightens SWCF by -0.6 W/m2 (not shown), with similar changes to RESTOM (see AMWG diags). So it's a minor change, but it's "climate changing."

[cacraigucar]

Will the upcoming PR which implements this need to be pushed through fairly quickly (ie will it be for Cecile's runs)?

[adamrher]

Yes, fairly quickly. Will plan to discuss the exact priority level tmrw.

[adamrher]

Adding results to this git thread because there are concerns about bringing in science changes this late in the game. Personally my opinion is that it has a small impact on clouds (see below -0.63 W/m2), and is the right thing to do (it makes code more consistent by using the same limiter for snow that is used for rain).

AMWG diags w/ and w/o the new limiter.

[adamrher]

Now I remember @PeterHjortLauritzen and I discussed a plan to submit a new coupled run with those mod once we take a look at 66. I will take care of this and provide an update in this thread.