Radaer api la side - part one run time

[Alan J Hewitt (alanjhewitt)]

PR Summary

Sci/Tech Reviewer: Mohit Dalvi (@mcdalvi)
Code Reviewer: Yaswant Pradhan (@yaswant)

I developed the original lfric to radaer functionality in 2020. Like a lot of developments back then, it was needed in a hurry and so I developed a fudged solution with technical debt.

We will need user flexibility soon to be able to run radaer with different user settings, hence this part one of two development.

In part one, I have introduced the user flexibility that is required for run time. Its already a very large change, and this is a natural break point for splitting a big PR.

Some hard-coded settings remain in the development (on the lfric_apps side the lists used by the case statement), which will be removed in the part two ticket to develop the initialisation stage functionality.

The KGO changes result from differences in 32bit r_def and 64bit r_um variables being added together in a slightly different way. Revision ab953f8 , which preserved the old adding method maintains KGO (see run1).
https://cylchub/services/cylc-review/taskjobs/alan.j.hewitt/?suite=radaer_api_la_side_3%2Frun1
Revision 4bf2d99, makes changes to the new case statement adding method which cause bit level differences.
4bf2d99
In hindsight, I should have wrapped all the elements of the old adding method with real( ... , r_um) as there would not have been a KGO change here.

• linked Radaer api ukca side - part one run time ukca#41

• blocks Radaer api la side - part two initialisation #683

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.)
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trac.log

Test Suite Results - lfric_apps - radaer_api_la_side_3/run2

Suite Information

Item	Value
Suite Name	radaer_api_la_side_3/run2
Suite User	alan.j.hewitt
Workflow Start	2026-07-28T14:03:13
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@2026.07.1	True
lfric_apps	alanjhewitt/lfric_apps@radaer_api_la_side_3	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	alanjhewitt/ukca@radaer_api_ukca_side_3	True

Task Information

✅ succeeded tasks - 1594

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Documentation

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Sci/Tech Review

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(Please alert the code reviewer via a tag when you have approved the SR)

Code Review

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• Tests are adequate and have passed
• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable

[iboutle]

Hi Alan,

Thanks for showing clearly where the KGO changes come from - I'm now happy with this.

Just one minor comment on the code itself below.

Cheers

Ian

[iboutle]

Rather than adding _um here, I would suggest removing it here and elsewhere from variable names - there is now really nothing to do with the UM in this code, so labelling fields with it makes no sense. I don't think they need any subscript after because they are all unique anwyay.

[Alan J Hewitt (alanjhewitt)]

this is probably a legacy thing. occasionally we had to distinguish variables that were both r_def and r_um. I will get rid of it. Maybe at some point we will rename r_um

[Mohit Dalvi (mcdalvi)]

Minor comments, but agree with Ian that the '_um' should be dropped. I expect this is for mapping to the names inside the core ''ukca_radaer_xx'' routines?

[Mohit Dalvi (mcdalvi)]

Subroutine needs to be accessed via ukca_api_mod as per the API design.

[Alan J Hewitt (alanjhewitt)]

We spoke offline and agreed that this is not required because this is separate from the UKCA API.

[Mohit Dalvi (mcdalvi)]

Same comment as the UKCA-side change, could use of '-1' for the 3rd index be made more robust? Is there a difference in start index 0 vs 1 in the arrays?

[iboutle]

My understanding is this arises from the fact that radaer excludes the 1st UKCA mode (nucleation) and so operates on modes 2-7 in the UKCA indexing, but accommodates these in arrays of size 6, hence the -1. The only way this is likely to become an issue is if radaer wanted to include the nucleation mode ever - but I imagine lots of hard-wired things might break if we wanted to do that!

[Alan J Hewitt (alanjhewitt)]

Ian is correct. Nucleation has almost zero impact on radiation properties, so we will never use nucleation mode in RADAER.

[Mohit Dalvi (mcdalvi)]

Thanks Alan (& Ian). It will help to add a comment for this, something like "to adjust indexing for soluble nucleation mode values that are at the first position in the array but not used here"

[Mohit Dalvi (mcdalvi)]

Thanks Alan. Sci-tech Review passed
Yaswant Pradhan (@yaswant)

[Mohit Dalvi (mcdalvi)]

Sci-tech Review passed