Fix averaging kernel used in stochastic physics filter

[tommbendall]

PR Summary

Sci/Tech Reviewer: @thomasmelvin
Code Reviewer: @cameronbateman-mo

This fixes the wth_to_w0_average kernel, which was using the incorrect multiplicity to average a field from Wtheta to W0. This kernel is used in a 1-2-1 filter in stochastic physics -- the filter is performed a number of times, and the impact of the incorrect multiplicity is that the stochastic physics tendencies were an order of magnitude too small.

• linked Stochastic Physics Fixes lfric_apps#148

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trac.log

Test Suite Results - lfric_core - fix_w0_wth_filter/run1

Suite Information

Item	Value
Suite Name	fix_w0_wth_filter/run1
Suite User	thomas.bendall
Workflow Start	2026-01-15T17:31:52
Groups Run	developer

Dependency	Reference	Main Like
lfric_core	tommbendall/lfric_core@TBendall/FixW0WthFilter	False
SimSys_Scripts	MetOffice/SimSys_Scripts@2025.12.1	True

Task Information

✅ succeeded tasks - 372

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[thomasmelvin]

The changes are correct and will fix the incorrect increments. I was surprised that no KGO's have changed with this fix. do no tests in the developer suite use stochastic physics? do more of the tests need to be run to pick up any other KGO changes?

[tommbendall]

The changes are correct and will fix the incorrect increments. I was surprised that no KGO's have changed with this fix. do no tests in the developer suite use stochastic physics? do more of the tests need to be run to pick up any other KGO changes?

I should've add the KGO label. It does changed KGOs, but I have bundled these into the linked Apps PR which has a handful of other fixes to the stochastic fixes.

[thomasmelvin]

I missed this was a linked ticket, I'm happy with the changes here and will let the scientific impact of them be reviewed in the related lfric_apps pull request