Split mol with transport_efficiency

[cjohnson-pi]

PR Summary

Sci/Tech Reviewer: James Kent @jameskent-metoffice
Code Reviewer: @mo-marqh

• Adds split transport to the linear and adjoint code (to give e.g. VHV vertical horizontal vertical rather than 3d)
• Adds a transport_efficiency switch to the split transport and the mol (method of lines) transport code
• Tests in the linear_model app and adjoint_tests

Associated with #108 and #113

Code Quality Checklist

• I have performed a self-review of my own code
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Testing

• I have tested this change locally, using the LFRic Core rose-stem suite
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trac.log

Test Suite Results - lfric_apps - test_split_mol/run15

Suite Information

Item	Value
Suite Name	test_split_mol/run15
Suite User	christine.johnson
Workflow Start	2026-01-16T12:23:18
Groups Run	suite_default

Dependency	Reference	Main Like
casim	MetOffice/casim@2025.12.1	True
jules	MetOffice/jules@2025.12.1	True
lfric_apps	cjohnson-pi/lfric_apps@split_mol	False
lfric_core	MetOffice/lfric_core@5d4d72f	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

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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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[jameskent-metoffice]

"substep" instead of "substeps" in comment "field at the start of each substep"

[cjohnson-pi]

Corrected

[jameskent-metoffice]

"substep" instead of "substeps" in comment "field at the start of each substep"

[cjohnson-pi]

Corrected

[jameskent-metoffice]

Are you hardwiring the flux_last_step to always be W2V? Is it possible to put in a check based on the splitting used? If not is it worth commenting that this is only suitable for testing VHV splitting?

[cjohnson-pi]

Comment added

[jameskent-metoffice]

This work sets up the split transport option for the linear model. This is vital as it will speed up the MoL linear transport, but is also necessary for FFSL linear transport (when we come to implement it).

I "pre-reviewed" this work when we were on trac, and I can see that the developer has addressed any issues I had. I have left 3 small comments in the code and I will leave it up to the developer to decide whether they warrant any changes.

It is good that the split MoL transport is tested in the linear suite, and it is the default config. It is also good that the old 3D MoL is still tested. I have looked at the plots for the kgo change tests and everything is looking good.

I am happy to approve.