Linear MoL horizontal theta transport

[James Kent (jameskent-metoffice)]

PR Summary

Sci/Tech Reviewer: cjohnson-pi
Code Reviewer:

There is a small error in the linear model MoL transport scheme. This only occurs for horizontal transport of Wtheta variables. In tl_wt_advective_update_alg_mod.x90 the algorithm hori_wt_update_alg is called twice, once using the linearisation wind and the pertubation field, and once using the perturbation wind and the linearisation field. This gives both terms required in the linear transport. However, the algorithm is not incremental, and so the first call is ovewriten and not updated.

This PR fixes this issue (for both 3D and horizontal MoL for Wtheta variables). It also tidies up the code in that algorithm. It also changes the adjoint to take this change into account.

I have run the test suites and there are kgo changes which I've updated. This includes kgo changes to Jedi tests. I've also changed the tolerance on some integration tests. Below are some plots to show some of the changes resulting from the PR.

DCMIP test on stable, showing theta.

DCMIP test from PR, showing theta (now propagating in the correct direction)

C224 NWP test on stable, showing u

C224 NWP test from PR, showing u

Linked PRs

• linked JCSDA-internal/lfric-jedi#1286
• linked MetOffice/jjtests#178

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 - linear_mol/run2

Suite Information

Item	Value
Suite Name	linear_mol/run2
Suite User	james.kent
Workflow Start	2026-06-08T13:15:14
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	jameskent-metoffice/lfric_apps@linear_mol	False
lfric_core	MetOffice/lfric_core@2026.03.2	True
moci	MetOffice/moci@2026.03.2	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.2	True
socrates	MetOffice/socrates@2026.03.2	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.2	True
ukca	MetOffice/ukca@2026.03.2	True

Task Information

✅ succeeded tasks - 1511

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

[DrTVockerodtMO]

Thanks for making the adjoint changes as well. Some clarification is needed before approval.

[James Kent (jameskent-metoffice)]

Thanks DrTVockerodtMO for spotting that. I must have introduced that as an error when trying to get the adjoint to work. I've removed the incorrect code and run the test suites and everything passes (including adjoint and jedi tests).

[DanStoneMO]

There will need to be an accompanying LFRic-JEDI PR for this, I'll link it once ready

[DrTVockerodtMO]

Thank you for the adjoint changes!

[DanStoneMO]

Linked JEDI PRs now live at:
lfric-jedi: https://github.com/JCSDA-internal/lfric-jedi/pull/1286
jjtests: https://github.com/MetOffice/jjtests/pull/178

[cjohnson-pi]

Many thanks for identifying and correcting this bug. I have checked the plots associated with the kgo changes, checked the linear and adjoint code, and am happy with the changes in the tolerances. So passes science review.