Fix coupled checkpointing

[Tim Graham (timgraham-Met)]

PR Summary

Sci/Tech Reviewer: Mike Hobson (@mike-hobson)
Code Reviewer: mo-marqh

Since core/apps vn3.0, coupled checkpointing has been broken in LFRic as a result of the implementation of capability to output restarts at any time in the model. In this core branch a change is implemented to allow checking of whether the next time step is a coupling time step. This is needed by lfric_apps to avoid writing a checkpoint at timesteps that are not coupling timesteps causing incorrect results.

• linked Fix writing and reading coupled checkpoints lfric_apps#505

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)
  There is a failure in the metadata check that doesn't seem to be linked to anything I have changed.
• 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)

This has been extensively tested (and used) in GC coupled climate workflows as part of the GC6 project. Future tests will be added in lfric_atm to protect this functionality. At this time the priority is being able to reat and write checkpoints with the coupled model.

trac.log

Test Suite Results - lfric_core - core_fix_cpl_chkpt/run1

Suite Information

Item	Value
Suite Name	core_fix_cpl_chkpt/run1
Suite User	tim.graham
Workflow Start	2026-05-20T10:20:03
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	timgraham-Met/lfric_core@fix_cpl_chkpt	False
SimSys_Scripts	MetOffice/SimSys_Scripts@2025.12.1	True

Task Information

❌ failed tasks - 1

Task	State
validate_rose_meta	failed

✅ succeeded tasks - 382

⌛ waiting tasks - 1

Task	State
housekeep_azspice	waiting

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

[Mike Hobson (mike-hobson)]

Like its Apps companion, this change is good enough to go on main. At a later time, we might want to see if it's possible to reduce some of the duplication between the new is_coupling_time_next and the existing is_coupling_time, but as it stands, it is easy to read and understand. It provides a logical API for the Apps code to call into.

I am happy to approve this.

Note for the code reviewer: It may not be important, but the changes in this Core PR will not break Apps. As long as the the Core PR is committed before the Apps PR, they don't have to go on at the same time.