Ztodt comment fix

doc/ChangeLog

@@ -1,5 +1,56 @@
 ===============================================================
 
+Tag name: cam6_4_190
+Originator(s): pel
+Date: 21 Jul 2026
+One-line Summary: Correct outdated ztodt "2 delta-t" comments
+Github PR URL:
+
+Purpose of changes (include the issue number and title text for each relevant GitHub issue):
+
+ Issue 1626: The ztodt argument passed to physics is documented in many routines as
+ "2 delta-t" / "two times model timestep", a leftover from the leapfrog
+ (Eulerian spectral) dycore.  All current dycores pass the plain physics
+ time step (get_step_size()).  Comments only; no code changes.
+
+Describe any changes made to build system: N/A
+
+Describe any changes made to the namelist: N/A
+
+List any changes to the defaults for the boundary datasets: N/A
+
+Describe any substantial timing or memory changes: N/A
+
+Code reviewed by:
+
+List all files eliminated: N/A
+
+List all files added and what they do: N/A
+
+List all existing files that have been modified, and describe the changes:
+M       doc/ChangeLog
+M       src/chemistry/aerosol/aero_convproc.F90
+M       src/physics/cam/check_energy.F90
+M       src/physics/cam/convect_deep.F90
+M       src/physics/cam/convect_shallow.F90
+M       src/physics/cam/molec_diff.F90
+M       src/physics/cam/physpkg.F90
+M       src/physics/cam/qneg_module.F90
+M       src/physics/cam/vertical_diffusion.F90
+M       src/physics/cam/waccmx_phys_intr.F90
+M       src/physics/cam/zm_conv_intr.F90
+M       src/physics/cam7/convect_diagnostics.F90
+M       src/physics/cam7/physpkg.F90
+M       src/physics/simple/held_suarez_cam.F90
+M       src/physics/simple/physpkg.F90
+M       src/physics/waccmx/majorsp_diffusion.F90
+ - correct ztodt comment: model physics timestep, not 2 delta-t
+
+Summarize any changes to answers: bit-for-bit unchanged (comments only)
+
+===============================================================
+===============================================================
+
 Tag name: cam6_4_189
 Originator(s): fvitt
 Date: 21 Jul 2026

src/chemistry/aerosol/aero_convproc.F90

@@ -330,7 +330,7 @@ subroutine aero_convproc_intr( aero_props, aero_state, state, ptend, pbuf, ztodt
    type(physics_state),target,intent(in )   :: state      ! Physics state variables
    type(physics_ptend),       intent(inout) :: ptend      ! %lq set in aero_model_wetdep
    type(physics_buffer_desc), pointer       :: pbuf(:)
-   real(r8), intent(in) :: ztodt                          ! 2 delta t (model time increment)
+   real(r8), intent(in) :: ztodt                          ! model physics timestep [s]
 
    integer,  intent(in)    :: nsrflx_mzaer2cnvpr
    real(r8), intent(in)    :: qsrflx_mzaer2cnvpr(pcols,ncnstaer,nsrflx_mzaer2cnvpr)

src/physics/cam/check_energy.F90

@@ -290,7 +290,7 @@ subroutine check_tracers_chng(state, tracerint, name, nstep, ztodt, cflx)
     type(check_tracers_data), intent(inout) :: tracerint! tracers integrals and boundary fluxes
     character*(*),intent(in) :: name               ! parameterization name for fluxes
     integer , intent(in   ) :: nstep               ! current timestep number
-    real(r8), intent(in   ) :: ztodt               ! 2 delta t (model time increment)
+    real(r8), intent(in   ) :: ztodt               ! model physics timestep [s]
     real(r8), intent(in   ) :: cflx(pcols,pcnst)       ! boundary flux of tracers       (kg/m2/s)
 
 !---------------------------Local storage-------------------------------

src/physics/cam/convect_deep.F90

@@ -185,7 +185,7 @@ subroutine convect_deep_tend( &
 
 
    type(physics_buffer_desc), pointer :: pbuf(:)
-   real(r8), intent(in) :: ztodt               ! 2 delta t (model time increment)
+   real(r8), intent(in) :: ztodt               ! model physics timestep [s]
    real(r8), intent(in) :: landfrac(pcols)     ! Land fraction
 
