8.5. Activating variables

There are some variables in the SICOPOLIS code, for example c_slide, the basal sliding coefficient, that are constant in time and reinitialized at each time step in the subroutines in src/subroutines/general/calc_vxy_m.F90. This reinitialization has an “erasing” effect on the the dependency of cost functions to such variables, resulting in identically zero adjoint fields. From a physics standpoint, it is pretty clear that c_slide should have affect various kinds of cost functions.

The workaround to this issue is to complete the following steps (the code snippets are based on the c_slide example):

  1. Declare a dummy variable in src/subroutines/general/sico_variables_m.F90.

real(dp), dimension(0:JMAX,0:IMAX) :: xxc_slide

  1. Initialize the dummy variable to 0. at the end of src/subroutines/{domain}/sico_init_m.F90. Here {domain} can either be grl or ant.

xxc_slide = 0.

  1. Add the dummy variable to the actual variable. In the case of c_slide, add it at the point where it’s initialization is complete.

c_slide = c_slide + xxc_slide

  1. Evaluate sensitivities to the dummy variable i.e. the {ind_vars} should contain xxc_slide for a non-trivial senstivity with respect to c_slide.

This procedure has the effect of continuity of the variable xxc_slide in time, preventing the “erasing” effect caused by the reinitialization of c_slide to the same value for each time step. Also, since xxc_slide is initialized to 0, the results in the normal simulations are unaffected, because for the purposes of the non-linear forward model, all we did was add zeros to c_slide.