Fix for using tidal amplitude when determining the BBL thickness

[adcroft]

Setting BBL_USE_TIDAL_BG in OM5_b003 revealed non-conservation and non-reproducibility across layouts. A local scalar variable was correctly being set based on the tidal amplitude but was being re-used after a break between loops.

• Replaced the scalar variable U_bg_sq with a vector u2_bg(:) that is set to either a constant or the square of the tidal amplitude.
• The module parameter CS%drag_bg_vel was not set if using the tidal amplitude but WAS being used for conditionals. We now set it to 1e30 because it should NOT be used or impact the solution. Setting it to zero, or anything meaningful would allow code to use it inadvertently. This "constant" does not need to satisfy any dimensional testing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.12%. Comparing base (3a3baf1) to head (d71ed45).

❗ Current head d71ed45 differs from pull request most recent head 8734901. Consider uploading reports for the commit 8734901 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##           dev/gfdl     #595      +/-   ##
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+ Coverage     37.07%   37.12%   +0.04%     
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  Files           271      271              
  Lines         80968    80804     -164     
  Branches      15118    15084      -34     
============================================
- Hits          30021    30000      -21     
+ Misses        45324    45203     -121     
+ Partials       5623     5601      -22     

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[Hallberg-NOAA]

Overall this is a valuable addition that addresses a real bug in the code. I am happy with this change, provided that a comment is added to line 2983 reminding someone looking at the code why the enormous hard-coded value does not actually break dimensional consistency (as was already explained in the description of the PR).

[Hallberg-NOAA]

With the recent minor change, I am now happy to approve this PR.

[marshallward]

Gaea regression: https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/pipelines/23134 ✔️