Allow lfric_atm to compile for oasis coupled models

[Paul Earnshaw (pdearnshaw)]

PR Summary

Sci/Tech Reviewer: Mike Hobson (@mike-hobson)
Code Reviewer: Alistair Pirrie (@mo-alistairp)

This PR will allow the lfric_atm app to be used for oasis coupled models. The changes add a conditional statement to the lfric_atm makefile controlled by the environment variable COUPLED. If set and true then the internal dependency $(CORE_ROOT_DIR)/components/coupling is added to the list of INTERNAL_DEPENDENCIES while also adding to the list of PRE_PROCESS_MACROS the values MCT and COUPLED. If not set or not true then nothing happens.

The lfric_atm.f90 program file is modified to a F90 file in order to allow of pre-processing, and under the macro MCT extra statements are added to the code that allow for the initialistion of the coupled context.

No new tests have been added to cover this functionality as these will added a later date to cover all GC6 requirements.

fixes #92 (auto-closes the issue)

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)

This branch (and previous versions of) has been used extensively in GC6 testing over a number of releases and has worked fine.

trac.log

Test Suite Results - lfric_apps - lfric_apps_92_coupled_compilation_fix/run1

Suite Information

Item	Value
Suite Name	lfric_apps_92_coupled_compilation_fix/run1
Suite User	paul.earnshaw
Workflow Start	2026-05-07T12:19:39
Groups Run	developer

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.03.2	True
jules	MetOffice/jules@2026.03.2	True
lfric_apps	pdearnshaw/lfric_apps@92_coupled_compilation_fix	False
lfric_core	MetOffice/lfric_core@2026.03.2	True
moci	MetOffice/moci@2026.03.2	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.03.2	True
socrates	MetOffice/socrates@2026.03.2	True
socrates-spectral	MetOffice/socrates-spectral@2026.03.2	True
ukca	MetOffice/ukca@2026.03.2	True

Task Information

✅ succeeded tasks - 1164

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Performance Impact

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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)

Documentation

• Where appropriate I have updated documentation related to this change and confirmed that it builds correctly

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Sci/Tech Review

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• 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
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• Documentation is complete and accurate
• Security considerations have been addressed
• Performance impact is acceptable

[Mike Hobson (mike-hobson)]

This is a straightforward change that will more easily allow lfric_atm jobs to run as coupled systems. I just have one comment about the use of #ifdefs.

[Mike Hobson (mike-hobson)]

Thanks for looking at the ifdefs I'm happy that they stay in. That means the the PR is ready for code review with Alistair Pirrie (@mo-alistairp).

[Alistair Pirrie (mo-alistairp)]

Can I check whether this is ready for Code Review, because it seems to have been moved out?

[Mike Hobson (mike-hobson)]

This has passed sci/tech review and is ready for code review - so you're free to start looking at it. I don't know why the tracker script isn't moving the state on - I'll have a word with the script's maintainer.

[Matthew Hambley (MatthewHambley)]

Code Owner (Build System) Review: Just a quick question.

[Matthew Hambley (MatthewHambley)]

Normally we would just test for the existence of the macro. What is the reason for requiring it to have a particular value?

[Paul Earnshaw (pdearnshaw)]

The COUPLED here is not a macro, it is an environment variable. What we want in this test is to make sure firstly that it has been set, and then that it has been set to True. The idea being that if it is not set, or set to False then the coupled model infrastructure is not applied.

As I understand it the ifdef statement can test for environment variables or keyword-like arguments in the make command, it cannot act on PRE_PROCESS_MACRO entries. So the choice in how to identify if we wanted to compile as a coupled model was to use an optional environment variable.

Happy to consider other ways of doing this.

[Matthew Hambley (MatthewHambley)]

The PRE_PROCESS_MACROS are converted into a set of -D<key>=<value> arguments to the compiler. So they will appear to the preprocessor as macros. In which case you can just test on their presence. Unless you think there will be cases where people are setting that value to False.

There's no correctness reason not to do it the way you have, it just runs the risk of people getting unexpected behaviour if they say COUPLED=YES.

I don't mind which you do so feel free to resolve this conversation when you have made a decision.

[Matthew Hambley (MatthewHambley)]

Code owner (Build system) further commentary.

[Matthew Hambley (MatthewHambley)]

The PRE_PROCESS_MACROS are converted into a set of -D<key>=<value> arguments to the compiler. So they will appear to the preprocessor as macros. In which case you can just test on their presence. Unless you think there will be cases where people are setting that value to False.

There's no correctness reason not to do it the way you have, it just runs the risk of people getting unexpected behaviour if they say COUPLED=YES.

I don't mind which you do so feel free to resolve this conversation when you have made a decision.