Add new stretching transformation to mesh tools

[cjohnson-pi]

PR Summary

Sci/Tech Reviewer: James Kent (@jameskent-metoffice)
Code Reviewer: Ricky Wong (@mo-rickywong)

Documentation:
#270 (comment)

linked MetOffice/lfric_apps#344

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Testing

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LFRic Apps Testing still to be completed.

trac.log

Test Suite Results - lfric_core - test_stretching/run20

Suite Information

Item	Value
Suite Name	test_stretching/run20
Suite User	christine.johnson
Workflow Start	2026-06-29T15:18:53
Groups Run	suite_default

Dependency	Reference	Main Like
lfric_core	cjohnson-pi/lfric_core@stretching	False
SimSys_Scripts	MetOffice/SimSys_Scripts@cab3315	True

Task Information

✅ succeeded tasks - 433

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[cjohnson-pi]

[cjohnson-pi]

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

[cjohnson-pi]

I've renamed apply_shift as recentre_high_resolution_region and added further Doxygen comments. Yes it would be possible to restructure to have an apply_shift with an argument detailing the type of offset to shift with. But I decided that it would be clearer to have these routines as separate routines.

Regarding the fine-meshes, the planar mesh needs to know about fine meshes because the stretch transform parameters are always derived using the fine mesh, and then applied to the coarse mesh.

A new issue regarding associated documentation has been added.

[Ricky Wong (mo-rickywong)]

I've renamed apply_shift as recentre_high_resolution_region and added further Doxygen comments. Yes it would be possible to restructure to have an apply_shift with an argument detailing the type of offset to shift with. But I decided that it would be clearer to have these routines as separate routines.

Regarding the fine-meshes, the planar mesh needs to know about fine meshes because the stretch transform parameters are always derived using the fine mesh, and then applied to the coarse mesh.

A new issue regarding associated documentation has been added.

I think these issues should be discussed as a follow-up issue/PR. It is all about scope and what should be aware of what.
The name recentre_high_resolution_region is specific to a particular stretching option(s), but is the code? I still don't see why the fine mesh is require by the planar mesh generation, I think there is some miscommunication going on.

Though it's something to discuss offline.

[Ricky Wong (mo-rickywong)]

Checked out the current branch and applied the upgrade macros and ran the test-suite.
The resulting changes from the upgrade macros cause validation failures (lbc_demo) and significant KGO failures in the new test task for your stretched ticket. Can you confirm? whether the KGOs/macros need updating?

[Ricky Wong (mo-rickywong)]

Might be best to bring up to head of main

[Ricky Wong (mo-rickywong)]

Still some issues on the code, mainly on the documentation and comments. I'm not a fan of referencing symbols in the science paper, explanatory variable names are clearer for the reader. Though I'll settle for clearer comments due to this having become a priority.

Please see comments, The current branch has significant diffs on KGOs for the new tasks when moved to head of trunk. Please move to head and confirm when the KGOs of the new tasks are as expected.

[cjohnson-pi]

Tests

Showing that the domain centre (the centre of the high resolution region) is correct in situations where there is more stretching on the W,S, E and N boundaries respectively.

 edge_cells_x    = 20
 edge_cells_y    = 20
 domain_centre   = 20.0, 10.0
 cell_size_inner = 0.01, 0.01
 cell_size_outer = 0.05, 0.05
 n_cells_outer_wsen   = 2, 2, 2, 5
 n_cells_stretch_wsen = 1, 1, 1, 5
 poly_power      = 3

West

n_cells_outer_wsen=5,2,2,2
n_cells_stretch_wsen=5,1,1,1

South

n_cells_outer_wsen=2,5,2,2
n_cells_stretch_wsen=1,5,1,1

East

n_cells_outer_wsen=2,2,5,2
n_cells_stretch_wsen=1,1,5,1

North

n_cells_outer_wsen=2,2,2,5
n_cells_stretch_wsen=1,1,1,5

[cjohnson-pi]

Test completes (trac.log above) and I have verified that the new WSEN metadata gives the correct response. kgos were updated. (As I did them twice they could be the same! - although I'm not sure how to check with github.)

Ready for review again. Thank you.

[Ricky Wong (mo-rickywong)]

Looks good, James is going to take over testing. I'm happy for this to go on, There are some outstanding issues/queries that can be followed up later.