Add runtime parameter SET_DTBT_USE_BT_CONT

The runtime parameter allows to use BT_CONT optional input argument in
subroutine set_dtbt if BT_CONT type is available.

Originally, when BT_CONT is used, set_dtbt is estimated with zero
surface height. The eta optinoal input argument has no effect since it
only works with NONLINEAR_BT_CONTINUITY is true, which is by default
false when BT_CONT is used.

The added option allows accurate estimate of dtbt when the mean surface
height of the domain is not at the same level, while maintains old
answers.

Also, an unused field calc_dtbt in type MOM_dyn_split_RK2_CS is removed.

Author: herrwang0

src/core/MOM_dynamics_split_RK2.F90

@@ -154,8 +154,7 @@ module MOM_dynamics_split_RK2
   logical :: split_bottom_stress  !< If true, provide the bottom stress
                                   !! calculated by the vertical viscosity to the
                                   !! barotropic solver.
-  logical :: calc_dtbt            !< If true, calculate the barotropic time-step
-                                  !! dynamically.
+  logical :: dtbt_use_bt_cont     !< If true, use BT_cont to calculate DTBT.
   logical :: store_CAu            !< If true, store the Coriolis and advective accelerations at the
                                   !! end of the timestep for use in the next predictor step.
   logical :: CAu_pred_stored      !< If true, the Coriolis and advective accelerations at the
@@ -648,7 +647,15 @@ subroutine step_MOM_dyn_split_RK2(u_inst, v_inst, h, tv, visc, Time_local, dt, f
   endif
 
   call cpu_clock_begin(id_clock_btstep)
-  if (calc_dtbt) call set_dtbt(G, GV, US, CS%barotropic_CSp, CS%pbce, eta=eta)
+  if (calc_dtbt) then
+    if (CS%dtbt_use_bt_cont .and. associated(CS%BT_cont)) then
+      call set_dtbt(G, GV, US, CS%barotropic_CSp, CS%pbce, BT_cont=CS%BT_cont)
+    else
+      ! In the following call, eta is only used when NONLINEAR_BT_CONTINUITY is True. Otherwise, dtbt is effectively
+      ! calculated with eta=0. Note that NONLINEAR_BT_CONTINUITY is False if BT_CONT is used, which is the default.
+      call set_dtbt(G, GV, US, CS%barotropic_CSp, CS%pbce, eta=eta)
+    endif
+  endif
   if (showCallTree) call callTree_enter("btstep(), MOM_barotropic.F90")
   ! This is the predictor step call to btstep.
   ! The CS%ADp argument here stores the weights for certain integrated diagnostics.
@@ -1410,7 +1417,8 @@ subroutine initialize_dyn_split_RK2(u, v, h, tv, uh, vh, eta, Time, G, GV, US, p
                  "If SPLIT is false and USE_RK2 is true, BEGW can be "//&
                  "between 0 and 0.5 to damp gravity waves.", &
                  units="nondim", default=0.0)
-
+  call get_param(param_file, mdl, "SET_DTBT_USE_BT_CONT", CS%dtbt_use_bt_cont, &
+                 "If true, use BT_CONT to calculate DTBT if possible.", default=.false.)
   call get_param(param_file, mdl, "SPLIT_BOTTOM_STRESS", CS%split_bottom_stress, &
                  "If true, provide the bottom stress calculated by the "//&
                  "vertical viscosity to the barotropic solver.", default=.false.)

src/core/MOM_dynamics_split_RK2b.F90

@@ -157,8 +157,7 @@ module MOM_dynamics_split_RK2b
   logical :: split_bottom_stress  !< If true, provide the bottom stress
                                   !! calculated by the vertical viscosity to the
                                   !! barotropic solver.
-  logical :: calc_dtbt            !< If true, calculate the barotropic time-step
-                                  !! dynamically.
+  logical :: dtbt_use_bt_cont     !< If true, use BT_cont to calculate DTBT.
   logical :: calculate_SAL        !< If true, calculate self-attraction and loading.
   logical :: use_tides            !< If true, tidal forcing is enabled.
   logical :: remap_aux            !< If true, apply ALE remapping to all of the auxiliary 3-D
@@ -672,7 +671,15 @@ subroutine step_MOM_dyn_split_RK2b(u_av, v_av, h, tv, visc, Time_local, dt, forc
   uh_ptr => uh_in ; vh_ptr => vh_in ; u_ptr => u_inst ; v_ptr => v_inst
 
   call cpu_clock_begin(id_clock_btstep)
-  if (calc_dtbt) call set_dtbt(G, GV, US, CS%barotropic_CSp, CS%pbce, eta=eta)
+  if (calc_dtbt) then
+    if (CS%dtbt_use_bt_cont .and. associated(CS%BT_cont)) then
+      call set_dtbt(G, GV, US, CS%barotropic_CSp, CS%pbce, BT_cont=CS%BT_cont)
+    else
+      ! In the following call, eta is only used when NONLINEAR_BT_CONTINUITY is True. Otherwise, dtbt is effectively
+      ! calculated with eta=0. Note that NONLINEAR_BT_CONTINUITY is False if BT_CONT is used, which is the default.
+      call set_dtbt(G, GV, US, CS%barotropic_CSp, CS%pbce, eta=eta)
+    endif
+  endif
   if (showCallTree) call callTree_enter("btstep(), MOM_barotropic.F90")
   ! This is the predictor step call to btstep.
   ! The CS%ADp argument here stores the weights for certain integrated diagnostics.
@@ -1335,7 +1342,8 @@ subroutine initialize_dyn_split_RK2b(u, v, h, tv, uh, vh, eta, Time, G, GV, US,
                  "If SPLIT is false and USE_RK2 is true, BEGW can be "//&
                  "between 0 and 0.5 to damp gravity waves.", &
                  units="nondim", default=0.0)
-
+  call get_param(param_file, mdl, "SET_DTBT_USE_BT_CONT", CS%dtbt_use_bt_cont, &
+                 "If true, use BT_CONT to calculate DTBT if possible.", default=.false.)
   call get_param(param_file, mdl, "SPLIT_BOTTOM_STRESS", CS%split_bottom_stress, &
                  "If true, provide the bottom stress calculated by the "//&
                  "vertical viscosity to the barotropic solver.", default=.false.)