Refactor code implementing initial chi transform with respect to North Pole and Schmit stretching

[Ricky Wong (mo-rickywong)]

PR Summary

Sci/Tech Reviewer: Reviewer requested, awaiting confirmation
Code Reviewer: Mike Hobson (@mike-hobson)

While generating the chi fields from meshes, the routine init_chi_transform is called from init_fem. The implementation of this is rather complicated and has in the past resulted in changes to test-suite KGOs if altered. This PR refactors/simplifies the implementation.

Issues with the existing implementation.

The initial chi transform as it stands:

• Passes in a mesh collection by argument (when it is available in module scope), only to extract the 1st mesh (which is only dependent on load order) to obtain the North Pole, Null Island, Equator latitude from the mesh. It doesn't allow for the case where there may be different meshes in the collection such as with the lfric2lfric application.
• The variables in the module init_chi_transform_mod are uninitialized and require that init_chi_transform is called in order to prevent random data. The transform is only for certain meshes (spherical geometry), however, omitting the call to init_chi_transform when not required still resulted in some KGO changes in the test-suite tasks.
• While the initialization is required only once, calls are made from init_fem for each mesh in the mesh_collection.

PR changes

• Simplify the API, only the key reference points are sent to the routine, north_pole, null_island, equator_latitiude. The determination of what those values are should be made by the application, though for the purposes of this PR, that determination is still applied in init_fem.
• Initialise (on declaration) and finalise the settings in init_chi_transform_mod so that they are for the default case, i.e. unrotated, unstretched mesh, to prevent random data.
• Only require the init_chi_transform to be called once for spherical geometry meshes. Even then its only necessary if the mesh has been rotated or stretched. This removes the bulk of required calls to init_chi_transform in the unit-tests routines.

Out-of-Scope issue

The current change refactors and tidies the existing code. Though there is still the issue of assuming that all spherical meshes in an application will use the pole/stretching criteria held in the module sci_chi_transforms_mod. Specifically in the general case with lfric2lfric where source/destination meshes could potentially have different pole location/stretching criteria.

Linked PRs

• linked Changes to Apps repository impacted by refactoring of init_chi_transforms lfric_apps#749

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
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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
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trac.log

Test Suite Results - lfric_core - RefactorChiTrans/run1

Suite Information

Item	Value
Suite Name	RefactorChiTrans/run1
Suite User	ricky.wong
Workflow Start	2026-09-03T07:04:11
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	mo-rickywong/lfric_core@RefactorChiTrans	False
SimSys_Scripts	MetOffice/SimSys_Scripts@cab3315	True

Task Information

✅ succeeded tasks - 433

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