+*Non-Boussinesq revision of MOM_vertvisc.F90

[Hallberg-NOAA]

Revised the code in MOM_vertvisc.F90 to work internally with thickness-based units for the viscous coupling between layers, which eliminates any dependence on the value of GV%Rho_0 in non-Boussinesq mode. Various kinematic viscosities are replaced with dynamic viscosities in non-Boussinesq configurations, including revising the scaled units of the viscosities to [H Z T-1 ~> m2 s-1 or Pa s]. This commit also modifies the code to explicitly use vertical distances rather than thicknesses when calculating the vertical viscous coupling coefficients in vertvisc_coef and find_coupling_coef.

This commit involves changing the units of numerous variables to use thickness, vertical distance, dynamic viscosity or other related units, including:

• 14 elements in the vertvisc_CS type
• 6 arguments to the private routine find_coupling_coef
• 2 arguments to the private routine find_coupling_coef_gl90
• 1 arguments ("a") to the public routine write_u_accel
• 1 arguments ("a") to the public routine write_v_accel
• 1 internal variable in vertvisc
• 1 internal variable in vertvisc_remnant
• 23 internal variables in vertvisc_coef,
• 7+4+4+3 internal variables in find_coupling_coef,
• 1 internal variable in find_coupling_coef_gl90

Local variables that are no longer needed were eliminated in vertvisc and vertvisc_remnant, while 2 new local variables were added to find_coupling_coef and 6 new local variables were added to vertvisc_coef. In 6 places the Boussinesq reference density was replaced with GV%H_to_RZ, which is equivalent to the reference density in Boussinesq mode, but scales to 1 in non-Boussinesq mode.

The previous dimensional rescaling factor for KD_GL90 was incorrect (and inconsistent with the correct scaling factor used when reading in the analagous spatially varying kappa_gl90_2d); this has been corrected in this commit.

A total of 59 GV%H_to_Z or GV%Z_to_H unit conversion factors or references to GV%Rho_0 were eliminated with these changes, and in non-Boussinesq mode there is no longer any dependence on the Boussinesq reference density.

Replaced the forces argument to find_coupling_coef with an array of the friction velocities and use find_ustar to set them. When in non-Boussinesq mode, this has the effect of using forces%tau_mag and tv%SpV_avg instead of forces%ustar and GV%Rho0 when interpolating the friction velocity and stress magnitude in find_coupling_coef.

Revised units used to set the GL90 viscosities and rescale to convert diagnostics of the vertical viscosities in the MOM_vert_friction module, so that they do not depend on RHO_0 when in non-Boussinesq mode.

To accomodate this change in vertvisc, the units of the "a" arguments to write_u_accel and write_v_accel were also changed to use thickness-based arguments.

Because GV%Z_to_H is an exact power of 2 in Boussinesq mode, all answers are bitwise identical in that mode. In non-Boussinesq mode, the answers are changed by the replacement of the Boussinesq reference density by expressions using the layer-averaged specific volumes. This commit changes the units of 2 arguments in public (diagnostic) subroutine interfaces.

[codecov]

Codecov Report

Merging #428 (a0bb33b) into dev/gfdl (597bbf1) will increase coverage by 0.00%.
The diff coverage is 66.41%.

❗ Current head a0bb33b differs from pull request most recent head 6956caf. Consider uploading reports for the commit 6956caf to get more accurate results

@@            Coverage Diff            @@
##           dev/gfdl     #428   +/-   ##
=========================================
  Coverage     38.05%   38.06%           
=========================================
  Files           269      269           
  Lines         77064    77133   +69     
  Branches      14225    14233    +8     
=========================================
+ Hits          29327    29358   +31     
- Misses        42419    42456   +37     
- Partials       5318     5319    +1     

Files Changed	Coverage Δ
src/diagnostics/MOM_PointAccel.F90	3.85% <0.00%> (ø)
...c/parameterizations/vertical/MOM_vert_friction.F90	57.63% <67.42%> (-0.79%)	⬇️

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[Hallberg-NOAA]

I just pushed a revised version that eliminates some of the extra logical branches that had been added to reproduce answers when fused-multiply-adds (FMAs) are added with one particular compiler, because these could not be guaranteed to be helpful with FMAs or other forms of aggressive optimization on other compilers, and they were just adding some ugly code complexity.

[MJHarrison-GFDL]

I approve this PR.

[marshallward]

OBO @MJHarrison-GFDL

[marshallward]

Gaea regression: https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/pipelines/20230 ✔️