 
@@ -293,7 +293,7 @@ subroutine convect_deep_tend_2( state,  ptend,  ztodt, pbuf)
 
    type(physics_buffer_desc), pointer :: pbuf(:)
 
-   real(r8), intent(in) :: ztodt                          ! 2 delta t (model time increment)
+   real(r8), intent(in) :: ztodt                          ! model physics timestep [s]
 
 
    if ( deep_scheme .eq. 'ZM' ) then  ! Zhang-McFarlane

src/physics/cam/convect_shallow.F90

@@ -334,7 +334,7 @@ subroutine convect_shallow_tend( ztodt  , cmfmc   , &
    ! ---------------------- !
    type(physics_buffer_desc), pointer :: pbuf(:)
    type(physics_state), intent(in)    :: state                           ! Physics state variables
-   real(r8),            intent(in)    :: ztodt                           ! 2 delta-t  [ s ]
+   real(r8),            intent(in)    :: ztodt                           ! model physics timestep [s]
 
    type(physics_ptend), intent(out)   :: ptend_all                       ! Indivdual parameterization tendencies
    real(r8),            intent(out)   :: rliq2(pcols)                    ! Vertically-integrated reserved cloud condensate [ m/s ]

src/physics/cam/molec_diff.F90

@@ -249,7 +249,7 @@ function vd_lu_qdecomp( &
     type(BoundaryType), intent(in) :: interface_boundary ! Boundary on grid edge.
     type(BoundaryType), intent(in) :: molec_boundary ! Boundary at edge of molec_diff region.
     real(r8), intent(in)    :: tint(:,:)    ! Air temperature [ K ], interfaces
-    real(r8), intent(in)    :: ztodt                 ! 2 delta-t [ s ]
+    real(r8), intent(in)    :: ztodt                 ! model physics timestep [s]
 
     real(r8), intent(in)    :: t(:,:)	    ! Air temperature [ K ]
     integer,  intent(in)    :: m 		    ! cnst index

src/physics/cam/physpkg.F90

@@ -1099,7 +1099,7 @@ subroutine phys_run1(phys_state, ztodt, phys_tend, pbuf2d,  cam_in, cam_out)
     !
     ! Input arguments
     !
-    real(r8), intent(in) :: ztodt            ! physics time step unless nstep=0
+    real(r8), intent(in) :: ztodt            ! model physics timestep [s]
     !
     ! Input/Output arguments
     !
@@ -1223,7 +1223,7 @@ subroutine phys_run2(phys_state, ztodt, phys_tend, pbuf2d,  cam_out, &
     !
     ! Input arguments
     !
-    real(r8), intent(in) :: ztodt                       ! physics time step unless nstep=0
+    real(r8), intent(in) :: ztodt                       ! model physics timestep [s]
     !
     ! Input/Output arguments
     !
@@ -1436,7 +1436,7 @@ subroutine tphysac (ztodt,   cam_in,  &
     !
     ! Arguments
     !
-    real(r8), intent(in) :: ztodt                  ! Two times model timestep (2 delta-t)
+    real(r8), intent(in) :: ztodt                  ! model physics timestep [s]
 
     type(cam_in_t),      intent(inout) :: cam_in
     type(cam_out_t),     intent(inout) :: cam_out
@@ -2198,7 +2198,7 @@ subroutine tphysbc (ztodt, state,  &
 
     ! Arguments
 
-    real(r8), intent(in) :: ztodt                          ! 2 delta t (model time increment)
+    real(r8), intent(in) :: ztodt                          ! model physics timestep [s]
 
     type(physics_state), intent(inout), target :: state
     type(physics_tend ), intent(inout) :: tend

src/physics/cam/qneg_module.F90

@@ -337,7 +337,7 @@ subroutine qneg4 (subnam, lchnk, ncol, ztodt,                            &
     integer, intent(in) :: lchnk             ! chunk index
     integer, intent(in) :: ncol              ! number of atmospheric columns
     !
-    real(r8), intent(in) :: ztodt            ! two times model timestep (2 delta-t)
+    real(r8), intent(in) :: ztodt            ! model physics timestep [s]
     real(r8), intent(in) :: qbot(ncol,pcnst) ! moisture at lowest model level
     real(r8), intent(in) :: srfrpdel(ncol)   ! 1./(pint(K+1)-pint(K))
     !

src/physics/cam/vertical_diffusion.F90

@@ -710,7 +710,7 @@ subroutine vertical_diffusion_tend( &
   type(physics_state), intent(inout) :: state                     ! Physics state variables
   type(cam_in_t),      intent(in)    :: cam_in                    ! Surface inputs
 
-  real(r8),            intent(in)    :: ztodt                     ! 2 delta-t [ s ]
+  real(r8),            intent(in)    :: ztodt                     ! model physics timestep [s]
   real(r8),            intent(in)    :: cldn(pcols,pver)          ! New stratus fraction [ fraction ]
 
   ! ---------------------- !

src/physics/cam/waccmx_phys_intr.F90

@@ -42,7 +42,7 @@ end subroutine waccmx_phys_mspd_init
   !------------------------------------------------------------------------------
   subroutine waccmx_phys_mspd_tend(ztodt, state, ptend)
 
-    real(r8), intent(in) :: ztodt                  ! 2 delta-t
+    real(r8), intent(in) :: ztodt                  ! model physics timestep [s]
     type(physics_state), intent(in)     :: state   ! Physics state variables
     type(physics_ptend), intent(inout)  :: ptend   ! indivdual parameterization tendencies
 

src/physics/cam/zm_conv_intr.F90

@@ -388,7 +388,7 @@ subroutine zm_conv_tend(pblh    ,mcon    ,cme     , &
    type(physics_ptend), intent(out)         :: ptend_all      ! individual parameterization tendencies
    type(physics_buffer_desc), pointer       :: pbuf(:)
 
-   real(r8), intent(in) :: ztodt                       ! 2 delta t (model time increment)
+   real(r8), intent(in) :: ztodt                       ! model physics timestep [s]
    real(r8), intent(in) :: pblh(pcols)                 ! Planetary boundary layer height
    real(r8), intent(in) :: tpert(pcols)                ! Thermal temperature excess
    real(r8), intent(in) :: landfrac(pcols)             ! RBN - Landfrac
@@ -817,7 +817,7 @@ subroutine zm_conv_tend_2( state,  ptend,  ztodt, pbuf)
 
    type(physics_buffer_desc), pointer :: pbuf(:)
 
-   real(r8), intent(in) :: ztodt                          ! 2 delta t (model time increment)
+   real(r8), intent(in) :: ztodt                          ! model physics timestep [s]
 
 ! Local variables
    integer :: i, lchnk, istat

src/physics/cam7/convect_diagnostics.F90

@@ -107,7 +107,7 @@ subroutine convect_diagnostics_calc( ztodt  , cmfmc    , &
    ! ---------------------- !
    type(physics_buffer_desc), pointer :: pbuf(:)
    type(physics_state), intent(in)    :: state                           ! Physics state variables
-   real(r8),            intent(in)    :: ztodt                           ! 2 delta-t  [ s ]
+   real(r8),            intent(in)    :: ztodt                           ! model physics timestep [s]
 
    real(r8),            intent(out)   :: rliq2(pcols)                    ! Vertically-integrated reserved cloud condensate [ m/s ]
    real(r8),            intent(out)   :: qc2(pcols,pver)                 ! Same as qc but only from shallow convection scheme

src/physics/cam7/physpkg.F90

@@ -1095,7 +1095,7 @@ subroutine phys_run1(phys_state, ztodt, phys_tend, pbuf2d,  cam_in, cam_out)
     !
     ! Input arguments
     !
-    real(r8), intent(in) :: ztodt            ! physics time step unless nstep=0
+    real(r8), intent(in) :: ztodt            ! model physics timestep [s]
     !
     ! Input/Output arguments
     !
@@ -1221,7 +1221,7 @@ subroutine phys_run2(phys_state, ztodt, phys_tend, pbuf2d,  cam_out, &
     !
     ! Input arguments
     !
-    real(r8), intent(in) :: ztodt                       ! physics time step unless nstep=0
+    real(r8), intent(in) :: ztodt                       ! model physics timestep [s]
     !
     ! Input/Output arguments
     !
@@ -1457,7 +1457,7 @@ subroutine tphysac (ztodt,   cam_in,  &
     !
     ! Arguments
     !
-    real(r8), intent(in) :: ztodt                  ! Two times model timestep (2 delta-t)
+    real(r8), intent(in) :: ztodt                  ! model physics timestep [s]
 
