[Thomas Bendall (tommbendall)]

PR Summary

Sci/Tech Reviewer: iboutle
Code Reviewer: Ed Hone (@EdHone)

This adds the capability to nudge LFRic prognostic fields to external data (e.g. ERA or AIFS). The technical infrastructure for this was added in #474 -- this PR adds the scientific capability.

This adds:

• a new algorithm called nudging_alg_mod.x90, which is called from external_forcing
• new namelist options to control the nudging
• two methods of nudging: "spectral" and "newtonian"
• kernels for performing the spectral nudging (to pre-compute weights and to compute the convolution)
• a kernel for calculating vertical nudging weights
• a kernel to perform the vertical interpolation of the external data onto the model's mesh
• tests for Newtonian and spectral nudging, on the dynamics mesh or coarser mesh
• documentation(!)

Test branch: https://github.com/tommbendall/lfric_apps/pull/16/changes

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 - test_spectral_nudging/run2

Suite Information

Item	Value
Suite Name	test_spectral_nudging/run2
Suite User	thomas.bendall
Workflow Start	2026-08-19T09:40:35
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@2026.07.1	True
lfric_apps	tommbendall/lfric_apps@test_spectral_nudging	False
lfric_core	MetOffice/lfric_core@2026.07.1	True
moci	MetOffice/moci@2026.07.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.07.1	True
socrates	MetOffice/socrates@2026.07.1	True
socrates-spectral	MetOffice/socrates-spectral@2026.07.1	True
ukca	MetOffice/ukca@2026.07.1	True

Task Information

✅ succeeded tasks - 1600

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)

Met Office Github Copilot Enterprise used in creating documentation.

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

[iboutle]

Hi Tom,

Passing this back now, although I'm still reading through the documentation etc, as I think there are a few changes that we probably do want to build into this, in-case they change the way anything is implemented:

• we need a nudging_stop namelist entry, to turn off the nudging after a set period of time. It needs to accommodate very large numbers, which is why I wondered if it would be better to specify in hours instead of seconds, and make the start times consistent with this
• we need the ability to stop the nudging above MAX(tropopause_level, nudging_min_tropopause_level) where nudging_min_tropopause_level=48 and tropopause_level is calculated via locate_tropopause_kernel_mod. I think you'll need to call locate_tropopause_kernel on the nudging mesh - possibly you could do this when calculating the weights, and then alter the weights calculation kernel to set them to 0 based on this condition. Essentially this means that the weights calculation will always need to happen when nudging is on, because the tropopause will change every timestep.

[Thomas Bendall (tommbendall)]

iboutle I think this is now ready for review again. I'm afraid there were quite a few changes, but the diff to see them all together is here: https://github.com/tommbendall/lfric_apps/compare/e5a2430..05797a3

What I've done:

• updated vertical levels in upgrade macro as suggested
• changed nudging_time variables to use hours rather than seconds
• separated out nudging_relax_time into an option for each variable
• added nudging_stop_time variable
• restored nudging_source as this will be used to specify nudging to UM/LFRic output
• added nudging_min_tropopause_level option
• changed the nudging weights to be a 1D array for the vertical weights. This now doesn't need a kernel to calculate it, so it is just calculated in a routine in physics_constants_mod.x90
• the temporal weights are pulled out into a single number, so weights no longer need updating. The update_nudging_weights routine has been removed
• added calls so that tropopause level to be used to cap vertical extent of nudging
• changed convolution_2d_kernel to include vertical and temporal weights, and to have its vertical extent limited
• introduced newtonian_nudging_kernel to perform Newtonian nudging routine, using vertical and temporal weights
• psy_lite versions of kernels that use scalar array needed
• renamed nudging_vert_interp_kernel to include ecmwf since it is specific to ECMWF pressure levels

[iboutle]

A few minor comments but nothing too serious!

[Thomas Bendall (tommbendall)]

A few minor comments but nothing too serious!

Thanks for the suggestions: I've made these now so this is back to you

[iboutle]

Looks good - over you you Ed Hone (@EdHone)

[thomasmelvin]

All looks fine from a code owner view

[Ed Hone (EdHone)]

Some minor changes requested for some formatting, but otherwise all looks good

[Thomas Bendall (tommbendall)]

Some minor changes requested for some formatting, but otherwise all looks good

Thanks for the review! Style fixes made in a1815c5

I've then merged up to head-of-main and fixed conflicts