Revise and optimise PMSL calculation

[iboutle]

PR Summary

Sci/Tech Reviewer: @tommbendall
Code Reviewer: @MatthewHambley

The calculation of pressure at mean sea-level (PMSL) is currently exceedingly expensive in the operational global model setup. This PR makes 2 changes to dramatically reduce its cost:

• reduce the number of iterations of the solver by a factor of 4 - this gives better agreement of the final field with the current UM calculation, and is being adopted by GC6 as it gives significant improvement in verification against UM analysis (it also gives slight improvements in data assimilation trials, as the improved PMSL field allows more observations to be assimilated)
• Implement an option to compute the PMSL field to the depth of the large halos, reducing the number of halo exchanges to once for every n iterations (where n is the large halo depth), rather than once for every iteration. The large halo depth is typically around 9, so this reduces the halo exchanges by a factor of 9

• linked Infrastructure for PMSL calculation lfric_core#303

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 - vn3.1_pmsl/run4

Suite Information

Item	Value
Suite Name	vn3.1_pmsl/run4
Suite User	ian.boutle
Workflow Start	2026-03-09T13:57:00
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	iboutle/lfric_apps@vn3.1_pmsl	False
lfric_core	iboutle/lfric_core@core3.1_pmsl	False
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 - 1164

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

[thomasmelvin]

Looks good to me

[tommbendall]

Science Review

There's a small typo in a comment but otherwise this looks good, so I would be happy for this to proceed to code review