     type(cam_in_t),      intent(inout) :: cam_in
     type(cam_out_t),     intent(inout) :: cam_out
@@ -2682,7 +2682,7 @@ subroutine tphysbc (ztodt, state,  &
 
     ! Arguments
 
-    real(r8), intent(in) :: ztodt                          ! 2 delta t (model time increment)
+    real(r8), intent(in) :: ztodt                          ! model physics timestep [s]
 
     type(physics_state), intent(inout), target :: state
     type(physics_tend ), intent(inout) :: tend

src/physics/simple/held_suarez_cam.F90

@@ -68,7 +68,7 @@ subroutine held_suarez_tend(state, ptend, ztodt)
     ! Input arguments
     !
     type(physics_state), intent(inout) :: state
-    real(r8),            intent(in)    :: ztodt            ! Two times model timestep (2 delta-t)
+    real(r8),            intent(in)    :: ztodt            ! model physics timestep [s]
                                                            !
                                                            ! Output argument
                                                            !

src/physics/simple/physpkg.F90

@@ -314,7 +314,7 @@ subroutine phys_run1(phys_state, ztodt, phys_tend, pbuf2d,  cam_in, cam_out)
     !
     ! Input arguments
     !
-    real(r8), intent(in) :: ztodt            ! physics time step unless nstep=0
+    real(r8), intent(in) :: ztodt            ! model physics timestep [s]
     !
     ! Input/Output arguments
     !
@@ -403,7 +403,7 @@ subroutine phys_run2(phys_state, ztodt, phys_tend, pbuf2d,  cam_out, cam_in)
     !
     ! Input arguments
     !
-    real(r8), intent(in) :: ztodt                       ! physics time step unless nstep=0
+    real(r8), intent(in) :: ztodt                       ! model physics timestep [s]
     !
     ! Input/Output arguments
     !
@@ -510,7 +510,7 @@ subroutine tphysac (ztodt, cam_in, cam_out, state, tend, pbuf)
 
     ! Arguments
     !
-    real(r8),                  intent(in)    :: ztodt ! Two times model timestep (2 delta-t)
+    real(r8),                  intent(in)    :: ztodt ! model physics timestep [s]
 
     type(cam_in_t),            intent(inout) :: cam_in
     type(cam_out_t),           intent(inout) :: cam_out

src/physics/waccmx/majorsp_diffusion.F90

@@ -139,7 +139,7 @@ subroutine mspd_intr(ztodt    ,state    ,ptend)
     use air_composition, only: rairv, mbarv
 
 !------------------------------Arguments--------------------------------
-    real(r8), intent(in) :: ztodt                  ! 2 delta-t
+    real(r8), intent(in) :: ztodt                  ! model physics timestep [s]
     type(physics_state), intent(in)     :: state   ! Physics state variables
     type(physics_ptend), intent(inout)  :: ptend   ! indivdual parameterization tendencies
 !---------------------------Local storage-------------------------------
@@ -228,7 +228,7 @@ subroutine mspdiff (lchnk      ,ncol       ,
     real(r8), intent(in) :: pmid(pcols,pver)       ! midpoint pressures
     real(r8), intent(in) :: pint(pcols,pverp)      ! interface pressures
     real(r8), intent(in) :: pdel(pcols,pver)       ! thickness between interfaces
-    real(r8), intent(in) :: ztodt                  ! 2 delta-t
+    real(r8), intent(in) :: ztodt                  ! model physics timestep [s]
     real(r8), intent(in) :: rairv(pcols,pver)                  ! composition dependent gas "constant"
     real(r8), intent(in) :: mbarv(pcols,pver)                  ! composition dependent mean mass