Use bottom pressure anomaly for self-attraction and loading #731
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Self-attraction and loading calculation in Boussinesq pressure gradient force is refactored to be consistent with the algorithm in non-Boussinesq version.
Using SSH to calculate self-attraction and loading (SAL) is only accurate for barotropic flows. Bottom pressure anomaly should really be used for general purposes. * New runtime parameter A runtime parameter SAL_USE_BPA is added to use bottom pressure anomaly. The option requires an input file for long term mean reference bottom pressure. The reference bottom pressure field is stored with SAL_CS. * Refactor SAL and tides in Boussinesq mode As the total bottom pressure is needed for bottom pressure anomaly, SAL calculation in Boussinesq mode needs to be refactored. In addition, there is a longstanding bug in Boussinesq mode that interface height is modified by SAL and tides, and the modified interface height is erroneously used for density and pressure calculation later on. Therefore the SAL and tides are refactored by moving their calculations after pressure is calculated. Tide answers before 20230630 is retained. Answers after 20230630 are changed for Boussinesq mode.
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## dev/gfdl #731 +/- ##
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I think that this code is technically correct, and a compiler should have no problem with it, but I found the use of the pbot_anom and bpa with different units and meanings to be very confusing.
I would strongly prefer that we simply have separate 2-d arrays, with different well-described units for SSH in [Z ~> m] and pbot_anom in [R L2 Z2 ~> Pa]. I think that the added computational cost of one added 2-d array that is not actually used would be negligible compared to the human-time cost of making this code harder to understand and more susceptible to bugs if we ever had to revise it!
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Upon further reflection, I think that there is a minor problem with this PR. Answers should only change when an answer_date is set to a higher value than the date at which a change is made or anticipated to be merged in. Failing to do so runs the risk of breaking regression testing.
This commit will change answers if TIDES_ANSWER_DATE > 20230630. I think that this PR needs to be modified so that if TIDES_ANSWER_DATE > 20230630 but TIDES_ANSWER_DATE < 20241201, and answers are only changed to the new values if TIDES_ANSWER_DATE >= 20241201 (or perhaps another date like 20250101).
* Local variable SSH is renamed to pbot_anom. * Tides related local variable names are changed to be less confusing. Notably, `e_sal_tide` is renamed to `e_sal_and_tide` (the summation of sal and tide), not to be confused with `e_tide_sal`, which is renamed to `e_tidal_sal` (sal from tides). * Fix e_tidal diagnostics in Boussinesq mode. The term was not assigned if tide answer date is >20230630.
Add an alternative method for SAL and tides in Boussinesq mode. The current method adjusts interface heights with geopotential height anomaly for SAL and tides. For non-Boussinesq mode, the current method is algebraically the same as taking the gradient of SAL and tide geopotential (body forcing). For Boussinesq mode, there is a baroclinic component of tidal forcing and SAL. The alternative method is added to calculate the gradient of tidal forcing and SAL directly at the cost of additional multiplications. The new method is controlled by runtime parameter BOUSSINESQ_SAL_TIDES.
* Instead of rescaling bottom pressure to height unit, calc_Loving_scaling is modified to be conditionally dimensional. When calculating self-attraction and loading, Love numbers are now dimensional when bottom pressure anomaly is used as an input. This change eliminate Love numbers' dependence on mean seawater density. * A new coefficient called linear_scaling is added to SAL CS for the same purpose, although to use bottom pressure anomaly for scalar approximation is not quite justifiable. A WARNING is given when users try to do that. * Two separate field, SSH (sea surface height anomaly) and pbot (total bottom pressure), are used for calculating self attraction and loading when SAL_USE_BPA = False and SAL_USE_BPA = True, respectively. The change is to enhance code readability.
A new date for TIDES_ANSWER_DATE is added to recover answers for tides in Boussinesq mode before 20250201.
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A new critical answer date (20250201) is added to recover answers in Boussinesq mode between [20230701, 20250201). Some thorough tests are done offline to assure that all answers can be recovered with the correct date value. I also squashed some of the minor middle commits and made sure each of the existing commit is compilable. It seems to me that each of the current six commits has its own purpose and is self-contained. |
This PR adds the option of using bottom pressure anomaly to calculate self-attraction and loading (SAL). Currently, sea surface height (SSH) is used for SAL, which is only accurate for barotropic flows. (SSH is the barotropic equivalent of bottom pressure anomaly.)
A new runtime parameter
SAL_USE_BPAis added to use bottom pressure anomaly. The option requires an input file for long term mean reference bottom pressure.SAL_CSrather thanPressureForce_FV_CS.SAL_USE_BPAis true, total bottom pressure field is used as the input for subroutinecalc_SAL(), the reference pressure is subtracted within the subroutine.SAL calculation in Boussinesq mode is refactored in order to
SSH_IN_EOS_PRESSURE_FOR_PGFalso fix this bug, the refactor avoids having SAL code appear in multiple places.Add an alternative method for SAL and tides in Boussinesq mode, controlled by runtime parameter
BOUSSINESQ_SAL_TIDES(default=False). The current method in Boussinesq mode has a baroclinic component of tidal forcing and SAL. While it is arguably consistent with the Boussinesq approximation, it might also be confusing. The alternative method directly calculate the gradients of tidal forcing and SAL and add them toPF[uv].Old answers withTIDES_ANSWER_DATE<=20230630 are not changed. But answers withTIDES_ANSWER_DATE>20230630 in Boussinesq mode are changed at bit-level. Since the default ofTIDES_ANSWER_DATEis currently 20230630, there is probably no compelling reason to preserve answers after that date.All old answers are preserved with the correct answer date value.