Conservative Moist Solver

[Thomas Bendall (tommbendall)]

PR Summary

Sci/Tech Reviewer:
Code Reviewer:

The linear solver in the dynamics only acts upon the dry dynamics prognostics: rho, theta, u and exner, and does not provide an update to the moisture mixing ratios. While the solver locally conserves dry mass, because it updates rho and not the moisture mixing ratios, the mass of moisture species is not conserved.

This PR implements a post-solver step that corrects the moisture mixing ratios to conserve mass. It does this by:

1. Building up the appropriate dry mass flux used to increment the density in the solver (built up over multiple semi-implicit inner iterations)
2. Multiplying the dry mass flux by the upwind mixing ratio, to create a moist mass flux for each species
3. Calculating a mass-conservative increment to each moisture species

Note that this process is also consistent, as the correction uses the same shifted mesh infrastructure as the transport code.

Details

The changes in this PR can be detailed as:

1. A new option is added to the formulation namelist, controlling the application of the new moisture correction
2. The solver now passes the dry flux (used to increment rho) back through the argument list
3. A new algorithm to perform the correction to the moist mass is added
4. The map_w2_to_sh_w2_alg is implemented at r_def precision
5. The fv_3d_divergence_kernel is implemented to work at different precisions
6. Two new kernels are added:
   (a) to calculate the upwind value of the tracer
   (b) to limit the flux to ensure non-negativity

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
• All automated checks in the CI pipeline have completed successfully

Testing

• I have tested this change locally, using the LFRic Apps rose-stem suite
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
• I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
• Any new tests have been assigned an appropriate amount of compute resource and have been allocated to an appropriate testing group (i.e. the developer tests are for jobs which use a small amount of compute resource and complete in a matter of minutes)

trac.log

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

Performance Impact

• Performance of the code has been considered and, if applicable, suitable performance measurements have been conducted

AI Assistance and Attribution

• Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

Documentation

• Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

PSyclone Approval

• If you have edited any PSyclone-related code (e.g. PSyKAl-lite, Kernel interface, optimisation scripts, LFRic data structure code) then please contact the TCD Team

Sci/Tech Review

• I understand this area of code and the changes being added
• The proposed changes correspond to the pull request description
• Documentation is sufficient (do documentation papers need updating)
• Sufficient testing has been completed

(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

• All dependencies have been resolved
• Related Issues have been properly linked and addressed
• CLA compliance has been confirmed
• Code quality standards have been met
• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable