From eaab2e00fb46fc2bfa8ac2b28de2dac6ae932b17 Mon Sep 17 00:00:00 2001
From: Haipeng Lin <hplin@ucar.edu>
Date: Thu, 4 Jun 2026 12:16:34 -0400
Subject: [PATCH 1/2] Complete CCPPization of CAM7 hb_free_atm

---
 .../vertical_diffusion/diffusion_stubs.F90    |  43 ++++++-
 .../suite_vdiff_holtslag_boville_free_atm.xml | 113 ++++++++++++++++++
 2 files changed, 154 insertions(+), 2 deletions(-)
 create mode 100644 test/test_suites/suite_vdiff_holtslag_boville_free_atm.xml

diff --git a/schemes/vertical_diffusion/diffusion_stubs.F90 b/schemes/vertical_diffusion/diffusion_stubs.F90
index 611514daa..e30f3173f 100644
--- a/schemes/vertical_diffusion/diffusion_stubs.F90
+++ b/schemes/vertical_diffusion/diffusion_stubs.F90
@@ -13,8 +13,9 @@ module diffusion_stubs
   ! CCPP-compliant subroutines
   public :: zero_upper_boundary_condition_init
 
-  public :: tms_beljaars_zero_stub_run
-  public :: beljaars_zero_stub_run
+  public :: tms_beljaars_zero_stub_run           ! Stub out both TMS and Beljaars (do_beljaars set to false)
+  public :: beljaars_zero_stub_run               ! Stub out Beljaars only and allows TMS to be read from snapshot (do_beljaars set to false)
+  public :: tms_zero_stub_run                    ! Stub out TMS only and allow Beljaars to be read from snapshot (do_beljaars set to true)
   public :: turbulent_mountain_stress_add_drag_coefficient_run
   public :: beljaars_add_wind_damping_rate_run
 
@@ -158,6 +159,44 @@ subroutine beljaars_zero_stub_run( &
 
   end subroutine beljaars_zero_stub_run
 
+  ! Stub for TMS to be set to zero while not implemented
+  ! but allow CAM7 Beljaars to be tested from snapshot.
+!> \section arg_table_tms_zero_stub_run Argument Table
+!! \htmlinclude tms_zero_stub_run.html
+  subroutine tms_zero_stub_run( &
+    ncol, pver, &
+    ksrftms, &
+    tautmsx, tautmsy, &
+    do_beljaars, &
+    errmsg, errflg)
+
+    ! Input arguments
+    integer,            intent(in)  :: ncol
+    integer,            intent(in)  :: pver
+
+    ! Output arguments
+    real(kind_phys),    intent(out) :: ksrftms(:)  ! Surface drag coefficient for turbulent mountain stress. > 0. [kg m-2 s-1]
+    real(kind_phys),    intent(out) :: tautmsx(:)  ! Eastward turbulent mountain surface stress [N m-2]
+    real(kind_phys),    intent(out) :: tautmsy(:)  ! Northward turbulent mountain surface stress [N m-2]
+    logical,            intent(out) :: do_beljaars
+    character(len=512), intent(out) :: errmsg      ! Error message
+    integer,            intent(out) :: errflg      ! Error flag
+
+    errmsg = ''
+    errflg = 0
+
+    ! Set TMS drag coefficient to zero (stub implementation)
+    ksrftms(:ncol) = 0._kind_phys
+
+    ! Set all TMS and Beljaars stresses to zero (stub implementation)
+    tautmsx(:ncol) = 0._kind_phys
+    tautmsy(:ncol) = 0._kind_phys
+
+    ! Set do_beljaars flag to true as it is being read from snapshot.
+    do_beljaars = .true.
+
+  end subroutine tms_zero_stub_run
+
   ! Add turbulent mountain stress to the total surface drag coefficient
 !> \section arg_table_turbulent_mountain_stress_add_drag_coefficient_run Argument Table
 !! \htmlinclude arg_table_turbulent_mountain_stress_add_drag_coefficient_run.html
diff --git a/test/test_suites/suite_vdiff_holtslag_boville_free_atm.xml b/test/test_suites/suite_vdiff_holtslag_boville_free_atm.xml
new file mode 100644
index 000000000..77cc0fd32
--- /dev/null
+++ b/test/test_suites/suite_vdiff_holtslag_boville_free_atm.xml
@@ -0,0 +1,113 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  CAM7 variant of the
+  Holtslag and Boville (HB) boundary layer scheme and vertical diffusion.
+
+  For CAM7 with CLUBB_SGS, the HB PBL scheme is only used in levels above
+  where CLUBB is active (the "free atmosphere"), via
+  hb_diff_free_atm_exchange_coefficients. The PBL-dependent coefficients
+  (hb_pbl_dependent_coefficients) are not computed; CLUBB handles the PBL.
+
+  The "prepare" interstitial zeros surface stresses and sensible heat flux
+  (CLUBB handles these), and optionally passes non-water-vapor constituent
+  fluxes (CAM6) or no fluxes at all (CAM7) to the diffusion solver via the
+  flag_for_vertical_diffusion_apply_ccpp_constituent_fluxes_excluding_water_vapor
+  flag.
+
+  The first part of this file is for HB to compute exchange coefficients
+  in the free atmosphere. The second part is the vertical diffusion solver.
+  TMS and Beljaars are stubbed as zeroes for now; when CCPPized they would
+  be separate schemes providing tautmsx/y, taubljx/y, and dragblj.
+-->
+<suite name="vdiff_holtslag_boville_free_atm" version="1.0">
+  <group name="physics_before_coupler">
+    <scheme>initialize_constituents</scheme>
+    <scheme>to_be_ccppized_temporary</scheme>
+
+    <!-- read namelist options -->
+    <scheme>holtslag_boville_diff_options</scheme>
+    <scheme>vertical_diffusion_options</scheme>
+
+    <!-- stub: zero UBC -->
+    <scheme>zero_upper_boundary_condition</scheme>
+
+    <!-- init phase only: for setting vertical diffusion top -->
+    <scheme>hb_diff_set_vertical_diffusion_top</scheme>
+
+    <!-- hb pre-interstitial: tautotx/y from coupler + TMS + Beljaars stresses -->
+    <scheme>tms_beljaars_zero_stub</scheme>
+
+    <!-- vdiff pre-interstitial 0: create p coords1d and potential temp. -->
+    <scheme>vertical_diffusion_prepare_inputs</scheme>
+
+    <!-- vdiff pre-interstitial 1: copy surfaces from coupler (cam_in)
+         For CAM7 with CLUBB: taux/tauy/shflux are zeroed (CLUBB handles surface).
+         Constituent fluxes depend on flag_for_cflux (CAM6: non-wv, CAM7: none). -->
+    <scheme>hb_free_atm_diff_prepare_vertical_diffusion_inputs</scheme>
+
+    <!-- vdiff pre-interstitial 2: interpolate t, rho, p to interfaces -->
+    <!-- not in WACCM-X mode so do not use rairv in vdiff. -->
+    <scheme>vertical_diffusion_not_use_rairv</scheme>
+    <scheme>vertical_diffusion_set_temperature_at_toa_default</scheme>
+    <scheme>vertical_diffusion_interpolate_to_interfaces</scheme>
+
+    <!-- vdiff pre-interstitial 3: calculate total surface stresses
+         used for surface momentum flux diagnostic.
+         here, also used for HB. -->
+    <scheme>vertical_diffusion_set_total_surface_stress</scheme>
+
+    <!-- Initialize Holtslag-Boville boundary layer scheme (no run phase) -->
+    <scheme>holtslag_boville_diff</scheme>
+
+    <!-- Holtslag-Boville boundary layer scheme: free atmosphere variant.
+         Only independent coefficients are computed (s2, ri).
+         PBL-dependent coefficients are NOT computed since CLUBB handles PBL. -->
+    <scheme>hb_pbl_independent_coefficients</scheme>
+    <scheme>compute_kinematic_fluxes_and_obklen</scheme>
+    <scheme>hb_diff_free_atm_exchange_coefficients</scheme>
+
+    <!-- sponge layer vertical diffusion (mutate kvm) -->
+    <scheme>vertical_diffusion_sponge_layer</scheme>
+
+    <!-- HB scheme diagnostics are not enabled for the free atmosphere variant,
+         as they are mostly zero when CLUBB is active.
+         TODO: ustar and obklen are still output by HB in CAM7; if needed,
+         a slimmed-down free_atm diagnostics scheme could be added here. -->
+
+    <!-- vdiff pre-interstitial 4: calculate drag coefficient -->
+    <scheme>implicit_surface_stress_add_drag_coefficient</scheme>
+
+    <!-- vdiff pre-interstitial 5: calculate damping rate from drag coef. -->
+    <scheme>vertical_diffusion_wind_damping_rate</scheme>
+    <scheme>beljaars_add_wind_damping_rate</scheme>
+
+    <!-- vdiff compute 1: horizontal momentum diffusion; dissipate KE -->
+    <scheme>vertical_diffusion_diffuse_horizontal_momentum</scheme>
+
+    <!-- set dry static energy top boundary condition to zero.
+         zero is used for: no molecular diffusion, or molecular diffusion + WACCM-X is active.
+         otherwise, use vertical_diffusion_set_dry_static_energy_at_toa_molecdiff -->
+    <scheme>vertical_diffusion_set_dry_static_energy_at_toa_zero</scheme>
+
+    <!-- vdiff compute 2: dry static energy diffusion -->
+    <scheme>vertical_diffusion_diffuse_dry_static_energy</scheme>
+
+    <!-- vdiff compute 3: tracer diffusion -->
+    <scheme>vertical_diffusion_diffuse_tracers</scheme>
+
+    <!-- vdiff post-interstitial 1: convert provisional updates to tendencies -->
+    <scheme>vertical_diffusion_tendencies</scheme>
+
+    <!-- diagnostics need to be outputted before tendencies are applied. -->
+    <scheme>vertical_diffusion_tendencies_diagnostics</scheme>
+
+    <!-- apply tendencies from vdiff -->
+    <scheme>apply_tendency_of_northward_wind</scheme>
+    <scheme>apply_tendency_of_eastward_wind</scheme>
+    <scheme>apply_constituent_tendencies</scheme>
+    <scheme>apply_heating_rate</scheme>
+    <scheme>qneg</scheme>
+    <scheme>geopotential_temp</scheme>
+    <scheme>update_dry_static_energy</scheme>
+  </group>
+</suite>

From 8993b739088b3e621dddc1a2307e1f0b36327bb8 Mon Sep 17 00:00:00 2001
From: Haipeng Lin <hplin+github@ucar.edu>
Date: Tue, 21 Jul 2026 13:30:51 -0400
Subject: [PATCH 2/2] Change len=512 to len=* for errmsg.

---
 schemes/vertical_diffusion/diffusion_stubs.F90  | 16 ++++++++--------
 schemes/vertical_diffusion/diffusion_stubs.meta | 14 +++++++-------
 2 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/schemes/vertical_diffusion/diffusion_stubs.F90 b/schemes/vertical_diffusion/diffusion_stubs.F90
index caa43e614..197fb33b3 100644
--- a/schemes/vertical_diffusion/diffusion_stubs.F90
+++ b/schemes/vertical_diffusion/diffusion_stubs.F90
@@ -50,7 +50,7 @@ subroutine zero_upper_boundary_condition_init( &
     ! Output arguments
     real(kind_phys),    intent(out) :: ubc_mmr(:,:)             ! Upper boundary condition mass mixing ratios [none]
     logical,            intent(out) :: cnst_fixed_ubc(:)        ! Flag for fixed upper boundary condition of constituents [flag]
-    character(len=512), intent(out) :: errmsg                   ! Error message
+    character(len=*)  , intent(out) :: errmsg                   ! Error message
     integer,            intent(out) :: errflg                   ! Error flag
 
     errmsg = ''
@@ -88,7 +88,7 @@ subroutine tms_beljaars_zero_stub_run( &
     real(kind_phys),    intent(out) :: dragblj(:,:)! Drag profile from Beljaars SGO form drag > 0. [s-1]
     real(kind_phys),    intent(out) :: taubljx(:)  ! Eastward Beljaars surface stress [N m-2]
     real(kind_phys),    intent(out) :: taubljy(:)  ! Northward Beljaars surface stress [N m-2]
-    character(len=512), intent(out) :: errmsg      ! Error message
+    character(len=*)  , intent(out) :: errmsg      ! Error message
     integer,            intent(out) :: errflg      ! Error flag
 
     errmsg = ''
@@ -134,7 +134,7 @@ subroutine beljaars_zero_stub_run( &
     real(kind_phys),    intent(out) :: dragblj(:,:)! Drag profile from Beljaars SGO form drag > 0. [s-1]
     real(kind_phys),    intent(out) :: taubljx(:)  ! Eastward Beljaars surface stress [N m-2]
     real(kind_phys),    intent(out) :: taubljy(:)  ! Northward Beljaars surface stress [N m-2]
-    character(len=512), intent(out) :: errmsg      ! Error message
+    character(len=*)  , intent(out) :: errmsg      ! Error message
     integer,            intent(out) :: errflg      ! Error flag
 
