6.4. Subglacial processes

6.4.1. Basal sliding

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6.4.2. Geothermal heat flux

The geothermal heat flux, assumed to be time-independent, can be specified in the run-specs header as either a constant value or a spatially varying distribution. For the example of simulation v5_grl16_bm5_ss25ka:

#define Q_GEO_MOD 2
!                       1 : Constant geothermal heat flux defined
!                           by parameter Q_GEO
!                       2 : Spatially varying geothermal heat flux
!                           read from file

#define Q_GEO 0.0d0
!                       Constant geothermal heat flux (for Q_GEO_MOD==1),
!                       in mW/m2

#define Q_GEO_FILE 'GHF_Greenland_Ver2.0_GridEPSG3413_16km.nc'
!                       Name of the file containing the spatially varying
!                       geothermal heat flux (for Q_GEO_MOD==2)
!                       (if NetCDF, variable name must be 'GHF')

If, like in the above example, a file with gridded data is provided, 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 from the “Lithosphere (bedrock) modelling” section in the run-specs header:

#define Q_LITHO 0
!                       0 : No coupled heat-conducting bedrock
!                       1 : Coupled heat-conducting bedrock

If set to 0, the geothermal heat flux 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 geothermal heat flux 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.

6.4.3. 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. [24, 25], Calov et al. [6]). The computations are carried out by the module hydro_m.

6.4.4. 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 [26] and Greve [12].


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.