.. _subglacial_processes:

Subglacial processes
********************

.. _basal_sliding:

Basal sliding
=============

Two options for basal sliding are implemented, to be chosen in the run-specs header by the parameter ``SLIDE_LAW``\:

* ``0``: No-slip conditions.

* ``1``: Weertman-Budd-type sliding law (Weertman :cite:`weertman_1957a`, Budd et al. :cite:`budd_etal_1979`, Budd and Jenssen :cite:`budd_jenssen_1987`).

Weertman-Budd-type sliding posits that the sliding velocity is proportional to the basal shear stress (drag) to some power (exponent ``P_WEERT``) divided by the reduced pressure (ice minus water) to some power (exponent ``Q_WEERT``). For the sliding coefficient (parameter ``C_SLIDE_DIMLESS``), a dimensionless formulation is chosen (Greve :cite:`greve_2025`) to avoid problems with varying units and numerical values.

The reduced pressure is the difference between the overburden normal stress of the ice and the basal water pressure. The latter can be chosen by the parameter ``BASAL_WATER_PRESSURE`` as follows\:

* ``0``: Zero everywhere.

* ``1``: Ocean pressure without cut-off (can become negative).

* ``2``: Ocean pressure with cut-off (non-negative values enforced).

Sub-melt sliding (Sato and Greve :cite:`sato_greve_2012`), water-film-enhanced sliding (requires ``BASAL_HYDROLOGY = 1``, see ":ref:`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.

.. _ghf:

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 ":ref:`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 the parameter ``H_R`` in the run-specs header), so that the thermal inertia of the lithosphere is properly accounted for.

.. _basal_hydrology:

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. :cite:`lebrocq_etal_2006, lebrocq_etal_2009`, Calov et al. :cite:`calov_etal_2018`). The computations are carried out by the module ``hydro_m``.

.. _gia:

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 :cite:`lemeur_huy_1996` and Greve :cite:`greve_2001`.

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 ":ref:`topography`") is required for the isostasy models. A special setting for generating this topography can be enabled by

  .. code-block:: fortran

    #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 :math:`t_\mathrm{final}`). The underlying assumption is that the present-day bed topography is approximately in equilibrium with the present-day ice load.