A new sea level pond scheme.

[dabail10]

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PR checklist

• Short (1 sentence) summary of your PR:
  This is meant as an alternative to the level ponds. Documentation of the new scheme is included here.
• Developer(s):
  @davidclemenssewall @dabail10
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  https://github.com/CICE-Consortium/Test-Results/wiki/cice_by_hash_forks#fa1b1b4101773826bc748e9f9da89df764e323c9
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please document the changes in detail, including why the changes are made. This will become part of the PR commit log.
  This is bfb with tr_pond_sealvl = .false. Separate CICE tag to come.

[eclare108213]

The rfracmax and rfracmin parameters bound the fraction of meltwater entering the ponds (rfrac). 1-rfrac would be the drainage of meltwater that doesn't make it into the ponds, so saying "the 'runoff' (rfrac) parameterization ... runoff is the same as drainage" might be confusing to users. Perhaps say something like "Instead of draining a portion of the total meltwater before it reaches the ponds via rfrac as in the topo and level-ice schemes, this water is handled by the macro-scale drainage in the sealvl scheme."

[davidclemenssewall]

I like the rephrasing, we'll plan to update this with the next commit.

[eclare108213]

It will take a bit more time for me to go through all of the changes -- I started with the documentation to understand what's happening. This PR is BFB since the parameterization is turned off by default, but has anyone done a QC comparison to see whether this might be climate-changing? I would expect not, but with strong albedo influences you never know.

[davidclemenssewall]

It will take a bit more time for me to go through all of the changes -- I started with the documentation to understand what's happening. This PR is BFB since the parameterization is turned off by default, but has anyone done a QC comparison to see whether this might be climate-changing? I would expect not, but with strong albedo influences you never know.

@dabail10 has been doing some testing in CESM3 development runs. We don't have an apples-to-apples comparison yet but we do have some comparisons with CESM1 and CESM2 large ensembles (LENS). In the plots below, the red line is the sealvl pond run and ignore the dark blue line.

Overall, the climatology of northern hemisphere for the CESM3 pond run tends to fall in between the CESM1-LENS and the CESM2-LENS during summer months. The ice extent is greater in the winter, although I expect that is related to changes other than the pond parameterization. The CESM3 pond run also has a more pronounced seasonal cycle than either the CESM1-LENS or CESM2-LENS. I wouldn't interpret too much from these comparisons due to the other changes in the model, but they suggest that the sealvl parameterization is not dramatically climate-changing.

Theoretically, the sealvl parameterization should produce a similar ice area-averaged albedo for ice that is ~1-3 m thick as the level pond parameterization. However, with the sealvl parameterization we should see higher transmission/solar heating of the mixed layer and reduced surface melt relative to the level parameterization. The sealvl parameterization should also result in lower albedo for thin ice in the summer, which may have impacts in the marginal ice zone and in recent and future sea ice states.

I will update here when we have a better comparison, but here are some summary figures comparing the CESM3 sealvl pond run (red line) with the large ensembles (and ignore the dark blue line):

[dabail10]

I will do a CICE QC test.

[dabail10]

Amazingly this passes the QC test.

[eclare108213]

Thank you, @dabail10. I am relieved that this modification of the melt pond scheme is not judged to be climate-changing in the QC tests. If it were, it would call into question all of the simulations we've done to date! That said, passing the QC test doesn't mean that it can't have a significant effect in a coupled system. It looks like @davidclemenssewall's tests are also showing that it's not climate-changing but is significant in some sense.

[dabail10]

I would agree with that assessment.

[davidclemenssewall]

@dabail10 Thank you for running the test!

@eclare108213, I didn't put this in the documentation, but the inspiration for these melt pond developments were the observations that CESM2 produces ice-area average albedo that is consistent with MOSAiC and SHEBA observations (Light et al., 2022) while simultaneously producing ponds with greater than observed albedo (Light et al., 2022) and area fraction (Webster et al., 2022). Our goal was to still produce reasonable ice-area average albedos while making the ponds more consistent with observations. The QC test confirms that we didn't totally throw off the albedo (which was consistent with other testing we did). I anticipate that we will see bigger impacts in coupled tests, especially in areas/climate states with thinner ice. But Dave is still running those tests.