*Non-Boussinesq revision of set_diffusivity

[Hallberg-NOAA]

This commit revises the internal routines called by set_diffusivity to work in fully non-Boussinesq mode, eliminating all dependencies on the Boussinesq reference density when in non-Boussinesq mode. The publicly visible interfaces to this module and the external routines it calls have already been revised, so only this file needs to be updated. The specific changes include:

• Refactored add_LOTW_BBL_diffusivity, add_MLrad_diffusivity, and set_BBL_TKE to work in units of layer vertical extent rather than layer thickness to give results in non-Boussinesq mode that avoid dependence on the Boussinesq reference density.

• Work with internal variables in vertical distances in the denominator of diffusive flux calculations in find_TKE_to_Kd, while other expressions in this routine are recast in terms of thicknesses to avoid conversions.

• Use tv%SpV_avg instead of 1/RHO_0 in find_TKE_to_Kd when in fully non-Boussinesq mode.

• Use layer target density differences in place of g_prime in set_density_ratios in mathematically equivalent expressions when non-Boussinesq.

• Use thickness_to_dz in 3 places to convert layer thicknesses into vertical distances.

• The thickness argument to add_MLrad_diffusivity (in [H ~> m or kg m-2]) has been replaced with a vertical extent argument (in [Z ~> m]).

• Use fluxes%tau_mag in place of fluxes%ustar in add_MLrad_diffusivity when in non-Boussinesq or semi-Boussinesq mode. There is a new thermo_var_ptrs type argument to the internal routine add_MLrad_diffusivity to permit this changes.

• Use the in situ near-bottom density when adding certain contributions to non-Boussinesq diffusivities. This change includes the addition of a new bottom density (rho_bot) argument to add_int_tide_diffusivity, add_LOTW_BBL_diffusivity and add_drag_diffusivity.

• Use GV%dZ_subroundoff in 4 spots in place of GV%H_to_Z*GV%H_subroundoff.

• A long-standing comment questioning whether there is double-counting of tidal mixing has been addressed (there is not) and the comment has been revised accordingly.

These changes involved changing the units of 21 internal variables and 1 element in the set_diffusivity_CS type. There are 11 new internal variables, while 9 internal variables were eliminated. A total of 44 thickness rescaling factors were eliminated, and there are 2 places where GV%Rho_0 was being used explicitly that were changed into equivalent rescaling factors.

All answers are bitwise identical in Boussinesq mode, but some solutions will change in non-Boussinesq mode with this commit.

[codecov]

Codecov Report

Merging #453 (b8ceee6) into dev/gfdl (546728a) will decrease coverage by 0.01%.
The diff coverage is 44.64%.

❗ Current head b8ceee6 differs from pull request most recent head 97315df. Consider uploading reports for the commit 97315df to get more accurate results

@@             Coverage Diff              @@
##           dev/gfdl     #453      +/-   ##
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- Coverage     38.06%   38.05%   -0.01%     
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  Files           269      269              
  Lines         77364    77389      +25     
  Branches      14286    14292       +6     
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+ Hits          29451    29453       +2     
- Misses        42572    42591      +19     
- Partials       5341     5345       +4     

Files Changed	Coverage Δ
...parameterizations/vertical/MOM_set_diffusivity.F90	52.43% <44.64%> (-1.11%)	⬇️

... and 1 file with indirect coverage changes

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[marshallward]

Gaea regression: https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/pipelines/20338 ✔️