Allow different meshes to have different maximum halo depths

[tommbendall]

PR Summary

Sci/Tech Reviewer: @jameskent-metoffice
Code Reviewer: @allynt

Our current infrastructure sets a single maximum halo depth for all meshes, irrespective of their relative resolution. This isn't is necessary and doesn't match the use of the meshes. In typical atmospheric configurations, the finest mesh needs a depth appropriate for the transport scheme, which is set by the expected maximum Courant number (e.g. 10). In contrast, the coarser meshes used for multigrid generally only need a halo depth of 2, and yet we still set their depth to 10. For these multigrid meshes, this means the halos can end up extending beyond the adjacent partitions (unnecessarily!)

This is a linked PR, which addresses this by allowing different halo depths to be set for different meshes. The change in this PR is:

• make get_required_stencil_depth a subroutine, returning the value for a particular mesh
• removing get_required_stencil_depth and replacing it with mesh%get_halo_depth() in appropriate science code
• picking up the change in API to the LFRic-Core init_mesh routine, so that stencil_depth is a rank-1 array (with different values passed for different meshes)

This changes the KGO of the Schar mountain test. This is because this test has a width in the y-direction of 8 cells, but uses a maximum halo depth of 4 -- meaning that the furthest cell in both positive and negative directions is the same. This triggers issue #175. A separate PR should change those vertical slice tests to avoid this issue.

Linked to: MetOffice/lfric_core#237

Plot of test with changed KGO:

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)

trac.log

Test Suite Results - lfric_apps - stencil_depths/run8

Suite Information

Item	Value
Suite Name	stencil_depths/run8
Suite User	thomas.bendall
Workflow Start	2026-01-22T21:03:01
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2025.12.1	True
jules	MetOffice/jules@2025.12.1	True
lfric_apps	tommbendall/lfric_apps@TBendall/StencilDepths	False
lfric_core	tommbendall/lfric_core@TBendall/MultiStencilDepths	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 - 1456

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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

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Sci/Tech Review

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• Sufficient testing has been completed

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

Code Review

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• 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

[jameskent-metoffice]

This work allows the different meshes to have different maximum halo depths. This means that the multigrid meshes do not have to have the same size halo depths as meshes used for transport.

There is a check whether the coarser meshes will be used for aerosol transport, which is good. The rest of the code is good. There is one small suggested alignment which I've put in the code.

The plot for the kgo changing test looks good, as does the explanation about the halo size in the 2D X-Z tests. I am happy this passes science review.

[tommbendall]

Thanks for the science review. @allynt this is now ready for code review