[Lottie Turner (mo-lottieturner)]

PR Summary

This ticket will need a new file added to $BIG_DATA_DIR
It will also add a new lfric2um kgo set

Sci/Tech Reviewer: Steve Mullerworth (@stevemullerworth)
Code Reviewer: allynt

This PR adds to LFRic2UM the functionality to regrid by the nearest neighbour method (as opposed to bilinear interpolation). I have set it up so that all fields that have a land-sea mask use this method for the moment, until such time as functionality for masked regridding is implemented [link to PR to go here].

The stashcodes for the relevant fields (and their respective lfric names) are

Stashcode	LFRic name
279	soil_sat_frac
278	water_table
281	wetness_under_soil
213	surface_conductance
49	sea_ice_temperature

• blocks Multidata fields/pseudo-levels in LFRic2UM #266 R2O testing in LFRic2UM #564 Fixing fixed length headers (and lookup headers) in lfric2um #838

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 Apps rose-stem suite
• 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 - t806_nearest_neighbour/run12

Suite Information

Item	Value
Suite Name	t806_nearest_neighbour/run12
Suite User	charlotte.turner
Workflow Start	2026-09-24T14:58:38
Groups Run	azspice_developer', 'lfric2um_azspice

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@c22aeea	True
lfric_apps	mo-lottieturner/lfric_apps@non_bilinear_weights_lfric2um	False
lfric_core	MetOffice/lfric_core@220a00c	True
moci	MetOffice/moci@2026.07.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@77a5166	True
socrates	MetOffice/socrates@2026.07.1	True
socrates-spectral	MetOffice/socrates-spectral@2026.07.1	True
ukca	MetOffice/ukca@c372b36	True

Task Information

❌ failed tasks - 2

Task	State
rose_ana_lfricinputs_lfric2um-nwp_gal9-C48L70_N48L70_azspice_gnu_fast-debug-64bit	failed
rose_ana_lfricinputs_lfric2um-nwp_gal9-C48L70_N48L70_azspice_gnu_full-debug-64bit	failed

✅ succeeded tasks - 602

⌛ waiting tasks - 1

Task	State
housekeep_azspice	waiting

# Test Suite Results - lfric_apps - t806_nearest_neighbour/run13

Suite Information

Item	Value
Suite Name	t806_nearest_neighbour/run13
Suite User	charlotte.turner
Workflow Start	2026-09-24T15:06:19
Groups Run	ex1a_developer', 'lfric2um_ex1a', 'lfric2um_ex1a_weekly

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@c22aeea	True
lfric_apps	mo-lottieturner/lfric_apps@non_bilinear_weights_lfric2um	False
lfric_core	MetOffice/lfric_core@220a00c	True
moci	MetOffice/moci@2026.07.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@77a5166	True
socrates	MetOffice/socrates@2026.07.1	True
socrates-spectral	MetOffice/socrates-spectral@2026.07.1	True
ukca	MetOffice/ukca@c372b36	True

Task Information

❌ failed tasks - 2

Task	State
rose_ana_lfricinputs_lfric2um-nwp_gal9-C48L70_N48L70_ex1a_gnu_fast-debug-64bit	failed
rose_ana_lfricinputs_lfric2um-nwp_gal9-C48L70_N48L70_ex1a_gnu_full-debug-64bit	failed

✅ succeeded tasks - 642

⌛ waiting tasks - 1

Task	State
housekeep_ex1a	waiting

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

PSyclone Approval

• If you have edited any PSyclone-related code (e.g. PSyKAl-lite, Kernel interface, optimisation scripts, LFRic data structure code) then please contact the HPC Optimisation Team

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

[Lottie Turner (mo-lottieturner)]

Masked field regridded with nearest neighbour method

Compare with below, which is what a masked field looks like when regridded with bilinear interpolation (note the scale)

[Lottie Turner (mo-lottieturner)]

This ticket uses the kgo file from the C48 um2lfric nwp_gal9 job as the start dump: $UMDIR/standard_jobs/lfricinputs/kgo/lfricinputs_um2lfric-nwp_gal9-N320L70_C48L70_azspice_gnu_full-debug-64bit/vn3.2_t565/checkpoint_um2lfric_000001.nc, which will need to be added to $BIG_DATA_DIR/lfric2um/apps3.2 and named nwp-gal9_N320L70_C48L70.nc.

As the kgo lives on azspice and not ex1a, the ex1a trac.log has been generated with the equivalent kgo file that is used by the ex1a tasks, hence the separate trac.logs

[Steve Mullerworth (stevemullerworth)]

Please provide a slightly more comprehensive description of the change in the PR template.

Implementation is in line with existing code, so sci/tech all OK subject to feedback being dealt with. I believe my role is to provide a tech, more than sci, review as the scientific result is not the final answer and subject to further change (it would be good for the PR description to discuss these aspects).

Spotted one minor spelling error while going through the change.

[Steve Mullerworth (stevemullerworth)]

Now approved

[allynt]

Code looks good. Just have a couple of questions.

[Lottie Turner (mo-lottieturner)]

Changed log level - also reordered task defs in tasks_lfric2um.cylc so the bigger job is after the smaller job

[allynt]

My concerns have been addressed. This looks good to me.