Reset thickness if the new concentration is zero

[simone-silvestri]

On ClimaOcean, ice is building up with a zero concentration in regions that should not generate sea ice. This is because if concentration is zero, both the atmospheric flux is zero and the melt flux is zero, so ice can build up over time.
It does not necessarily influence the ocean but still it is wrong.

This PR corrects this by resetting the thickness if the new concentration is identically zero (in main, we are resetting only the concentration if thickness is identically zero, but it should go both ways).

[simone-silvestri]

This is an example of such behavior on 5-year integration at 1/6th of a degree

Note that effective ice thickness is thickness * concentration

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.48%. Comparing base (8296ec2) to head (cbc2eff).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   65.36%   65.48%   +0.11%     
==========================================
  Files          28       28              
  Lines         898      901       +3     
==========================================
+ Hits          587      590       +3     
  Misses        311      311              

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[glwagner]

I didn't completely grasp your explanation why the ice thickness builds up. Can you explain it in a bit more detail?

[simone-silvestri]

I didn't completely grasp your explanation why the ice thickness builds up. Can you explain it in a bit more detail?

I am also not 100% sure, but I hypothesize that there are tiny bits of ice forming from the ocean. The concentration then drops to zero. Then the fluxes from the atmosphere are multiplied by (a > 0) so they are shut down, and the melting flux is multiplied by the concentration, so the only flux that remains is the freezing one. So, I think this PR is necessary but there might be something else wrong that will pop up later on that we have to correct.

[glwagner]

Is it a good idea to just bulid up a bunch of conditionals for the evolution? It seems like it would be better to have a simpler formulation without the conditionals. It would be easier to understand.

[glwagner]

I'd suggest that it isn't a good practice to throw in fixes for things that aren't understood because it doesn't actually represent real progress. It's just superficial progress that will ultimately cost more time in the future.