# Difference between revisions of "Adding dH/dt module"

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This page contains step-by-step instructions for adding a new module to Glimmer-CISM. In this case, the module is a first-order, upwinding advection scheme for mass transport (dH/dt) using velocities calculated from a higher-order dynamics model. The procedure, however, is generic and could apply to adding almost any module. The goal here is not to get lost in the details of the upwinding scheme (we'll provide the code for that later) but rather to understand the steps for adding a module in an incremental, structured way that allows you to track small errors and fix them at each step.

## Step 1

In step 1, we will create modules containing "empty" subroutines and add these modules to the build.

First, create a module for_upwind_advect_mod.F90 that contains the necessary subroutines. For now, these will just be "stubs", which we will fill in later.

module fo_upwind_advect_mod

! subroutines for mass advection scheme based on 1st order upwinding

contains

!----------------------------------------------------------------------

subroutine fo_upwind_advect_init( )

! initialization for 1st-order upwinding mass advection

end subroutine fo_upwind_advect_init

!----------------------------------------------------------------------

subroutine fo_upwind_advect_final( )

! finalization for 1st-order upwinding mass advection

end subroutine fo_upwind_advect_final

!----------------------------------------------------------------------

subroutine fo_upwind_advect_main( )

! 1st-order upwinding mass advection

end subroutine fo_upwind_advect_main

!----------------------------------------------------------------------

end module fo_upwind_advect_mod

Now create another module for_upwind_advect.F90 that will contain the "driver" subroutine - that which is called from the main code, which in this case is glide.F90 ...

module fo_upwind_advect

! this is the driver routine for the 1st order, upwind mass transport scheme

use fo_upwind_advect_mod, only : fo_upwind_advect_main

implicit none
private
public :: fo_upwind_advect_driver

contains

subroutine fo_upwind_advect_driver( )

call fo_upwind_advect_main( )

end subroutine fo_upwind_advect_driver

end module fo_upwind_advect

NOTE: This may seem like extra work ... why don't we just call the subroutine fo_upwind_advect_main directly from the main code? In this case, perhaps this is true. In general, however, having a "driver" routine is good practice. For example, there might be additional pre/post processing of variables that needs to take place before and after the call to the subroutine fo_upwind_advect_main. Making space for that here rather than in the main program, where only the driver is called, makes more sense and keeps the main code cleaner and easier to understand.

Add these new modules to the build by altering the file Makefile.am ...

libglide_a_SOURCES = glide.F90 glide_setup.F90 glide_types.F90 glide_temp.F90 \
glide_bwater.F90 glide_deriv.F90 xls.F90 ice3d_lib.F90 \
glide_velo_higher.F90 glide_thck.F90 glide_velo.F90 \
glide_mask.F90 glide_stop.F90 glide_io.F90 \
glide_nc_custom.F90 isostasy.F90 isostasy_el.F90\
isostasy_setup.F90 isostasy_types.F90 glide_lithot.F90\
glide_lithot3d.F90 glide_lithot1d.F90 glide_profile.F90\
glide_diagnostics.F90 glissade.F90 glissade_remap.F90\
glissade_velo.F90 glissade_constants.F90 glide_vertint.F90\
glide_thckmask.F90 glide_nonlin.F90 glide_grids.F90\
glam.F90 glam_strs2.F90 glam_thck_ppm.F90\
remap_advection.F90 remap_glamutils.F90 glide_ground.F90

becomes ...

libglide_a_SOURCES = glide.F90 glide_setup.F90 glide_types.F90 glide_temp.F90 \
glide_bwater.F90 glide_deriv.F90 xls.F90 ice3d_lib.F90 \
glide_velo_higher.F90 glide_thck.F90 glide_velo.F90 \
glide_mask.F90 glide_stop.F90 glide_io.F90 \
glide_nc_custom.F90 isostasy.F90 isostasy_el.F90\
isostasy_setup.F90 isostasy_types.F90 glide_lithot.F90\
glide_lithot3d.F90 glide_lithot1d.F90 glide_profile.F90\
glide_diagnostics.F90 glissade.F90 glissade_remap.F90\
glissade_velo.F90 glissade_constants.F90 glide_vertint.F90\
glide_thckmask.F90 glide_nonlin.F90 glide_grids.F90\
glam.F90 glam_strs2.F90 glam_thck_ppm.F90\
remap_advection.F90 remap_glamutils.F90 glide_ground.F90 \     ! CHANGED LAST TWO LINES
fo_upwind_advect_mod.F90, fo_upwind_advect.F90

Now we stop adding/editing things for a minute and type make to rebuild and make sure we haven't broken the code yet. If you get a successful build, then proceed to Step 2.

