Add theta perturbation parameters to namelist

[Adrian Lock (Adrian-Lock)]

PR Summary

Sci/Tech Reviewer: Thomas Bendall (@tommbendall)
Code Reviewer: Benjamin Went (@MetBenjaminWent)

The initial random perturbations used in idealised simulations to initiate fine scale motion are hardwired in the code. Here we add them to the initial_temperature namelist so they can be varied in magnitude (theta_pert_size) and location in the vertical (between the heights theta_pert_start and theta_pert_end). Sadly the code to generate the perturbations is in lfric_core so there has to be a linked PR.

• linked Use theta perturbation parameters from namelist lfric_core#374

• closes Add theta perturbation parameters to namelist #503

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.)
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trac.log

Test Suite Results - lfric_apps - test_apps_init_pert_nml_arglist_3264_all/run1

Suite Information

Item	Value
Suite Name	test_apps_init_pert_nml_arglist_3264_all/run1
Suite User	adrian.lock
Workflow Start	2026-05-21T15:55:23
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@test_apps_init_pert_nml_arglist	False
lfric_core	Adrian-Lock/lfric_core@tag3.1_core_init_pert_nml_arglist_3264	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

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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)

Documentation

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PSyclone Approval

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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

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

[Adrian Lock (Adrian-Lock)]

Sorry, got ahead of myself as I'm having trouble with accessing this apps namelist from code, see MetOffice/simulation-systems#634

[thomasmelvin]

All looks good to me

[Thomas Bendall (tommbendall)]

Science Review

This PR adds namelist options to specify the height and magnitude of theta perturbations, which were previously hardwired. Most of the changes involve rolling the namelist option out to the feign_initial_temperature_config in unit-tests

There is nothing controversial here and I'm happy to approve.

[mo-cjsmith]

Initialisation changes look fine.

[DanStoneMO]

Tested JEDI against this and it runs fine as-is. No linked lfric-jedi PR will be needed.