6.5. Ice-thickness evolution
Several options are available for the evolution of the ice thickness, selected by the run-specs-header parameter THK_EVOL
:
0
: No evolution of the ice thickness, kept fixed on the initial thickness.1
: Free evolution of the ice thickness.2
: Evolution of the ice thickness, but the ice topography (zs, zb, zl, H) is nugded towards a prescribed target with a time-dependent relaxation time read from the fileTARGET_TOPO_TAU0_FILE
(to be located insico_in/general
).3
: Evolution of the ice thickness, but the ice topography (zs, zb, zl, H) is nugded towards a prescribed target with the constant relaxation timeTARGET_TOPO_TAU0
.
For the cases THK_EVOL >= 1
, the ice-thickness equation
is solved, where \(H\) is the ice thickness, \((Q_x,Q_y)\) the volume flux (depth-integrated horizontal velocity), \(a_\mathrm{s}\) the surface mass balance (SMB) and \(a_\mathrm{b}\) the basal mass balance (BMB). Note that SMB and BMB are consistently counted as positive for a volume gain and negative for a volume loss.
For the cases THK_EVOL = 2, 3
, nudging of the ice topography towards a prescribed target topography (e.g., a slightly smoothed present-day topography) is employed. During each time step, after solving the ice-thickness equation, the relaxation equations
are applied, where \(h\) is the surface topography, \(b\) the ice-base topography (\(H=h-b\)), \(z_\mathrm{l}\) the lithosphere-surface (bedrock/seabed) topography, \((\cdot)_\mathrm{target}\) the respective nudging target and \(\tau\) the relaxation time (Rueckamp et al. [54]). The nudging target is specified by the parameter TARGET_TOPO_DAT_NAME
, to be assigned a text string with the file name (typically a time-slice output file from a previous simulation, see Section “Output files”). In addition, the path where this file is located must be given as an option -t /path/to/nudging/target/directory
to the script sico.sh
when running SICOPOLIS (see Section “How to run a simulation”).
Nudging is equivalent to applying an SMB correction, which is diagnosed by the model.
For the cases THK_EVOL >= 1
, the maximum ice extent can be constrained spatially by a prescribed mask file, specified by the parameter MASK_MAXEXTENT_FILE
. This file, in either NetCDF (*.nc
, recommended) or ASCII (any other file extension) format, is to be located in sico_in/ant
for Antarctica, sico_in/grl
for Greenland, etc. The mask must be a 2D integer array that matches the horizontal grid, with values 1
for grid points allowed to glaciate, and 0
for grid points not allowed to glaciate. For NetCDF, the required variable name of the mask is mask_maxextent
. If MASK_MAXEXTENT_FILE
is set to 'none'
or undefined, no spatial constraint is applied.
The numerical scheme for solving the ice-thickness equation can be chosen by the parameter CALCTHK
:
1
: Explicit scheme for the diffusive SIA ice-surface equation.
2
: Over-implicit scheme for the diffusive SIA ice-surface equation, iterative built-in SOR solver.
4
: Explicit scheme for the general ice-thickness equation.
For more details on the cases CALCTHK = 1, 2
, see Greve and Calov [33]. However, these cases are only applicable for SIA dynamics (see “Ice dynamics”), and they do not preserve mass well on complex bed topographies. Therefore, for most real-world problems, CALCTHK = 4
is the recommended setting.