## Step 2

In step 2, we will do some minor updates to other parts of the code so that there is an option to call our 1st order upwinding scheme when we want to evolve the ice thickness (that is, rather than using some other scheme).

In glide_types.F90, add the following to the transport scheme options around line 100 ...

  integer, parameter :: EVOL_PSEUDO_DIFF = 0
integer, parameter :: EVOL_ADI = 1
integer, parameter :: EVOL_DIFFUSION = 2
integer, parameter :: EVOL_INC_REMAP = 3
integer, parameter :: EVOL_FO_UPWIND = 4         ! ADDED THIS LINE

... similarly, update some lines around 207 for documentation ...

    !*FD Thickness evolution method:
!*FD \begin{description}
!*FD \item[0] Pseudo-diffusion approach
!*FD \item[2] Diffusion approach (also calculates velocities)
!*FD \item[3] Incremental remapping
!*FD \item[4] 1st-order upwinding scheme       ! ADDED THIS LINE
!*FD \end{description}

In glide_setup.F90, around line 475, change ...

    character(len=*), dimension(0:4), parameter :: evolution = (/ &    ! CHANGED 0:3 TO 0:4
'pseudo-diffusion                      ', &
'ADI scheme                            ', &
'iterated diffusion                    ', &
'remap thickness                       ', &
'1st order upwind                      ' /) 			! ADDED THIS LINE

Time to type make again and check for a successful build. If so, proceed to Step 3.

## Step 3

Now we will add use statements, if constructs, and dummy calls to the advection subroutines from the appropriate places in the code. Hence, we are setting up all the necessary structure to call the subroutines but are not passing any arguments to them yet (nor are they actually doing anything yet).

In 'glide.F90, add the following use statement to subroutine glide_initialise ...

    use glam_strs2, only : glam_velo_fordsiapstr_init
use remap_glamutils, only : horizontal_remap_init
use fo_upwind_advect_mod, only : fo_upwind_advect_init         ! ADDED

Now add if construct and call to the initialization stub in fo_upwind_advect_mod.F90, around line 190 of glide.F90, just after the if construct for EVOL_INC_REMAP ...

    if (model%options%whichevol== EVOL_FO_UPWIND ) then
call fo_upwind_advect_init( )
end if

In glide_stop.F90, add the following to the use statements ...

module glide_stop

use glide_types
use glimmer_log
use remap_glamutils
use fo_upwind_advect_mod, only : fo_upwind_advect_final       ! ADDED

Add the necessary if construct to glide_stop.F90 around line 150, as we did with the initialization routine ...

    if (model%options%whichevol== EVOL_FO_UPWIND ) then
call fo_upwind_advectfinal( )
endif

Finally, update the use statements in glide.F90 around line 340, and the case construct for ice sheet evolution, around line 380, so that the 1st order upwinding subroutines can be called ...

    use glide_thck
use glide_velo
use glide_ground
use glide_setup
use glide_temp
use glide_mask
use isostasy
use glam, only: inc_remap_driver
use fo_upwind_advect, only: fo_upwind_advect_driver      ! ADDED

... and add a call to the driver routine to the case construct ...

    case(EVOL_FO_UPWIND) ! Use first order upwind scheme for mass transport

call fo_upwind_advect_driver( model )

end select

Make sure to place it BEFORE the "end select" statement!

Type make again and check for a successful build. If so, proceed to Step 4.

## Step 4

At this point we've done nothing but build the necessary structures in the code. Now we will start filling in the subroutines with the necessary variable definitions. We'll follow this by passing arguments and, if everything still works, we can actually start to do something w/ those arguments within the subroutines themselves.

Note that, in practice, one doesn't always know ahead of time exactly what variables are needed within a particular subroutine, and there is some trial and error while figuring it out. Here, we'll assume we've thought this out really well ahead of time and we know exactly what we need to do before we ever started writing the code (like punch-card programmers of old), in which case we know exactly which arguments are needed when we start.

First, add argument definitions to the main subroutine in fo_upwind_advect_mode.F90 ...

Then, pass these same arguements from the driver routine fo_upwind_advect_driver in fo_upwind_advect.F90. Note that this requires accessing the variables from the derived type "model" ...

Finally, pass the derived type "model" during the call to the driver subroutine in glide.F90 ...