[Mohit Dalvi (mcdalvi)]

PR Summary

Sci/Tech Reviewer: Alan J Hewitt (@alanjhewitt)
Code Reviewer: James Bruten (@james-bruten-mo)

UKCA Diagnostics calculated via API are accessed by the parent model by passing in optional data arrays (diag_data_fullht_real, diag_data_flat_real) to ukca_step. Depending on the model/ type of requests there may be no diagnostics to be obtained at all, or on some timesteps. This information is passed to API via an earlier call to ukca_update_diagnostic_requests.
However, in SET_DIAG_PTRS_1D/3D_DOMAIN, there is an additional check that raises an error when the data arrays above are passed in but no diagnostics required in that call (which is a perfectly valid situation).

The error is encountered while requesting sample diagnostics from LFRic. It looks like the UM test/ real configurations had at least one diagnostic of each flat/fullht type requested on every timestep and hence did not encounter the issue so far.

This PR will remove the checks, and set up the pointers to internal diag arrays only when diagnostics are actually needed.

• linked Add sample Chemistry diagnostics lfric_apps#751
• linked MetOffice/um#137 (only for updating revision of UKCA trunk in UM builds).

Code Quality Checklist

(Some checks are automatically carried out via the CI pipeline)

• 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

Testing

• I have tested this change locally, using the UKCA rose-stem suite
• If shared files have been modified, I have run the UM and LFRic Apps rose stem suites
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (eg. kgo changes)
• I have added tests to cover new functionality as appropriate (eg. system tests, unit tests, etc.)

trac.log

Test Suite Results - um - vn14.2_update_ukca_rev/run2

Suite Information

Item	Value
Suite Name	vn14.2_update_ukca_rev/run2
Suite User	mohit.dalvi
Workflow Start	2026-09-23T22:32:09
Groups Run	developer', 'ukca

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@2026.07.1	True
moci	MetOffice/moci@2026.07.1	True
mule	MetOffice/mule@2026.07.1	True
shumlib	MetOffice/shumlib@2026.07.1	True
socrates	MetOffice/socrates@2026.07.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@393eddb	True
ukca	mcdalvi/ukca@rem_diag_array_check	True
um	mcdalvi/um@vn14.2_update_ukca_rev	False
um_aux	MetOffice/um_aux@2026.07.1	True
um_meta	MetOffice/um_meta@2026.07.1	True

Approvals

Code Owners

• No UM Code Owners Required

Config Owners

No UM Config Owners Required

Task Information

✅ succeeded tasks - 1306

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

Performance Impact

• Performance of the code has been considered and, if applicable, suitable
  performance measurements have been conducted

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

Sci/Tech Review

• I understand this area of code and the changes being added
• 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

[Alan J Hewitt (alanjhewitt)]

Looks good to me

[Luke Abraham (theabro)]

Many thanks - looks good to me

[Alan J Hewitt (alanjhewitt)]

scitech approved

[James Bruten (james-bruten-mo)]

I've just approved, but can you add yourself to the cla here please