Alternative congelation formulation following Plante et al. 2024

[eclare108213]

Draft for Plante et al. to confirm that these changes are all that are needed, separate from their flooding/snow-ice and forcing modifications. I still need to add tests to the test suite and update the Icepack documentation, then implement and test in CICE. Suggestions for better congel_freeze option labels are welcome.

I realigned some columns while working on this. Ignore white space when looking at diffs!

• Short (1 sentence) summary of your PR:
  Add alternative congelation formulation following Plante et al., 2024.
• Developer(s):
  @mathieuslplante et al., @eclare108213
• 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.
  congel_freeze = ‘two-step’ is BFB in the base_suite -- this is currently the default (as before).
  congel_freeze = 'one-step' is not BFB. Setting conserv_check = .true. does not turn up any issues.
  A QC comparison for CICE PR #965 shows the differences between congel_freeze = 'one-step' and 'two-step' are not climate changing in CICE standalone simulations.
• How much do the PR code changes differ from the unmodified code?
  • bit for bit (default)
  • different at roundoff level
  • more substantial when congel_freeze = 'one-step'
• 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.

Namelist flag congel_freeze chooses which formulation to use. The original is ‘two-step’, since only the mushy boundary moves in the first step and the phase change happens in the next step. Plante et al. (‘one-step’) moves the boundary and performs the phase change in a single time step.

Closes #481

[mathieuslplante]

I confirm that the new one-step option corresponds exactly to our changes in Plante et al. (2024).

[apcraig]

Will need some changes in Icepack documentation and maybe add a test case. Also need to update the icepack interface documentation (run ./icepack.setup --docintfc). CICE will also eventually need a new namelist added.

[apcraig]

See also CICE-Consortium/CICE#965

This PR and the CICE PR are ready to merge assuming we are happy leaving the old 'two-step' as the default. Please confirm that's how we want to proceed. We can always change the default later.

[dabail10]

I did do a fully-coupled test here with the one-step option and the solutions are quite different. Climate changing? Maybe not, but I think this is the correct way to go. We will definitely be using this in CESM3.