Remove rescaling bottom pressure in SAL

* Instead of rescaling bottom pressure to height unit,
calc_Loving_scaling is modified to be conditionally dimensional. When
calculating self-attraction and loading, Love numbers are now
dimensional when bottom pressure anomaly is used as an input. This
change eliminate Love numbers' dependence on mean seawater density.

* A new coefficient called linear_scaling is added to SAL CS for the
same purpose, although to use bottom pressure anomaly for scalar
approximation is not quite justifiable. A WARNING is given when users
try to do that.

* Two separate field, SSH (sea surface height anomaly) and pbot (total
bottom pressure), are used for calculating self attraction and loading
when SAL_USE_BPA = False and SAL_USE_BPA = True, respectively. The
change is to enhance code readability.

Author: herrwang0

src/core/MOM_PressureForce_FV.F90

@@ -145,7 +145,10 @@ subroutine PressureForce_FV_nonBouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_
                 ! the pressure anomaly at the top of the layer [R L4 T-4 ~> Pa m2 s-2].
   real, dimension(SZI_(G),SZJ_(G))  :: &
     dp, &       ! The (positive) change in pressure across a layer [R L2 T-2 ~> Pa].
-    pbot_anom, & ! Bottom pressure (anomaly) in depth units, used for self-attraction and loading [Z ~> m].
+    SSH, &      ! Sea surfae height anomaly for self-attraction and loading. Used if
+                ! CALCULATE_SAL is True and SAL_USE_BPA is False [Z ~> m].
+    pbot, &     ! Total bottom pressure for self-attraction and loading. Used if
+                ! CALCULATE_SAL is True and SAL_USE_BPA is True [R L2 T-2 ~> Pa].
     e_sal, &    ! The bottom geopotential anomaly due to self-attraction and loading [Z ~> m].
     e_tidal_eq,  & ! The bottom geopotential anomaly due to tidal forces from astronomical sources [Z ~> m].
     e_tidal_sal, & ! The bottom geopotential anomaly due to harmonic self-attraction and loading
@@ -222,7 +225,6 @@ subroutine PressureForce_FV_nonBouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_
   real :: dp_neglect         ! A thickness that is so small it is usually lost
                              ! in roundoff and can be neglected [R L2 T-2 ~> Pa].
   real :: I_gEarth           ! The inverse of GV%g_Earth [T2 Z L-2 ~> s2 m-1]
-  real :: I_g_rho            ! The inverse of the density times the gravitational acceleration [Z T2 L-2 R-1 ~> m Pa-1]
   real :: alpha_anom         ! The in-situ specific volume, averaged over a
                              ! layer, less alpha_ref [R-1 ~> m3 kg-1].
   logical :: use_p_atm       ! If true, use the atmospheric pressure.
@@ -408,19 +410,19 @@ subroutine PressureForce_FV_nonBouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_
   ! Calculate and add self-attraction and loading (SAL) geopotential height anomaly to interface height.
   if (CS%calculate_SAL) then
     if (CS%sal_use_bpa) then
-      I_g_rho = 1.0 / (GV%rho0*GV%g_Earth)
       !$OMP parallel do default(shared)
       do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1
-        pbot_anom(i,j) = p(i,j,nz+1) * I_g_rho
+        pbot(i,j) = p(i,j,nz+1)
       enddo ; enddo
+      call calc_SAL(pbot, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
     else
       !$OMP parallel do default(shared)
       do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1
-        pbot_anom(i,j) = (za(i,j,1) - alpha_ref*p(i,j,1)) * I_gEarth - G%Z_ref &
+        SSH(i,j) = (za(i,j,1) - alpha_ref*p(i,j,1)) * I_gEarth - G%Z_ref &
                   - max(-G%bathyT(i,j)-G%Z_ref, 0.0)
       enddo ; enddo
+      call calc_SAL(SSH, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
     endif
-    call calc_SAL(pbot_anom, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
 
