Add new stretching transformation to mesh tools

[cjohnson-pi]

PR Summary

Sci/Tech Reviewer: @jameskent-metoffice
Code Reviewer: @mo-rickywong

Documentation:
#270 (comment)

linked MetOffice/lfric_apps#344

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 Core rose-stem suite
• If required (e.g. API changes) I have also run the LFRic Apps test suite using this branch
• If any tests fail (rose-stem or CI) the reason is understood and acceptable (e.g. kgo changes)
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LFRic Apps Testing still to be completed.

trac.log

Test Suite Results - lfric_core - test_stretching/run11

Suite Information

Item	Value
Suite Name	test_stretching/run11
Suite User	christine.johnson
Workflow Start	2026-03-13T14:48:49
Groups Run	suite_default

Dependency	Reference	Main Like
lfric_core	cjohnson-pi/lfric_core@stretching	False
SimSys_Scripts	MetOffice/SimSys_Scripts@2025.12.1	True

Task Information

✅ succeeded tasks - 390

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Performance Impact

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• 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

[cjohnson-pi]

[cjohnson-pi]

Document describing the new stretching function and its implementation stretch.pdf

[cjohnson-pi]

The coordinates and the cell spacing for the new method (with various polynomial powers) and the old 'inflation' method that is currently used in the UM (in black).

So the new method gives similar answers to the UM inflation method.

[cjohnson-pi]

The ratio of the cell spacing between neighbouring cells.

[cjohnson-pi]

A non-symmetrical mesh, where the centre of the high resolution region is at (30,10).

Relevant Configuration is:
edge_cells_x = 24
edge_cells_y = 24
domain_centre = 30.0, 10.0
cell_size_inner = 0.0135,0.0135
cell_size_outer = 0.036,0.036
n_cells_outer_nsew = 1, 8, 5, 1
n_cells_stretch_nsew = 1, 1, 1, 1
poly_power = 3

[cjohnson-pi]

Updated documentation (including non-symmetrical meshes) stretch.pdf