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Allow substepping of just tracers #733

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4a7b1fc
Merge pull request #5 from MetOffice/stable
tommbendall Mar 9, 2026
92ac379
Merge pull request #13 from MetOffice/stable
tommbendall Jul 20, 2026
0fb29a0
bring over changes to implement substepping for tracers
tommbendall Aug 24, 2026
8143cc1
only substep tracers if namelist says so!
tommbendall Aug 25, 2026
351c569
tighten up substepping criteria
tommbendall Aug 25, 2026
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3 changes: 2 additions & 1 deletion science/gungho/source/algorithm/transport/control/gungho_transport_control_alg_mod.X90
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Original file line number Diff line number Diff line change
Expand Up @@ -244,6 +244,7 @@ contains
! Internal variables
logical(kind=l_def) :: do_moisture_diagnostics
logical(kind=l_def) :: cheap_update_step
logical(kind=l_def) :: safe_ref

! Temporary fields or pointers
type(field_type) :: fields_np1(bundle_size)
Expand Down Expand Up @@ -346,7 +347,7 @@ contains

! Check negative reference fields at this point
if (check_any_eqn_consistent()) then
call transport_controller%check_reference_field()
safe_ref = transport_controller%check_reference_field(fail_on_neg=.true.)
end if

! ------------------------------------------------------------------------ !
Expand Down
86 changes: 75 additions & 11 deletions science/gungho/source/algorithm/transport/control/transport_controller_mod.x90
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Original file line number Diff line number Diff line change
Expand Up @@ -557,7 +557,10 @@ contains
!> @brief Calculates the reference fields to be used in tracer transport, and
!! if any of these are going to be negative, may perform the Watkins
!! algorithm to adjust how the fluxes are distributed between steps.
subroutine check_reference_field(self)
!> @param[in] fail_on_neg If true, the model will stop if a reference field
!! is negative. If false, the model will continue.
!> @return True if the ref field is safe to use, false if not
function check_reference_field(self, fail_on_neg) result(ref_field_safe)

use transport_config_mod, only: ffsl_unity_3d, &
adjust_vhv_wind, &
Expand All @@ -572,6 +575,7 @@ contains
implicit none

class(transport_controller_type), target, intent(inout) :: self
logical(kind=l_def), intent(in) :: fail_on_neg

type(flux_precomputations_type), pointer :: flux_precomputations

Expand Down Expand Up @@ -688,22 +692,36 @@ contains
call self%flux_precomputations%initialise_step(step=(3+3*(i-1)), ref_flux=step_3_flux)
end do

ref_field_safe = .true.
else
! We're about to fail... log the Lipschitz numbers
ref_field_safe = .false.

call lipschitz_diag_alg( &
self%config, &
self%reference_splitting, self%transporting_wind, &
self%dt_substep, LOG_LEVEL_WARNING &
)

if (adjust_tracer_equation) then
if (.not. fail_on_neg &
.and. substep_transport == substep_transport_adaptive &
.and. self%num_substeps < 3) then
! Time to attempt substepping...
call log_event( &
'The Watkins algorithm failed to keep all reference fields ' // &
'positive for the transport of tracers. Now try substepping...', &
LOG_LEVEL_WARNING &
)

else if (adjust_tracer_equation) then
! Use advective form for tracer transport instead of consistent form
call log_event( &
'Reference field for tracer transport is negative, so ' // &
'making all tracers advective for this step. This will ' // &
'make tracer transport non-conservative. ', LOG_LEVEL_INFO &
'make tracer transport non-conservative. ', LOG_LEVEL_WARNING &
)
self%make_tracers_advective = .true.

else
! Fail gracefully
call log_event( &
Expand Down Expand Up @@ -734,7 +752,7 @@ contains
end if
end if

end subroutine check_reference_field
end function check_reference_field

! ============================================================================ !
! SETTERS
Expand Down Expand Up @@ -1021,12 +1039,17 @@ contains
!> @param[in] ref_flux The reference flux field
subroutine initialise_flux_precomputations(self, ref_flux)

use transport_config_mod, only: substep_transport, &
substep_transport_adaptive

implicit none

class(transport_controller_type), target, intent(inout) :: self
type(r_tran_field_type), intent(in) :: ref_flux

type(mesh_type), pointer :: mesh
logical(kind=l_def) :: safe_ref
integer(kind=i_def) :: i, start_substeps, end_substeps

if (self%flux_precomputations%is_initialised()) then
call log_event( &
Expand All @@ -1043,13 +1066,54 @@ contains
! coarser mesh. In either case, the fluxes calculated during the transport
! of the density are not the same as those that will be used to transport
! the tracers. We set the correct flux here.
call self%flux_precomputations%initialise( &
self%config, &
mesh, self%reference_splitting, self%num_substeps, &
self%transporting_wind, self%dt_substep, &
flux=ref_flux, ref_field=self%ref_field_rtran &
)
call self%check_reference_field()

! Loop through substeps, as we may need to increase the number to keep
! the reference fields positive
safe_ref = .false.
start_substeps = self%num_substeps
if (substep_transport == substep_transport_adaptive) then
end_substeps = 3 ! Force max substeps of 3
else
end_substeps = 1 ! No substepping
end if

do i = start_substeps, end_substeps
call self%flux_precomputations%initialise( &
self%config, &
mesh, self%reference_splitting, self%num_substeps, &
self%transporting_wind, self%dt_substep, &
flux=ref_flux, ref_field=self%ref_field_rtran &
)
if (i == end_substeps) then
! Fail if we have reached max substeps
safe_ref = self%check_reference_field(fail_on_neg=.true.)
else
safe_ref = self%check_reference_field(fail_on_neg=.false.)
end if

if (.not. safe_ref) then
! Watkins algorithm failed to keep reference field positive
! This is likely to be BDF tracer transport near start of run
! Try enforcing substepping for tracer transport
self%dt_substep = ( &
self%dt_substep * self%num_substeps &
/ real(self%num_substeps + 1, r_tran) &
)
self%num_substeps = self%num_substeps + 1

! Clear all precomputations in case they were set
call self%ffsl_precomputations%finalise()
call self%flux_precomputations%finalise()
call self%wind_precomputations%finalise()

write(log_scratch_space, '(A,I4)') &
'Transport: Reference field for tracer transport cannot stay ' // &
'positive. Increasing number of substeps to ', self%num_substeps
call log_event(log_scratch_space, LOG_LEVEL_WARNING)
else
EXIT
end if
end do

end subroutine initialise_flux_precomputations

Expand Down
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