     ! This gives new answers after the change of separating SAL from tidal forcing module.
     if ((CS%tides_answer_date>20230630) .or. (.not.GV%semi_Boussinesq) .or. (.not.CS%tides)) then
@@ -873,7 +875,10 @@ subroutine PressureForce_FV_Bouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_atm
     e_tidal_sal, & ! The bottom geopotential anomaly due to harmonic self-attraction and loading
                   ! specific to tides [Z ~> m].
     Z_0p, &       ! The height at which the pressure used in the equation of state is 0 [Z ~> m]
-    pbot_anom, &  ! Bottom pressure (anomaly) in depth units, used for self-attraction and loading [Z ~> m].
+    SSH, &      ! Sea surfae height anomaly for self-attraction and loading. Used if
+                ! CALCULATE_SAL is True and SAL_USE_BPA is False [Z ~> m].
+    pbot, &     ! Total bottom pressure for self-attraction and loading. Used if
+                ! CALCULATE_SAL is True and SAL_USE_BPA is True [R L2 T-2 ~> Pa].
     dM          ! The barotropic adjustment to the Montgomery potential to
                 ! account for a reduced gravity model [L2 T-2 ~> m2 s-2].
   real, dimension(SZI_(G)) :: &
@@ -1018,7 +1023,6 @@ subroutine PressureForce_FV_Bouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_atm
   G_Rho0 = GV%g_Earth / GV%Rho0
   GxRho = GV%g_Earth * GV%Rho0
   rho_ref = CS%Rho0
-  I_g_rho = 1.0 / (CS%rho0*GV%g_Earth) ! I think it should be I_g_rho = 1.0 / (GV%rho0*GV%g_Earth)
 
   if ((CS%id_MassWt_u > 0) .or. (CS%id_MassWt_v > 0)) then
     MassWt_u(:,:,:) = 0.0 ; MassWt_v(:,:,:) = 0.0
@@ -1033,17 +1037,17 @@ subroutine PressureForce_FV_Bouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_atm
   ! The following code is for recovering old answers only. The algorithm moves interface heights before density
   ! calculations, and therefore is incorrect without SSH_IN_EOS_PRESSURE_FOR_PGF=True (added in August 2024).
   ! See the code right after Pa calculation loop for the new method.
-  if (CS%tides_answer_date<=20230630) then
+  if (CS%tides .and. CS%tides_answer_date<=20230630) then
     !$OMP parallel do default(shared)
     do j=Jsq,Jeq+1
       do i=Isq,Ieq+1
-        pbot_anom(i,j) = min(-G%bathyT(i,j) - G%Z_ref, 0.0) ! reference bottom pressure
+        SSH(i,j) = min(-G%bathyT(i,j) - G%Z_ref, 0.0) ! reference bottom pressure
       enddo
       do k=1,nz ; do i=Isq,Ieq+1
-        pbot_anom(i,j) = pbot_anom(i,j) + h(i,j,k)*GV%H_to_Z
+        SSH(i,j) = SSH(i,j) + h(i,j,k)*GV%H_to_Z
       enddo ; enddo
     enddo
-    call calc_SAL(pbot_anom, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
+    call calc_SAL(SSH, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
     call calc_tidal_forcing_legacy(CS%Time, e_sal, e_sal_and_tide, e_tidal_eq, e_tidal_sal, &
                                    G, US, CS%tides_CSp)
     !$OMP parallel do default(shared)
@@ -1118,6 +1122,7 @@ subroutine PressureForce_FV_Bouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_atm
   endif
 
   if (CS%use_SSH_in_Z0p .and. use_p_atm) then
+    I_g_rho = 1.0 / (CS%rho0*GV%g_Earth)
     do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1
       Z_0p(i,j) = e(i,j,1) + p_atm(i,j) * I_g_rho
     enddo ; enddo
@@ -1201,15 +1206,16 @@ subroutine PressureForce_FV_Bouss(h, tv, PFu, PFv, G, GV, US, CS, ALE_CSp, p_atm
     if (CS%sal_use_bpa) then
       !$OMP parallel do default(shared)
       do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1
-        pbot_anom(i,j) = pa(i,j,nz+1) * I_g_rho - e(i,j,nz+1)
+        pbot(i,j) = pa(i,j,nz+1) - GxRho * e(i,j,nz+1)
       enddo ; enddo
+      call calc_SAL(pbot, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
     else
       !$OMP parallel do default(shared)
       do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1
-        pbot_anom(i,j) = e(i,j,1) - max(-G%bathyT(i,j) - G%Z_ref, 0.0) ! Remove topography above sea level
+        SSH(i,j) = e(i,j,1) - max(-G%bathyT(i,j) - G%Z_ref, 0.0) ! Remove topography above sea level
       enddo ; enddo
+      call calc_SAL(SSH, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
     endif
-    call calc_SAL(pbot_anom, e_sal, G, CS%SAL_CSp, tmp_scale=US%Z_to_m)
     if (.not.CS%bq_sal_tides) then ; do K=1,nz+1
       !$OMP parallel do default(shared)
       do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1

