Configure LFRic to receive meltpond fields as grid box mean

[Tim Graham (timgraham-Met)]

PR Summary

Sci/Tech Reviewer: Dan Copsey (@DanCopsey)
Code Reviewer: Alistair Pirrie (@mo-alistairp)

LFRic, and previously the UM, uses sea ice melt pond area fraction, and melt pond thickness, for surface albedo calculations (in JULES). Until now, these variables have been passed as mean over sea ice (melt pond area fraction) and as mean over melt pond area (melt pond thickness). Where there is covariance between these variables and sea ice area fraction within ocean cells underlying an LFRic grid cell, this can lead to surface albedo biases. Passing these variables as grid box means (GBM) is preferable, as it lets the atmosphere see a more representative sample of surface types.

A NEMO branch to allow melt pond variables to be sent as GBM has already been written; correspondingly, LFRic must be configured to receive them as such. After receipt, melt pond area fraction must be divided through by sea ice area fraction; melt pond thickness by sea ice area fraction and melt pond area fraction (or alternatively, by GBM melt pond area fraction in a single step). These changes have been applied to the NEMO package branch and the coupled rose stem test has been updated to the later revision of the branch.

Given that this change will be included in the first released coupled configuration using LFRic (GC6) it's suggested that the behaviour is included without a switch. There are no configurations that will need to use the old behaviour.

This code was originally developed by MO-AlexW but as it is a simple change that we have been using in GC workflows, I have picked it up to get on main.

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)

This code has been used in multiple development versions of GC6.

trac.log

Test Suite Results - lfric_apps - mpnds_as_gbm/run1

Suite Information

Item	Value
Suite Name	mpnds_as_gbm/run1
Suite User	tim.graham
Workflow Start	2026-04-14T12:37:50
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.1	True
jules	MetOffice/jules@2026.03.1	True
lfric_apps	timgraham-Met/lfric_apps@meltponds_as_gbm	False
lfric_core	MetOffice/lfric_core@2026.03.1	True
moci	MetOffice/moci@2026.03.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.1	True
socrates	MetOffice/socrates@2026.03.1	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.1	True
ukca	MetOffice/ukca@2026.03.1	True

Task Information

❌ failed tasks - 1

Task	State
check_lfric_coupled_nwp_gal9-C48_ex1a_cce_fast-debug-64bit	failed

✅ succeeded tasks - 1510

⌛ 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 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

[Tim Graham (timgraham-Met)]

KGOs for lfric_coupled have been updated and the check now passes.

[Tim Graham (timgraham-Met)]

⚠️ Hello Tim Graham (Tim Graham (@timgraham-Met))!

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 change to the CONTRIBUTORS.md file is because the code change was originally developed by Alex West but I have made the final version of the branch and created the pull request. When I discussed with you, you suggested that Alex should be added as a contributor because he did the original development but that causes this check to fail. Can you comment on the best way forward?

[Mike Hobson (mike-hobson)]

Hi Tim Graham (@timgraham-Met), I may have provided poor (or at least misleading) advice about the contributor's file. You are not allowed to add someone who is not yourself to that file. We can get around this, but in this case, GitHub doesn't know that Alex has contributed as it's your PR, so the easiest fix is just to not add Alex's name to the contributor's file.

[Tim Graham (timgraham-Met)]

Hi Tim Graham (Tim Graham (@timgraham-Met)), I may have provided poor (or at least misleading) advice about the contributor's file. You are not allowed to add someone who is not yourself to that file. We can get around this, but in this case, GitHub doesn't know that Alex has contributed as it's your PR, so the easiest fix is just to not add Alex's name to the contributor's file.

Thanks Mike Hobson (@mike-hobson). I've now removed Alex from the file and the CLA check is passing again.