Refactor fast physics

[iboutle]

PR Summary

Sci/Tech Reviewer: MichaelWhitall
Code Reviewer: Lottie Turner (@mo-lottieturner)

This PR refactors fast physics to enable 2 key pieces of science infrastructure (which are also implemented by this PR):

• allow the implicit boundary layer to be called before convection (needed by Comorph-B)
• allow different convection schemes to be called on different outer loops (specifically LLCS on the first outer loop)

To enable this, it has been necessary to properly separate the boundary layer and convection increments, which were previously badly inter-mixed. Fast physics now tracks "latest" fields which are updated by each scheme in turn (since the physics here is sequential by design), and then finally constructs an increment from the difference between the final and initial fields. Because this changes the calculation order, it changes many KGOs for lfric_atm

The calls to these schemes in slow physics have also had to be updated to be consistent, and diagnostics previously produced in bl_imp_alg but that depend on convection have had to be separated into their own algorithm to be called after BL and convection have definitely both been done.

Ideally, the new "BL before conv" functionality would be switched on in the comorph_dev tests, however these currently fail when doing so. I will investigate this separately, as the failure is not connected to the code introduced by this PR (particularly the refactoring, which we want to consolidate back to main asap), but is a feedback on model evolution. Doing so would also conflict with several other PRs in flight (#292, #326) so is best left as a separate PR.

closes #228

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 - fast_refactor/run1

Suite Information

Item	Value
Suite Name	fast_refactor/run1
Suite User	ian.boutle
Workflow Start	2026-03-17T14:04:49
Groups Run	developer', 'lfric_atm_extra

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	iboutle/lfric_apps@fast_refactor	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 - 1310

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

[iboutle]

Ah I just found the "submit review" button! I think its been hiding all my in-line comments from you until I press this button... Can you see the comments now?

Hopefully I've addressed all of these...

[MichaelWhitall]

Hopefully I've addressed all of these...

Yep this all looks good to me :) The bot seems to've already automatically moved this out of sci/tech review for me (I assumed I'd need to press a button somewhere to sign this off?) Maybe it counts as signed-off once all the comments are marked as "resolved"?)

Cheers!
Mike

[MichaelWhitall]

Oh no wait there actually is an "approve" button in here, which I hadn't pressed yet; I think the bot's activity was a bit premature? Anyhow I'm pressing the button now ;)

[iboutle]

Hopefully I've addressed all of these...

Yep this all looks good to me :) The bot seems to've already automatically moved this out of sci/tech review for me (I assumed I'd need to press a button somewhere to sign this off?) Maybe it counts as signed-off once all the comments are marked as "resolved"?)

Cheers! Mike

The bot moved it to "changes requested" - I should have moved it back to sci/tech after doing them. I think you need to tick the boxes in the science review section, and then tag the code reviewer in a comment saying your happy it's passed - the bot is supposed to move it to code review, although I've never actually got that to work!

[MichaelWhitall]

Sci/Tech review approved on this one, passing over to Lottie Turner (@mo-lottieturner) for code review...

Cheers!
Mike

[iboutle]

Suggested change

	subroutine stph_main_alg( du_stph, dtheta_stph, dmv_stph, dtheta, &
	subroutine stph_main_alg( du_stph, dtheta_stph, dmv_stph, &

changed name variable has been duplicated in merge

[Lottie Turner (mo-lottieturner)]

Well caught, thanks for pointing it out!

[iboutle]

Suggested change

	call spt_main_alg(dtheta_stph, dmv_stph, dtheta, &
	call spt_main_alg(dtheta_stph, dmv_stph, &

changed name variable has been duplicated in merge

[Lottie Turner (mo-lottieturner)]

Thanks Ian

[Lottie Turner (mo-lottieturner)]

Well caught, thanks for pointing it out!

[Lottie Turner (mo-lottieturner)]

Thanks Ian