Improved PC2 "smooth" initiation option

[MichaelWhitall]

PR Summary

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

In the PC2 cloud-scheme, "initiation" is the process of creating new cloud based on a call to the diagnostic cloud-scheme. Under the existing "smooth" initiation option, the cloud liquid water content from the diagnostic scheme is taken as a minimum limit on the prognostic cloud water each timestep (so the diagnostic scheme only applies if the prognosed cloud-water falls below the diagnostic scheme's prediction). However, the cloud fraction was only relaxed partially towards the diagnostic scheme's prediction, as a function of how much cloud water has been added by initiation. This led to a problem where initiation would systematically increase cloud-water by a larger proportion than cloud-fraction, so that in-cloud water content spuriously increases.

The new PC2 initiation option avoids this problem by imposing the diagnostic cloud-scheme's predicted cloud-fraction as a hard limit on the prognosed cloud-fraction instead of relaxing partially towards it, so that cloud water content and fraction are both handled consistently.

Branch at vn3.1: vn3.1_pc2_init_logic_cflim

Test branch: pc2_init_logic_cflim_test

closes #249

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 **

* I checked this by searching for the names of files I modified in the job.err file from the build_lfric_atm_azspice_gnu_full-debug-32bit compile in rose-stem. I could only find compiler warnings related to pre-existing issues (numerous instances of Possible change of value in conversion from REAL(8) to REAL(4) where arrays are copied in bl_imp2_kernel_mod).

** The check_cr_approved CI check is clearly not going to complete successfully until the code review has been approved.

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)

* The new pc2_init_logic option has been switched on in the comorph_dev rose-stem app to test the new functionality. Therefore all the rose-stem tests of this configuration have changes answers.

trac.log

Test Suite Results - lfric_apps - pc2_init_logic_cflim_test/run1

Suite Information

Item	Value
Suite Name	pc2_init_logic_cflim_test/run1
Suite User	michael.whitall
Workflow Start	2026-03-27T22:51:20
Groups Run	developer', 'lfric_atm_nwp_ex1a_extra', 'lfric_atm_nwp_azspice_extra

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.1	True
jules	MetOffice/jules@2026.03.1	True
lfric_apps	MichaelWhitall/lfric_apps@pc2_init_logic_cflim	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 - 6

Task	State
check_lfric_atm_nwp_comorph_dev-C12_ex1a_cce_fast-debug-32bit-crun1	failed
check_lfric_atm_scm_comorph_dev_bomex-BiP2x2-50000x50000_azspice_gnu_fast-debug-32bit	failed
check_lfric_atm_scm_comorph_dev_bomex-BiP2x2-50000x50000_ex1a_cce_fast-debug-32bit	failed
check_lfric_atm_scm_comorph_dev_toga-BiP2x2-50000x50000_azspice_gnu_fast-debug-32bit	failed
check_lfric_atm_scm_comorph_dev_toga-BiP2x2-50000x50000_ex1a_cce_fast-debug-32bit	failed
kgo_groups_checker	failed

✅ succeeded tasks - 1242

⌛ waiting tasks - 2

Task	State
housekeep_azspice	waiting
housekeep_ex1a	waiting

Note: I have intentionally left the update of KGO checksums out of the test branch, so that those tests which change KGO show-up above as failures in the check_lfric_atm tasks for clarity. These tasks would not fail on the dev branch as the KGO files have been updated there.

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable) NA
• Authentication and authorisation are properly implemented (if applicable) NA

Performance Impact

• Performance of the code has been considered and, if applicable, suitable performance measurements have been conducted

Not expecting any significant performance impact.

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)

NOTE: I initially tried to get Met Office Github Copilot to port the code-changes I had already made in a UM branch into the lfric_apps branch for this PR. Copilot's attempt at this can be seen in the first 2 commits in the branch, 87c9ffc and eed3190 (hence those commits are not verified). However, this was a failure; copilot inexplicably deleted most of the contents of one file, and then did not attempt the rest of the changes. I made the 3rd commit 7b102da which simply reverts the changes made by Copilot, and then ported the changes manually in the subsequent commits.

Documentation

• Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

Documentation pending...

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 'NA'

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

[MichaelWhitall]

This meta-data was copied from the UM previously. I just spotted that it still refers to the allowed integer values for this multi-option switch in the UM. The LFRic namelists now accept character strings for multi-option switches instead of the integers. So I've replaced the confusing integers with the appropriate character strings here.

[MichaelWhitall]

I have re-imported this file into lfric_apps from my equivalent UM branch and then ran the lfric code-styling script to make the code-style consistent (i.e. lower-case the UM's capitalised fortran keywords). However, clearly the lfric code-styling script has been modified since the original import of this file from the UM. The styling script formerly lower-cased fortran keywords within OMP directives (but not the other OMP keywords). But it now just leaves OMP directives alone. This means my re-import and rerunning of the styling script has re-capitalised the fortran keywords in OMP directives. But I've decided to just leave this as-is; the OMP directives are clearer if consistently all capitalised, rather than having a confusing mix of lower / upper case for different words!)

[James Bruten (james-bruten-mo)]

Macro approved