Issue 35 co2 interactive

[Rob Waters (RobWatersMet)]

PR Summary

Fixes #35

Sci/Tech Reviewer: Mohit Dalvi (@mcdalvi)
Code Reviewer: Sam Clarke-Green (@t00sa)

so the CO2 environmental field is held in a co2_interactive variable

REAL, ALLOCATABLE, TARGET, SAVE, PUBLIC :: co2_interactive(:,:,:)
  ! mass_fraction_of_carbon_dioxide_in_air [CF] {UM:00252}

CO2 is only added to fld_names if ukca_config%l_chem_environ_co2_fld

! CO2 field is required if option to use an external CO2 field is set
IF (ukca_config%l_chem_environ_co2_fld .AND. ANY(speci(:) == 'CO2       ')) THEN
  n = n + 1
  IF (n <= n_max) fld_names(n) = fldname_co2_interactive
END IF

It is then allocated

CASE (fldname_co2_interactive)
  CALL set_field_3d_real(i_field, i1, i2, j1, j2, k1, k2, field_data,          &
                         co2_interactive)

But then in ukca_main1 it is passed in unconditionally into ukca_chemistry_ctl and ukca_chemistry_ctl_full as an input variable. Inside both ctl files it has the following intent:

REAL, INTENT(IN) :: co2_interactive(tot_n_pnts)

so it is an explicit shaped array dummy variable - which needs to have been allocated.

Fortran standards:

• An unallocated allocatable variable does not represent an array object.
• A non-allocatable dummy argument requires an actual argument that is an object of the appropriate type, rank, etc.
• Therefore, an unallocated allocatable cannot be associated with a non-allocatable dummy.

However some compilers consider it fine and others don't - seems like GNU in particular doesn't allow it.

Code Quality Checklist

(Some checks are automatically carried out via the CI pipeline)

• I have performed a self-review of my own code
• My code follows the project's style guidelines
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  readability of the code
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Testing

• I have tested this change locally, using the UKCA rose-stem suite
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trac.log

Test Suite Results - lfric_apps - co2_interactive_testing/run2

Suite Information

Item	Value
Suite Name	co2_interactive_testing/run2
Suite User	robert.waters
Workflow Start	2026-07-27T15:01:48
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@2026.07.1	True
lfric_apps	RobWatersMet/lfric_apps@co2_interactive_testing	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	RobWatersMet/ukca@issue_35_co2_interactive	True

Task Information

✅ succeeded tasks - 1602

Test Suite Results - um - co2_interactive_testing_um/run2

Suite Information

Item	Value
Suite Name	co2_interactive_testing_um/run2
Suite User	robert.waters
Workflow Start	2026-07-29T10:15:48
Groups Run	all

Dependency	Reference	Main Like
casim	MetOffice/casim@2026.07.1	True
jules	MetOffice/jules@2026.07.1	True
moci	MetOffice/moci@2026.07.1	True
mule	MetOffice/mule@2026.07.1	True
shumlib	MetOffice/shumlib@2026.07.1	True
socrates	MetOffice/socrates@2026.07.1	True
SimSys_Scripts	MetOffice/SimSys_Scripts@2026.07.1	True
ukca	RobWatersMet/ukca@issue_35_co2_interactive	True
um	RobWatersMet/um@co2_interactive_testing	False
um_aux	MetOffice/um_aux@2026.07.1	True
um_meta	MetOffice/um_meta@2026.07.1	True

Approvals

Code Owners

Luke Abraham - approved

Config Owners

No UM Config Owners Required

Task Information

✅ succeeded tasks - 2770

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[Rob Waters (RobWatersMet)]

Option 1 - Always allocate it

• one solution would be to always have it allocated
• original branch had the following solution

! The co2_interactive array must be allocated before being passed to
! ukca_chemistry_ctl
IF (ALLOCATED(co2_interactive)) THEN
  temporary_co2_array=.FALSE.
ELSE
  ALLOCATE(co2_interactive(1,1,1))
  temporary_co2_array=.TRUE.
END IF

• I don't think we need to allocate it as a 1,1,1
• I think we can also do better

Option 2 - Make it a module variable

• current solution in column mode - call the co2_interactive variable from the module level

USE ukca_environment_fields_mod, ONLY: co2_interactive

• not preferred - unclear dependency

Option 3 - Make the dummy allocatable (preferred option)

real, allocatable, intent(in) :: co2_interactive(row_length,rows,model_levels)

• cleanest solution in my opinion
• note we have to have consistent ranks between dummy variable and actual variable
• hence it can no longer be co2_interactive(:) in the intent
• slightly more code required later.

[Rob Waters (RobWatersMet)]

reshaping required & allocation guard added

  ! CO2 as species
  IF (ANY(speci(:) == 'CO2       ')) THEN
    !  Copy the CO2 concentration into the asad module as VMR
    IF (ukca_config%l_chem_environ_co2_fld) THEN

      ! co2_interactive should be allocated if config option on
      IF (.NOT. ALLOCATED(co2_interactive)) THEN
        errcode = 1
        CALL ereport(ModuleName//':'//RoutineName, errcode,                    &
          'ERROR: co2_interactive array not allocated')
      END IF

      co2_1d(:) = RESHAPE(co2_interactive(:,:,k), [theta_field_size]) / c_co2
    ELSE
      co2_1d(:) = rmdi
    END IF

  END IF

I've also made it consistent against all 3 interfaces.

[Rob Waters (RobWatersMet)]

trac.log files added

Mohit Dalvi (@mcdalvi) can you be sci reviewer as you have been aware of the issue

[Mohit Dalvi (mcdalvi)]

co2_interactive(tot_n_pnts) => co2_interactive(:,:,:)

I presume the reshape from 3-D (in ukca_main) to 1-D used to happen implicitly via subroutine argument?

[Rob Waters (RobWatersMet)]

yeah exactly, I think by adding allocatable we now have to be explicit on the array shape coming into the routine

[Mohit Dalvi (mcdalvi)]

Sci-tech Review passed
Sam Clarke-Green (@t00sa) for Code Review

[Luke Abraham (theabro)]

I'm happy to approve this as Code Owner