     errmsg = ''
@@ -176,7 +176,7 @@ subroutine tms_zero_stub_run( &
     real(kind_phys),    intent(out) :: tautmsx(:)  ! Eastward turbulent mountain surface stress [N m-2]
     real(kind_phys),    intent(out) :: tautmsy(:)  ! Northward turbulent mountain surface stress [N m-2]
     logical,            intent(out) :: do_beljaars
-    character(len=512), intent(out) :: errmsg      ! Error message
+    character(len=*)  , intent(out) :: errmsg      ! Error message
     integer,            intent(out) :: errflg      ! Error flag
 
     errmsg = ''
@@ -212,7 +212,7 @@ subroutine turbulent_mountain_stress_add_drag_coefficient_run( &
     real(kind_phys),    intent(inout) :: ksrf(:)          ! total surface drag coefficient [kg m-2 s-1]
 
     ! Output arguments
-    character(len=512), intent(out)   :: errmsg  ! error message
+    character(len=*)  , intent(out)   :: errmsg  ! error message
     integer,            intent(out)   :: errflg  ! error flag
 
     errmsg = ''
@@ -256,7 +256,7 @@ subroutine turbulent_mountain_stress_add_updated_surface_stress_run( &
     ! Output arguments
     real(kind_phys), intent(out)      :: tautmsx(:)       ! Implicit zonal turbulent mountain surface stress [N m-2]
     real(kind_phys), intent(out)      :: tautmsy(:)       ! Implicit meridional turbulent mountain surface stress [N m-2]
-    character(len=512), intent(out)   :: errmsg  ! error message
+    character(len=*)  , intent(out)   :: errmsg  ! error message
     integer,            intent(out)   :: errflg  ! error flag
 
     errmsg = ''
@@ -297,7 +297,7 @@ subroutine vertical_diffusion_not_use_rairv_init( &
 
     ! Output arguments
     logical,            intent(out) :: use_rairv ! Flag for constituent-dependent gas constant [flag]
-    character(len=512), intent(out) :: errmsg
+    character(len=*)  , intent(out) :: errmsg
     integer,            intent(out) :: errflg
 
     errmsg = ''
@@ -342,7 +342,7 @@ subroutine dropmixnuc_apply_surface_fluxes_run( &
     real(kind_phys),    intent(inout) :: q1(:, :, :)               ! Constituent array after "vertical diffusion" [kg kg-1]
 
     ! Output arguments
-    character(len=512), intent(out)   :: errmsg
+    character(len=*)  , intent(out)   :: errmsg
     integer,            intent(out)   :: errflg
 
     ! Local variables
diff --git a/schemes/vertical_diffusion/diffusion_stubs.meta b/schemes/vertical_diffusion/diffusion_stubs.meta
index 109c1cc1e..c4010dfb6 100644
--- a/schemes/vertical_diffusion/diffusion_stubs.meta
+++ b/schemes/vertical_diffusion/diffusion_stubs.meta
@@ -33,7 +33,7 @@
   standard_name = ccpp_error_message
   units = none
   dimensions = ()
-  type = character | kind = len=512
+  type = character | kind = len=*
   intent = out
 [ errflg ]
   standard_name = ccpp_error_code
@@ -107,7 +107,7 @@
   standard_name = ccpp_error_message
   units = none
   dimensions = ()
-  type = character | kind = len=512
+  type = character | kind = len=*
   intent = out
 [ errflg ]
   standard_name = ccpp_error_code
@@ -164,7 +164,7 @@
   standard_name = ccpp_error_message
   units = none
   dimensions = ()
-  type = character | kind = len=512
+  type = character | kind = len=*
   intent = out
 [ errflg ]
   standard_name = ccpp_error_code
@@ -208,7 +208,7 @@
   standard_name = ccpp_error_message
   units = none
   dimensions = ()
-  type = character | kind = len=512
+  type = character | kind = len=*
   intent = out
 [ errflg ]
   standard_name = ccpp_error_code
@@ -305,7 +305,7 @@
 [ errmsg ]
   standard_name = ccpp_error_message
   units = none
-  type = character | kind = len=512
+  type = character | kind = len=*
   dimensions = ()
   intent = out
 [ errflg ]
@@ -331,7 +331,7 @@
 [ errmsg ]
   standard_name = ccpp_error_message
   units = none
-  type = character | kind = len=512
+  type = character | kind = len=*
   dimensions = ()
   intent = out
 [ errflg ]
@@ -407,7 +407,7 @@
 [ errmsg ]
   standard_name = ccpp_error_message
   units = none
-  type = character | kind = len=512
+  type = character | kind = len=*
   dimensions = ()
   intent = out
 [ errflg ]