PC2 response to horizontal mixing

[iboutle]

PR Summary

Sci/Tech Reviewer: MichaelWhitall
Code Reviewer: Oakley Brunt (@oakleybrunt)

Closes #242

When using the PC2 cloud-scheme in model-configurations which include horizontal turbulent mixing (aka 3D Smagorinsky or blended boundary-layer), currently the horizontal mixing calculation does not update the PC2 cloud-fraction prognostic consistent with cloud water content. This will be allowing the cloud water and fraction prognostics to become inconsistent with eachother, which can cause numerical problems elsewhere (e.g. increasing cloud water without increasing fraction can lead to silly large in-cloud water contents, which rain-out spuriously).

For vertical turbulent mixing (aka the boundary-layer scheme), PC2 already updates the cloud water content and fraction consistently by assuming the mixing increment acts as a homogeneous forcing. For now we will just implement the same approach for the horizontal mixing, for consistency.

I have switched on Smagorinsky + the new option in the comorph_dev test to protect the new code, hence this has changed KGOs. The KGO change to other jobs on the ex1a arises because we have split up a single line of code that multiplied the Smagorinsky increment by dt and added it back to the main increment into 2 lines - the multiplication by dt needs to happen before the PC2 code is called, and adding the increment back needs to happen after this.

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
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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)
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trac.log

Test Suite Results - lfric_apps - pc2_smag/run1

Suite Information

Item	Value
Suite Name	pc2_smag/run1
Suite User	ian.boutle
Workflow Start	2026-04-30T11:51:37
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@pc2_smag	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

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Performance Impact

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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)

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Sci/Tech Review

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• 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
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• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable

[James Bruten (james-bruten-mo)]

Macro all fine

[DanStoneMO]

Can confirm JEDI works fine with this, no linked PR is needed for lfric-jedi

[MichaelWhitall]

Hi thanks lots for implementing this change! I think I'm broadly happy with this, just a few questions below...

Another more general thing is how much work would it be to add diagnostics for the diffusion increments (excluding PC2) and the PC2 homog increments added by this PR? Given this is not something I can just eyeball in the SCM, it would be nice to be able to look at example maps of the increments at some height where there's cloud and check that they look as expected?

[iboutle]

Hi thanks lots for implementing this change! I think I'm broadly happy with this, just a few questions below...

Another more general thing is how much work would it be to add diagnostics for the diffusion increments (excluding PC2) and the PC2 homog increments added by this PR? Given this is not something I can just eyeball in the SCM, it would be nice to be able to look at example maps of the increments at some height where there's cloud and check that they look as expected?

I've added the diagnostics as requested, and changed the test job. Back to MichaelWhitall for science review

[MichaelWhitall]

The added stuff to output the increment diagnostics look good to me; wow it really is a lot less work to add these in LFRic compared to the UM! I'm happy with this now, approving the sci/tech review...

[iboutle]

Oakley Brunt (@oakleybrunt) ready for code review

[Oakley Brunt (oakleybrunt)]

Just one comment about adding timing callipers. Otherwise, happy with this change.

[Oakley Brunt (oakleybrunt)]

Thanks!

[thomasmelvin]

Looks good to me