6.8. Subglacial processes

6.8.1. Basal sliding

Basal sliding is implemented by a Weertman-Budd-type sliding law (Weertman [51], Budd et al. [7], Budd and Jenssen [6]). Three variants are available, to be chosen in the run-specs header by the parameter SLIDE_LAW:

  • 1: Weertman-Budd-type sliding, full ice pressure in denominator.

  • 2: Weertman-Budd-type sliding, reduced pressure (ice minus water) in denominator, limiter RED_PRES_LIMIT_FACT applied for SIA and SStA.

  • 3: Weertman-Budd-type sliding, reduced pressure (ice minus water) in denominator, limiter RED_PRES_LIMIT_FACT applied for SIA only.

Sub-melt sliding (Sato and Greve [47]), water-film-enhanced sliding (requires BASAL_HYDROLOGY = 1, see “Basal hydrology” below) and regionally varying sliding parameters can be added. The detailed settings are controlled by additional parameters as described in the run-specs headers.

6.8.2. Geothermal heat flux

The geothermal heat flux (GHF), assumed to be time-independent, can be specified in the run-specs headers as either a constant value or a spatially varying distribution via the parameters Q_GEO and Q_GEO_FILE:

  • If Q_GEO_FILE = 'none' (or undefined): Constant GHF defined by parameter Q_GEO.

  • Otherwise: Spatially varying GHF read from file specified by Q_GEO_FILE.

If a file with gridded data is provided (second case), it must match the chosen horizontal grid (see “Spatial grid”). The format can either be NetCDF (*.nc) or ASCII (any other file extension).

A further, relevant parameter is Q_LITHO:

  • 0: No coupled heat-conducting bedrock.

  • 1: Coupled heat-conducting bedrock.

If set to 0, the GHF is imposed directly at the grounded ice base, which is suitable for steady-state simulations because it reduces the time required to reach the steady state. However, for transient simulations, 1 is the preferred setting. The GHF is then imposed at the base of the lithosphere layer (thickness defined by variable H_R in the physical-parameter file, see “Physical-parameter files”), so that the thermal inertia of the lithosphere is properly accounted for.

6.8.3. Basal hydrology

Basal hydrology can be selected in the run-specs header by the parameter BASAL_HYDROLOGY:

  • If set to 0, basal hydrology is ignored.

  • If set to 1, it is assumed that basal water exists and moves in a thin (order of millimetres) and distributed water film. The film thickness is computed by a steady-state routing scheme for subglacial water that receives its input from the basal melting rate under grounded ice (Le Brocq et al. [37, 38], Calov et al. [8]). The computations are carried out by the module hydro_m.

6.8.4. Glacial isostatic adjustment

Three options are available for glacial isostatic adjustment, which can be selected in the run-specs header by the parameter REBOUND:

  • 0: Rigid lithosphere, no adjustment.

  • 1: Local-lithosphere–relaxing-asthenosphere (LLRA) model.

  • 2: Elastic-lithosphere–relaxing-asthenosphere (ELRA) model.

These models are described by Le Meur and Huybrechts [39] and Greve [21].

The detailed settings are controlled by additional parameters (FRAC_LLRA, TIME_LAG_MOD, TIME_LAG, TIME_LAG_FILE, FLEX_RIG_MOD, FLEX_RIG, FLEX_RIG_FILE, DTIME_WSS0) as described in the run-specs headers.

Note

The isostatically relaxed lithosphere surface topography (parameter ZL0_FILE, see “Topography”) is required for the isostasy models. A special setting for generating this topography can be enabled by

#define EXEC_MAKE_ZL0

It should be used together with ANF_DAT = 1 (present-day topography used as initial topography), computes the isostatically relaxed lithosphere surface topography, writes it on file and then stops the simulation (irrespective of the setting for the final time \(t_\mathrm{final}\)). The underlying assumption is that the present-day bed topography is approximately in equilibrium with the present-day ice load.