src/parameterizations/lateral/MOM_self_attr_load.F90

@@ -34,8 +34,7 @@ module MOM_self_attr_load
   real :: eta_prop
     !< The partial derivative of eta_sal with the local value of eta [nondim].
   real :: linear_scaling
-    !< A dimensional coefficient for scalar approximation SAL, equal to eta_prop * unit_convert
-    !! [nondim or L2 Z-1 T-2 ~> m s-2 or R-1 ~> m3 kg-1 or L2 Z-1 T-2 R-1 ~> m4 s-2 kg-1]
+    !< Dimensional coefficients for scalar SAL [nondim or Z T2 L-2 R-1 ~> m Pa-1]
   type(sht_CS), allocatable :: sht
     !< Spherical harmonic transforms (SHT) control structure
   integer :: sal_sht_Nd
@@ -44,7 +43,7 @@ module MOM_self_attr_load
     !< Reference bottom pressure scaled by Rho_0 and G_Earth[Z ~> m]
   real, allocatable :: Love_scaling(:)
     !< Dimensional coefficients for harmonic SAL, which are functions of Love numbers
-    !! [nondim or L2 Z-1 T-2 ~> m s-2 or R-1 ~> m3 kg-1 or L2 Z-1 T-2 R-1 ~> m4 s-2 kg-1]
+    !! [nondim or Z T2 L-2 R-1 ~> m Pa-1]
   real, allocatable :: Snm_Re(:), &    !< Real SHT coefficient for SHT SAL [Z ~> m]
                        Snm_Im(:)       !< Imaginary SHT coefficient for SHT SAL [Z ~> m]
 end type SAL_CS
@@ -56,24 +55,22 @@ module MOM_self_attr_load
 !> This subroutine calculates seawater self-attraction and loading based on either sea surface height (SSH) or bottom
 !! pressure anomaly.  Note that the SAL calculation applies to all motions across the spectrum. Tidal-specific methods
 !! that assume periodicity, i.e. iterative and read-in SAL, are stored in MOM_tidal_forcing module.
-!! The input field is always assume to have the unit of [Z ~> m], which can be either SSH or total bottom pressure
-!! scaled by mean seawater density Rho_0 and earth gravity G_Earth. For spherical harmonics, the mean seawater density
-!! would be cancelled by the same parameter in Love number scalings. If total bottom pressure is used as input, bottom
-!! pressure anomaly is calculated in the subroutine by subtracting a reference pressure from the input bottom pressure.
+!! The input field can be either SSH [Z ~> m] or total bottom pressure [R L2 T-2 ~> Pa].  If total bottom pressure is
+!! used, bottom pressure anomaly is first calculated by subtracting a reference bottom pressure from an input file.
 !! The output field is expressed as geopotential height anomaly, and therefore has the unit of [Z ~> m].
 subroutine calc_SAL(eta, eta_sal, G, CS, tmp_scale)
-  type(ocean_grid_type), intent(in)  :: G  !< The ocean's grid structure.
+  type(ocean_grid_type), intent(in) :: G  !< The ocean's grid structure.
   real, dimension(SZI_(G),SZJ_(G)), intent(in)  :: eta     !< The sea surface height anomaly from
-              !! a time-mean geoid or total bottom pressure scaled by mean density and earth gravity [Z ~> m].
+              !! a time-mean geoid or total bottom pressure [Z ~> m] or [R L2 T-2 ~> Pa].
   real, dimension(SZI_(G),SZJ_(G)), intent(out) :: eta_sal !< The geopotential height anomaly from
               !! self-attraction and loading [Z ~> m].
   type(SAL_CS), intent(inout) :: CS !< The control structure returned by a previous call to SAL_init.
   real, optional, intent(in)  :: tmp_scale !< A rescaling factor to temporarily convert eta
               !! to MKS units in reproducing sumes [m Z-1 ~> 1]
 
   ! Local variables
+  real, dimension(SZI_(G),SZJ_(G)) :: bpa ! SSH or bottom pressure anomaly [Z ~> m or R L2 T-2 ~> Pa]
   integer :: n, m, l
-  real, dimension(SZI_(G),SZJ_(G)) :: bpa ! [Z ~> m]
   integer :: Isq, Ieq, Jsq, Jeq
   integer :: i, j
 
@@ -90,7 +87,7 @@ subroutine calc_SAL(eta, eta_sal, G, CS, tmp_scale)
   ! use the scalar approximation and/or iterative tidal SAL
   if (CS%use_sal_scalar .or. CS%use_tidal_sal_prev) then
     do j=Jsq,Jeq+1 ; do i=Isq,Ieq+1
-      eta_sal(i,j) = CS%eta_prop * bpa(i,j)
+      eta_sal(i,j) = CS%linear_scaling * bpa(i,j)
     enddo ; enddo
 
