Added Stability with Advective Tracers

[jameskent-metoffice]

PR Summary

Sci/Tech Reviewer: @thomasmelvin
Code Reviewer: @cameronbateman-mo

This ticket fixes a potential stability issue, where departure points can't be found for consistent tracers with FFSL transport schemes. This issue is very rare, and has only been observed once when running Climate Case Studies with very large time steps. The fix is to revert to advective form (and thus break conservation) for the tracers on this time step only.

I've put this on a switch as some users might desire conservation above all else. None of the tests in the test suites see this stability issue, so there are no kgo changes whether the switch is on or off. (I've tested it worked on the Climate Case Study that failed).

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 Core rose-stem suite
• If required (e.g. API changes) I have also run the LFRic Apps test suite using this branch
• 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)

I have run the gungho_model and lfric_atm suites with the switch set to true. As expected nothing changed and all the kgos passed. I have left the switch set to false.

trac.log

Test Suite Results - lfric_apps - advective_tracer_stability_test/run1

Suite Information

Item	Value
Suite Name	advective_tracer_stability_test/run1
Suite User	james.kent
Workflow Start	2026-01-21T12:45:35
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2025.12.1	True
jules	MetOffice/jules@2025.12.1	True
lfric_apps	jameskent-metoffice/lfric_apps@advective_tracer_stability_test	False
lfric_core	MetOffice/lfric_core@2025.12.1	True
moci	MetOffice/moci@2025.12.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2025.12.1	True
socrates	MetOffice/socrates@2025.12.1	True
socrates-spectral	MetOffice/socrates-spectral@2025.12.1	True
ukca	MetOffice/ukca@2025.12.1	True

Task Information

✅ succeeded tasks - 1106

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

[thomasmelvin]

A couple of minor suggestions to comments strings otherwise it is all fine

[thomasmelvin]

Can you explicitly state here that this breaks conservation

[jameskent-metoffice]

Thanks, I have made this change

[thomasmelvin]

again, can you make it clear in the log statement that this will make the transport non-conservative

[jameskent-metoffice]

Thanks, I have made this change

[thomasmelvin]

Looks good to me

[cameronbateman-mo]

All looks good, moving to approved.

[cameronbateman-mo]

Apologies for all the commits having an issue with commit signature 😊

[ss421]

It looks like wee need to make a configuration update in the JEDI testing to add: adjust_tracer_equation=.false.. @DanStoneMO, can you make a linked JEDI update for this? Thanks.