ice_dyn_vp: improve default parameters for VP solver

During QC testing of the previous commit, the 5 years QC test with the
updated VP solver failed twice with "bad departure points" after a few
years of simulation. Simply bumping the number of nonlinear iterations
(maxits_nonlin) from 4 to 5 makes these failures disappear and allow the
simulations to run to completion, suggesting the solution is not
converged enough with 4 iterations.

We also noticed that in these failing cases, the relative tolerance for
the linear solver (reltol_fmgres = 1E-2) is too small to be reached in
less than 50 iterations (maxits_fgmres), and that's the case at each
nonlinear iteration. Other papers mention a relative tolerance of 1E-1
for the linear solver, and using this value also allows both cases to
run to completion (even without changing maxits_nonlin).

Let's set the default tolerance for the linear solver to 1E-1, and let's
be conservative and bump the number of nonlinear iterations to 10. This
should give us a more converged solution and add robustness to the
default settings.

cicecore/cicedynB/general/ice_init.F90

@@ -419,7 +419,7 @@ subroutine input_data
       deltaminEVP = 1e-11_dbl_kind ! minimum delta for viscosities (EVP, Hunke 2001)
       deltaminVP  = 2e-9_dbl_kind  ! minimum delta for viscosities (VP, Hibler 1979)
       capping_method  = 'max'  ! method for capping of viscosities (max=Hibler 1979,sum=Kreyscher2000)
-      maxits_nonlin = 4        ! max nb of iteration for nonlinear solver
+      maxits_nonlin = 10       ! max nb of iteration for nonlinear solver
       precond = 'pgmres'       ! preconditioner for fgmres: 'ident' (identity), 'diag' (diagonal),
                                ! 'pgmres' (Jacobi-preconditioned GMRES)
       dim_fgmres = 50          ! size of fgmres Krylov subspace
@@ -431,7 +431,7 @@ subroutine input_data
       monitor_pgmres = .false. ! print pgmres residual norm
       ortho_type = 'mgs'       ! orthogonalization procedure 'cgs' or 'mgs'
       reltol_nonlin = 1e-8_dbl_kind ! nonlinear stopping criterion: reltol_nonlin*res(k=0)
-      reltol_fgmres = 1e-2_dbl_kind ! fgmres stopping criterion: reltol_fgmres*res(k)
+      reltol_fgmres = 1e-1_dbl_kind ! fgmres stopping criterion: reltol_fgmres*res(k)
       reltol_pgmres = 1e-6_dbl_kind ! pgmres stopping criterion: reltol_pgmres*res(k)
       algo_nonlin = 'picard'        ! nonlinear algorithm: 'picard' (Picard iteration), 'anderson' (Anderson acceleration)
       fpfunc_andacc = 1        ! fixed point function for Anderson acceleration:

configuration/scripts/ice_in

@@ -167,7 +167,7 @@
     kridge          = 1
     ktransport      = 1
     ssh_stress      = 'geostrophic'
-    maxits_nonlin   = 4
+    maxits_nonlin   = 10
     precond         = 'pgmres'
     dim_fgmres       = 50
     dim_pgmres       = 5
@@ -178,7 +178,7 @@
     monitor_pgmres  = .false.
     ortho_type      = 'mgs'
     reltol_nonlin   = 1e-8
-    reltol_fgmres   = 1e-2
+    reltol_fgmres   = 1e-1
     reltol_pgmres   = 1e-6
     algo_nonlin     = 'picard'
     use_mean_vrel   = .true.

doc/source/user_guide/ug_case_settings.rst

@@ -469,7 +469,7 @@ dynamics_nml
    "``k1``", "real", "1st free parameter for landfast parameterization", "7.5"
    "``k2``", "real", "2nd free parameter (N/m\ :math:`^3`) for landfast parameterization", "15.0"
    "``maxits_fgmres``", "integer", "maximum number of restarts for FGMRES solver", "1"
-   "``maxits_nonlin``", "integer", "maximum number of nonlinear iterations for VP solver", "4"
+   "``maxits_nonlin``", "integer", "maximum number of nonlinear iterations for VP solver", "10"
    "``maxits_pgmres``", "integer", "maximum number of restarts for PGMRES preconditioner", "1"
    "``monitor_fgmres``", "logical", "write velocity norm at each FGMRES iteration", "``.false.``"
    "``monitor_nonlin``", "logical", "write velocity norm at each nonlinear iteration", "``.false.``"
@@ -482,7 +482,7 @@ dynamics_nml
    "", "``ident``", "Don't use a preconditioner for the FGMRES solver", ""
    "", "``pgmres``", "Use GMRES as preconditioner for FGMRES solver", ""
    "``Pstar``", "real", "constant in Hibler strength formula (N/m\ :math:`^2`)", "2.75e4"
-   "``reltol_fgmres``", "real", "relative tolerance for FGMRES solver", "1e-2"
+   "``reltol_fgmres``", "real", "relative tolerance for FGMRES solver", "1e-1"
    "``reltol_nonlin``", "real", "relative tolerance for nonlinear solver", "1e-8"
    "``reltol_pgmres``", "real", "relative tolerance for PGMRES preconditioner", "1e-6"
    "``revised_evp``", "logical", "use revised EVP formulation", "``.false.``"