smooth lambda

[Adrian Lock (Adrian-Lock)]

PR Summary

Sci/Tech Reviewer: P-Burns
Code Reviewer: Pierre Siddall (@Pierre-siddall)

Add a new option for free_atm_mix to calculate a vertically smoother mixing length. Under the existing options, the mixing length is constant within turbulent layers (defined as where the Richardson number is subcritical), except as it approaches the surface where it tends towards the height above the surface. This reduction represents the suppression of the scale of vertical mixing by the physical presence of the boundary. In this new option we apply the same principle to the top and bottom boundaries of every turbulent layer, thereby making smoother transitions in the mixing length between layers of the atmosphere.

Equivalent changes in the UM at vn14.1: vn14.1_smooth_lambda

closes #159

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)

I have activated the new option in the comorph_dev configuration and so the kgo is broken for those tasks.

trac.log

Test Suite Results - lfric_apps - main905c_smooth_lambda/run7

Suite Information

Item	Value
Suite Name	main905c_smooth_lambda/run7
Suite User	adrian.lock
Workflow Start	2026-05-06T11:50:57
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	Adrian-Lock/lfric_apps@main905c_smooth_lambda	False
lfric_core	MetOffice/lfric_core@3c8ccc6	True
moci	MetOffice/moci@2026.03.2	True
SimSys_Scripts	MetOffice/SimSys_Scripts@cab3315	True
socrates	MetOffice/socrates@2026.03.2	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.2	True
ukca	MetOffice/ukca@1cdb9c2	True

Task Information

❌ failed tasks - 6

Task	State
check_lfric_atm_nwp_comorph_dev-C12_azspice_gnu_fast-debug-32bit-crun1	failed
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

✅ succeeded tasks - 1529

⌛ waiting tasks - 2

Task	State
housekeep_azspice	waiting
housekeep_ex1a	waiting

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

Needs to be added to the Boundary Layer documentation paper but will have to wait until it is ported. I've opened issue #481 to do this separately.

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

[github-actions]

⚠️ Hello Adrian Lock (@Adrian-Lock)!

Your CLA signature was found on the base branch, but you appear to have modified the CONTRIBUTORS.md file in this PR.

Please do not edit the CONTRIBUTORS.md file. If you have already signed the CLA, revert changes to the file and your signature will be picked up.