Fix mistaken uses of splitting fraction

[Thomas Bendall (tommbendall)]

PR Summary

Sci/Tech Reviewer: James Kent (@jameskent-metoffice)
Code Reviewer:

Our transport scheme is designed to use various "splittings", which divide the transport step up into substeps, and "split" vertical or horizontal steps. Each of these levels of step has its own timestep (dt), which is set by information about the substepping and splitting.

A key routine is get_splitting_fraction, which sets the value in the denominator for the factor for that splitting, e.g. in a VHV splitting this would return 2 for the V steps and 1 for the H step.

It appears that this is used incorrectly in a couple of places! Instead of dividing by this value, we multiply by this value. We have managed not to notice because the timestep fortuitously cancels out in those two cases.

This PR fixes the problem by using get_splitting_factor instead.

Although hypothetically KGOs could be changed, they appear not to.

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

Test Suite Results - lfric_apps - fix_splitting_fraction/run2

Suite Information

Item	Value
Suite Name	fix_splitting_fraction/run2
Suite User	thomas.bendall
Workflow Start	2026-06-01T16:30:45
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	tommbendall/lfric_apps@fix_splitting_fraction	False
lfric_core	MetOffice/lfric_core@2026.03.2	True
moci	MetOffice/moci@2026.03.2	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.2	True
socrates	MetOffice/socrates@2026.03.2	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.2	True
ukca	MetOffice/ukca@2026.03.2	True

Task Information

✅ succeeded tasks - 1511

Security Considerations

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• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

Performance Impact

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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

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• 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

[James Kent (jameskent-metoffice)]

This ticket fixes a small bug where the wrong splitting fraction/factor was used in a few places. Luckily things cancelled out, but it is good to see it fixed. This is a very small ticket with only 4 changes and no kgo changes. I am happy we now have the correct splitting factor, and so approve this PR.