Add a COSP timestep so diagnostics can be sampled less often

[mo-jmanners]

PR Summary

Sci/Tech Reviewer: @Petzi1
Code Reviewer: @yaswant

COSP requires an array of lit points (where the sun is above the horizon) so that SW diagnostics can be appropriately sampled. Currently the lit points are calculated for the model timestep. If COSP diagnostics are sampled less often than every model timestep then it would be more accurate to calculate the lit points based on the "COSP timestep".

A namelist option is added: n_cosp_step

This sets the number of model timesteps over which to calculate lit_fraction_cosp in illuminate_kernel_mod.F90. The lit_fraction_cosp is then passed to cosp_alg_mod.x90 in place of lit_fraction (the lit fraction for the model timestep).

• closes Add a COSP timestep so diagnostics can be sampled less often #135

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)

I have run tests using the standard rose-stem climate configuration with n_cosp_step=1 and n_cosp_step=12. The COSP sunlit_mask diagnostic was plotted in each case and correctly showed the increase in sunlit area when n_cosp_step=12.

trac.log

Test Suite Results - lfric_apps - cosp_timestep_test/run1

Suite Information

Item	Value
Suite Name	cosp_timestep_test/run1
Suite User	james.manners
Workflow Start	2026-01-20T10:38:56
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2025.12.1	True
jules	MetOffice/jules@2025.12.1	True
lfric_apps	mo-jmanners/lfric_apps@cosp_timestep_test	False
lfric_core	MetOffice/lfric_core@2025.12.1	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

Security Considerations

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Performance Impact

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Documentation

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Sci/Tech Review

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• Sufficient testing has been completed

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Code Review

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• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable

[Petzi1]

I am happy with the science changes to enable variable timesteps for calls to cosp with appropriate lit_fractions. Over to @yaswant for a code review.