Only use mp_czero_rp if using the RP2B scheme

[Marc Stringer (marcstring)]

PR Summary

Sci/Tech Reviewer: yongmingtang
Code Reviewer: Cameron Bateman (@cameronbateman-mo)

MetOffice/um#73 describes how UM ticket #6980 (https://code.metoffice.gov.uk/trac/um/ticket/6980) broke the option l_rp2=true and i_rp_scheme=0, which is used by our prototype UKESM2 model. Our prototype UKESM2 uses the UM, but during the approval process Paul Barrett has pointed out that the fix in MetOffice/um#73 should also be applied to LFRic, hence this pull request.

MetOffice/um#73 modifies three FORTRAN files: large_scale_cloud/bm_calc_tau.F90, large_scale_precipitation/mphys_turb_gen_mixed_phase.F90 and stochastic_physics/stph_rp-stph_rp2.F90. The first two files can be found in lfric_apps, but not the third (stochastic_physics/stph_rp-stph_rp2.F90), so I've only mirrored the changes from MetOffice/um#73 for large_scale_cloud/bm_calc_tau.F90 and large_scale_precipitation/mphys_turb_gen_mixed_phase.F90 for the lfric_apps branch here.

I don't work on LFRic and I'm not aware of any LFRic jobs which can be used to test the option l_rp2=true and i_rp_scheme=0, so my testing for lfric_apps is limited to just running rose-stem, which passes fine.

• linked MetOffice/um#73

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)
• [ ]*1 I have added tests to cover new functionality as appropriate (e.g. system tests, unit tests, etc.)
• [ ]*2 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)
  *1 No extra tests here. Protection of the option l_rp2=true and i_rp_scheme=0 is being added to our UKESM2 UM rose-stem jobs (UKESM-curr & UKESM-beta).
  *2 Not applicable

trac.log

Test Suite Results - lfric_apps - test_lfric_mp_czero_fix/run1

Suite Information

Item	Value
Suite Name	test_lfric_mp_czero_fix/run1
Suite User	marc.stringer
Workflow Start	2026-05-06T08:42:38
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	marcstring/lfric_apps@mp_czero_fix	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

• [x]*3 Performance of the code has been considered and, if applicable, suitable performance measurements have been conducted
  *3 This is just expanding a couple of if statements for routines not called that often. Impact will be negligible.

AI Assistance and Attribution

• [ ]*4 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)
  *4 No AI here

Documentation

• [ ]*5 Where appropriate I have updated documentation related to this change and confirmed that it builds correctly
  *5 No documentation update required

PSyclone Approval

• [ ]*6 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
  *6 Not applicable

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

[Marc Stringer (marcstring)]

i_rp_scheme=0 scheme unavailable at LFRic

In https://code.metoffice.gov.uk/trac/UKESM/wiki/ticket/1101/lfric_converse I've recorded an e-mail conversation between me and Anne McCabe where she says that the i_rp_scheme=0 is unavailable at LFRic, which explains why there's no stochastic_physics/stph_rp-stph_rp2.F90 in LFRic apps.

[paulfield2024]

seems fine. Passes rose-stems

[Paul Barrett (paul-barrett)]

seems fine, as UM

[yongmingtang]

all is fine -- approved