   ! use the spherical harmonics method
@@ -132,21 +129,21 @@ end subroutine scalar_SAL_sensitivity
 !> This subroutine calculates coefficients of the spherical harmonic modes for self-attraction and loading.
 !! The algorithm is based on the SAL implementation in MPAS-ocean, which was modified by Kristin Barton from
 !! routine written by K. Quinn (March 2010) and modified by M. Schindelegger (May 2017).
-subroutine calc_love_scaling(nlm, rhoW, rhoE, Love_scaling)
-  integer, intent(in) :: nlm  !< Maximum spherical harmonics degree [nondim]
-  real,    intent(in) :: rhoW !< The average density of sea water [R ~> kg m-3]
-  real,    intent(in) :: rhoE !< The average density of Earth [R ~> kg m-3]
-  real, dimension(:), intent(out) :: Love_scaling !< Scaling factors for inverse spherical harmonic
-    !! transforms [nondim]
+subroutine calc_love_scaling(rhoW, rhoE, grav, CS)
+  real, intent(in) :: rhoW !< The average density of sea water [R ~> kg m-3]
+  real, intent(in) :: rhoE !< The average density of Earth [R ~> kg m-3]
+  real, intent(in) :: grav !< The gravitational acceleration [L2 Z-1 T-2 ~> m s-2]
+  type(SAL_CS), intent(inout) :: CS !< The control structure returned by a previous call to SAL_init.
 
   ! Local variables
   real, dimension(:), allocatable :: HDat, LDat, KDat ! Love numbers converted in CF reference frames [nondim]
   real :: H1, L1, K1 ! Temporary variables to store degree 1 Love numbers [nondim]
-  integer :: n_tot ! Size of the stored Love numbers
+  integer :: n_tot ! Size of the stored Love numbers [nondim]
+  integer :: nlm  ! Maximum spherical harmonics degree [nondim]
   integer :: n, m, l
-  real :: unit
 
   n_tot = size(Love_Data, dim=2)
+  nlm = CS%sal_sht_Nd
 
   if (nlm+1 > n_tot) call MOM_error(FATAL, "MOM_tidal_forcing " // &
     "calc_love_scaling: maximum spherical harmonics degree is larger than " // &
@@ -165,7 +162,11 @@ subroutine calc_love_scaling(nlm, rhoW, rhoE, Love_scaling)
 
   do m=0,nlm ; do n=m,nlm
     l = order2index(m,nlm)
-    Love_scaling(l+n-m) = (3.0 / real(2*n+1)) * (rhoW / rhoE) * (1.0 + KDat(n+1) - HDat(n+1))
+    if (CS%use_bpa) then
+      CS%Love_scaling(l+n-m) = (3.0 / real(2*n+1)) * (1.0 / (rhoE * grav)) * (1.0 + KDat(n+1) - HDat(n+1))
+    else
+      CS%Love_scaling(l+n-m) = (3.0 / real(2*n+1)) * (rhoW / rhoE) * (1.0 + KDat(n+1) - HDat(n+1))
+    endif
   enddo ; enddo
 end subroutine calc_love_scaling
 
@@ -184,14 +185,12 @@ subroutine SAL_init(G, GV, US, param_file, CS)
   real :: rhoE    ! The average density of Earth [R ~> kg m-3].
   character(len=200) :: filename, ebot_ref_file, inputdir ! Strings for file/path
   character(len=200) :: ebot_ref_varname                  ! Variable name in file
-  real :: I_g_rho  ! The inverse of the density times the gravitational acceleration [Z T2 L-2 R-1 ~> m Pa-1]
   logical :: calculate_sal=.false.
   logical :: tides=.false., use_tidal_sal_file=.false., bq_sal_tides_bug=.false.
   integer :: tides_answer_date=99991203 ! Recover old answers with tides
   real :: sal_scalar_value, tide_sal_scalar_value ! Scaling SAL factors [nondim]
-
   integer :: isd, ied, jsd, jed
-  integer :: i, j
+
   isd = G%isd ; ied = G%ied ; jsd = G%jsd ; jed = G%jed
 
