Read weights on pe0

[Steve Mullerworth (stevemullerworth)]

PR Summary

Sci/Tech Reviewer: Mike Hobson (@mike-hobson)
Code Reviewer: Lottie Turner (@mo-lottieturner)

When run in parallel, all ranks of lfric2um and um2lfric read the regridding weights. However, regridding is only done on rank 0. The main purpose of this change is to read the weights only on rank 0. Doing so saves significant memory which is important when regridding at high resolution: the memory use of a C896-N1280 lfric2um run is reduced by 150GB when running on 108 ranks with 32 IO servers.

Unfortunately, to enable this to be done, some significant reordering of the application set-up needed to be done as MPI is not set up by the LFRic component till after the weights are read in. The reordering of the setup forms the bulk of the code changes in this branch.

The branch breaks the set-up into two. In the first part sets up MPI, but also the LFRic logger and XIOS. Setting up the LFRic logger early gives the benefit that a lot of log messages from the applications now go to the PET output whereas previously they ended up in stdout because they were sent before the logger had been initialised.

While lfric2um is the main target of this PR, parallel changes are made to um2lfric and the sciintelapi to maintain consistency between the applications.

closes #590
closes #642

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 - read_weights_on_pe0/run1

Suite Information

Item	Value
Suite Name	read_weights_on_pe0/run1
Suite User	steve.mullerworth
Workflow Start	2026-07-29T11:00:24
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@2026.07.1	True
lfric_apps	stevemullerworth/lfric_apps@read_weights_on_pe0	False
lfric_core	MetOffice/lfric_core@2026.07.1	True
moci	MetOffice/moci@2026.07.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.07.1	True
socrates	MetOffice/socrates@2026.07.1	True
socrates-spectral	MetOffice/socrates-spectral@2026.07.1	True
ukca	MetOffice/ukca@2026.07.1	True

Task Information

✅ succeeded tasks - 1208

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 TCD 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

[github-actions]

⚠️ Hello Steve Mullerworth (@stevemullerworth)!

Your CLA signature was found on the base branch, but you appear to have modified the CONTRIBUTORS.md file in this PR.

Please do not edit the CONTRIBUTORS.md file. If you have already signed the CLA, revert changes to the file and your signature will be picked up.

[Mike Hobson (mike-hobson)]

This looks good to me, This is exactly how I would have approached the issue. So. on to a code review with Lottie Turner (@mo-lottieturner).