#474: LFRic-Nudging: Part-A: Introduce ability to read nudging datafiles : vn3.1.1 read nudging data

[Mohit Dalvi (mcdalvi)]

PR Summary

Sci/Tech Reviewer: Thomas Bendall (@tommbendall)
Code Reviewer: Matthew Hambley (@MatthewHambley)

This PR introduces the ability to read in datafiles that will be used for Nudging the Momentum models (Ref: Nudging NWP to AI/ML models - possibly targeting PS49).
The handling of Nudging data is modelled on existing lateral boundary condition handling in LFRic since they are both required to be interpolated from high time resolution file data on every model timestep.
Nudging changes summary:

• Ability to read in nudging data for temperature, U-wind, V-wind and Log surface pressure (required for vertical remapping to model levels).

• New [namelist:nudging], which will be active when theta or wind are 'forced by nudging' (in namelist:external_forcing). A corresponding upgrade_macro to add the namelist and I/O related items.

• Introduce routine create_nudging_fields to create the nudging input fields as '2-D with user-specified (nudge_data) levels'. Corresponding grid_def_nudging.xml and grid_def_nudging_coarse.xml added to lfric_atm/metadata/.

• Introduce ability to read in nudging data on native ('dynamics') or a coarser grid, with rationalising of routines that create the model fields ('make_spec', 'create_model_data') to accept both aerosol and nudging coarse mesh definitions.

• Note that the above requires a LFRIC_CORE change to allow defining 3-D coarse grid fields as ''multigrid_ll'' x ndata.

• Define a nudging_time_axis which will be populated based on the actual number of time records in the specified nudging file.

• Introduce two new rose-stem tests lfric_atm_nwp_gal9_nudging-C48 that reads nudging files on the model grid and lfric_atm_nwp_gal9_nudging_coarse-C48 that reads in data at a lower resolution (C12) grid. This currently uses nudging data created from ERA5 source, but aim is to expand this to AIFS and other models.

• At this stage the reference fields are only made available to LFRic and do not modify temperature/ winds.

• linked #369: Allow coarse-grid fields with ndata levels: vn3.1 read coarse 2d lfric_core#369

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)

Test Branch: test_vn31_nudging_data

trac.log

Test Suite Results - lfric_apps - test_vn31_nudging_data/run2

Suite Information

Item	Value
Suite Name	test_vn31_nudging_data/run2
Suite User	mohit.dalvi
Workflow Start	2026-05-14T14:06:15
Groups Run	developer', 'lfric_atm_nudging

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	mcdalvi/lfric_apps@test_vn31_nudging_data	False
lfric_core	mcdalvi/lfric_core@vn3.1_read_coarse_2d	True
moci	MetOffice/moci@2026.03.2	True
SimSys_Scripts	MetOffice/SimSys_Scripts@cab3315	True
socrates	MetOffice/socrates@2026.03.2	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.2	True
ukca	MetOffice/ukca@1cdb9c2	True

Task Information

✅ succeeded tasks - 1197

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)

VsCode - code completion/ snippets.

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

[DanStoneMO]

Is there any chance main could be merged into this branch? I'm currently unable to test the changes with JEDI due to the conflicts

[Mohit Dalvi (mcdalvi)]

Apologies, I was not aware creating a PR triggers all this activity ! I have changed it to draft till all my testing is complete.

[Mohit Dalvi (mcdalvi)]

Merged main into branch at [08455b0](url).

Sample plots

Comparing data from ERA file (left) with lfric_atm_nudging diagnostic .
*Note that ERA data are single time snapshots while output fields are interpolated between ERA snapshots to model timestep and then averaged before output.

[Mohit Dalvi (mcdalvi)]

Input test files for $BIGDATA

The input files for the lfric_atm_nwp_gal9_nudging_xx tests have been generated using ERA5 reanalysts data provided by by the Copernicus Climate Change Service distributed via the Copernicus Data Service. Dataset information, including usage terms can be found here: https://cds.climate.copernicus.eu/datasets/reanalysis-era5-complete?tab=documentation

The files proposed to be added to $BIGDATA are:
/data/users/mohit.dalvi/lfric/data/nudge/TomB/era/C12/era5_nudge_L137_C12_2021032400-2706.nc
/data/users/mohit.dalvi/lfric/data/nudge/TomB/era/C48/era5_nudge_L137_C48_MG_2021032400-2706.nc

Both the files contain the following attributes:

		:licence = "The ERA5 data from Copernicus Climate Change Service is licensed under the CC-BY licence. 
Highlights and key features of the licence are here: https://creativecommons.org/licenses/by/4.0/. 
Full legal text is provided here:https://creativecommons.org/licenses/by/4.0/legalcode" ;

		:attribution = "Generated using or contains modified Copernicus Climate Change Service information .
 Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus
 information or data it contains." ;

		:references = "Hersbach, H. et al, (2017): Complete ERA5 from 1940: Fifth generation of ECMWF
 atmospheric reanalyses of the global climate. Copernicus Climate Change Service (C3S) Data Store (CDS). DOI: 
10.24381/cds.143582cf" 

[iboutle]

Hi Mohit

I know this isn't fully ready yet, but I'm on leave next week so wanted to give some comments and code-owner approval so you can get this into code review by next Friday. Hopefully nothing too challenging, but a few requests and suggestions below.

Cheers

Ian

[James Bruten (james-bruten-mo)]

Thanks

[DanStoneMO]

Tested this and can confirm this works fine with JEDI, no linked JEDI PR will be needed

[DanStoneMO]

Only thing to note from me is this rogue space here

[Mohit Dalvi (mcdalvi)]

Thanks DanStoneMO . I have fixed that in my copy

[Thomas Bendall (tommbendall)]

Science Review
This PR adds the technical infrastructure for nudging to ERA or AIFS data, and will be followed by a PR to add the science to perform the nudging itself (including only nudging certain length scales).

Thanks so much for this Mohit -- it looks great. I have two more major thoughts, and then a handful of other suggestions that I've commented in the code.

1. Do you think we should reorder the ERA ancil data vertically before we add it to BIG_DATA_DIR? (I'm assuming we haven't done this yet)... That would make the vertical interpolation more efficient when we get to that. If that's too much work before the deadline then we can potentially do it later when adding AIFS data?
2. I have an annoying suggestion to change the settings of the nudging-era-coarse test, which is to set the aerosols to be on the dynamics mesh rather than keeping them on a coarse mesh. I just thought this would be a bit cleaner in the test-suite, so have added suggestions on how to do this

[Mohit Dalvi (mcdalvi)]

Thomas Bendall (@tommbendall) I have reversed the ERA file data using 'ncpdq -a hybrid' and using the new files in the tests now.
Also reveted nudging_era_coarse test to use coarse grid only for Nudging. 642225b

Tests underway.