   ! Read all relevant parameters and write them to the model log.
@@ -230,10 +229,6 @@ subroutine SAL_init(G, GV, US, param_file, CS)
     allocate(CS%ebot_ref(isd:ied, jsd:jed), source=0.0)
     call MOM_read_data(filename, trim(ebot_ref_varname), CS%ebot_ref, G%Domain,&
                        scale=US%Pa_to_RL2_T2)
-    I_g_rho = 1.0 / (GV%g_Earth * GV%Rho0)
-    do j=jsd,jed ; do i=isd,ied
-      CS%ebot_ref(i,j) = CS%ebot_ref(i,j) * I_g_rho
-    enddo ; enddo
     call pass_var(CS%ebot_ref, G%Domain)
   endif
   if (tides_answer_date<=20230630 .and. CS%use_bpa) &
@@ -242,9 +237,9 @@ subroutine SAL_init(G, GV, US, param_file, CS)
   call get_param(param_file, mdl, "SAL_SCALAR_APPROX", CS%use_sal_scalar, &
                  "If true, use the scalar approximation to calculate self-attraction and "//&
                  "loading.", default=tides .and. (.not. use_tidal_sal_file))
-  ! if (CS%use_sal_scalar .and. CS%use_bpa) &
-  !   call MOM_error(WARNING, trim(mdl) // ", SAL_init: Using bottom pressure anomaly for scalar "//&
-  !                  "approximation SAL is unsubstantiated.")
+  if (CS%use_sal_scalar .and. CS%use_bpa) &
+    call MOM_error(WARNING, trim(mdl) // ", SAL_init: Using bottom pressure anomaly for scalar "//&
+                   "approximation SAL is unsubstantiated.")
   call get_param(param_file, '', "TIDE_SAL_SCALAR_VALUE", tide_sal_scalar_value, &
                  units="m m-1", default=0.0, do_not_log=.True.)
   if (tide_sal_scalar_value/=0.0) &
@@ -270,22 +265,29 @@ subroutine SAL_init(G, GV, US, param_file, CS)
                  default=5517.0, scale=US%kg_m3_to_R, do_not_log=(.not. CS%use_sal_sht))
 
   ! Set scaling coefficients for scalar approximation
-  if (CS%use_sal_scalar .and. CS%use_tidal_sal_prev) then
-    CS%eta_prop = 2.0 * sal_scalar_value
-  elseif (CS%use_sal_scalar .or. CS%use_tidal_sal_prev) then
-    CS%eta_prop = sal_scalar_value
+  if (CS%use_sal_scalar .or. CS%use_tidal_sal_prev) then
+    if (CS%use_sal_scalar .and. CS%use_tidal_sal_prev) then
+      CS%eta_prop = 2.0 * sal_scalar_value
+    else
+      CS%eta_prop = sal_scalar_value
+    endif
+    if (CS%use_bpa) then
+      CS%linear_scaling = CS%eta_prop / (GV%Rho0 * GV%g_Earth)
+    else
+      CS%linear_scaling = CS%eta_prop
+    endif
   else
-    CS%eta_prop = 0.0
+    CS%eta_prop = 0.0 ; CS%linear_scaling = 0.0
   endif
 
   ! Set scaling coefficients for spherical harmonics
   if (CS%use_sal_sht) then
     lmax = calc_lmax(CS%sal_sht_Nd)
-    allocate(CS%Snm_Re(lmax)); CS%Snm_Re(:) = 0.0
-    allocate(CS%Snm_Im(lmax)); CS%Snm_Im(:) = 0.0
+    allocate(CS%Snm_Re(lmax), source=0.0)
+    allocate(CS%Snm_Im(lmax), source=0.0)
 
-    allocate(CS%Love_scaling(lmax)); CS%Love_scaling(:) = 0.0
-    call calc_love_scaling(CS%sal_sht_Nd, GV%Rho0, rhoE, CS%Love_scaling)
+    allocate(CS%Love_scaling(lmax), source=0.0)
+    call calc_love_scaling(GV%Rho0, rhoE, GV%g_Earth, CS)
 
     allocate(CS%sht)
     call spherical_harmonics_init(G, param_file, CS%sht)

src/parameterizations/lateral/MOM_spherical_harmonics.F90

@@ -1,12 +1,12 @@
 !> Laplace's spherical harmonic transforms (SHT)
 module MOM_spherical_harmonics
+use MOM_coms_infra,    only : sum_across_PEs
+use MOM_coms,          only : reproducing_sum
 use MOM_cpu_clock,     only : cpu_clock_id, cpu_clock_begin, cpu_clock_end, &
                               CLOCK_MODULE, CLOCK_ROUTINE, CLOCK_LOOP
 use MOM_error_handler, only : MOM_error, FATAL
 use MOM_file_parser,   only : get_param, log_version, param_file_type
 use MOM_grid,          only : ocean_grid_type
-use MOM_coms_infra,    only : sum_across_PEs
-use MOM_coms,          only : reproducing_sum
 
 implicit none ; private