Bugfix to bimodal cloud scheme

[iboutle]

PR Summary

Sci/Tech Reviewer: Paul Barrett (@paul-barrett)
Code Reviewer: Andrew Coughtrie (@andrewcoughtrie)

There is a trivial bug in the bimodal cloud scheme's lengthscale calculation, where arrays without a level 0 in bm_tau_kernel are passed into arrays with level 0 in bm_calc_tau (tdims%k_start=0). To fix this, we redefine the arrays in bm_calc_tau to not have the 0th level, since this is not needed.

This changes KGOs in most configurations. Martin W has approved the KGO change for GC6, based on the AMIP and Case-study results below:

• https://wwwspice/~ian.boutle/GES/GC6_GAL10b3_bmfix/imt/page.html?DDV
• https://wwwspice/~ian.boutle/autoassess/u-dx955_vs_u-dx491/index.html

These were produced using the fcm branch:

• https://code.metoffice.gov.uk/trac/lfric_apps/log/main/branches/dev/ianboutle/git_migration_bm_fix

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 - bm_lev_fix/run2

Suite Information

Item	Value
Suite Name	bm_lev_fix/run2
Suite User	ian.boutle
Workflow Start	2026-03-25T12:03:39
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@bm_lev_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 - 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]

Ready for science review Paul Barrett (@paul-barrett)