ISMIP6 Greenland¶

Running ISMIP6-Greenland projections required implementing some additional sub-models as well as several modifications needed to follow ISMIP6 conventions. This section describes these modifications and explains how to use PISM to run ISMIP6 projections.

Top surface mass balance and temperature¶

Use the ismip6 surface model to implement ISMIP6 surface mass balance forcing.

pismr -surface ismip6 \
      -surface_ismip6_file climate_forcing.nc \
      -surface_ismip6_reference_file climate_forcing_reference.nc

Here climate_forcing.nc should contain time-dependent variables

climatic_mass_balance_anomaly (units: \(kg / (m^2 s)\)) and
ice_surface_temp_anomaly (units: Kelvin).

The file climate_forcing_reference.nc should contain time-independent (2D) variables

climatic_mass_balance_reference (units: \(kg / (m^2 s)\)),
climatic_mass_balance_gradient (units: \((kg / (m^2 s)) / m\)),
ice_surface_temp_reference (units: Kelvin),
ice_surface_temp_gradient (units: Kelvin / m),
surface_elevation (units: m)

The surface mass balance is computed using the following formula:

SMB(x,y,t) = SMB_ref(x,y) + aSMB(x,y,t) + dSMBdz(x,y) * [h(x,y,t) - h_ref(x,y)]

Frontal melt parameterization¶

Use the discharge_given frontal melt model to implement the ISMIP6 frontal melt parameterization.

pismr -frontal_melt discharge_given \
      -frontal_melt_discharge_given_file forcing.nc ...

The file forcing.nc has to contain variables theta_ocean (potential temperature of adjacent ocean, degrees Celsius) and subglacial_discharge (water flux per unit area of submerged ice front, \(kg / (m^2 s)\)).

These inputs are used in the frontal melt parameterization described in [115]:

\[q_m = (A\, h\, q_{sg}^{\alpha} + B)\, \theta^{\beta}\]

Here \(q_m\) is the frontal melt rate in m/day, \(h\) is the water depth at an ice front, \(\theta\) in the thermal forcing and \(A\), \(B\), \(\alpha\), \(\beta\) are model parameters.

Parameterized front retreat¶

To use the parameterized front retreat mechanism use the Prescribed front retreat mechanism.

pismr -front_retreat_file retreat_forcing.nc ...

The file retreat_forcing.nc should contain the variable land_ice_area_fraction_retreat which defines the maximum ice extent at a given time.

Mass losses resulting from applying this mechanism are reported as a part of tendency_of_ice_amount_due_to_discharge and related diagnostics (i.e. they are not attributed to calving or frontal melt).

Output variables¶

See Table 30 for a list of variables requested by ISMIP6. Note that they have names different from the ones listed in Spatially-variable fields and use MKS units. To reduce the amount of post-processing output files require PISM can follow these conventions.

Setting output.ISMIP6 makes PISM save diagnostics using MKS units and recognize ISMIP6 variable names.

To save all the diagnostics requested by ISMIP6 use the short-cut

pismr -extra_vars ismip6 ...

The list of variables is stored in the configuration parameter output.ISMIP6_extra_variables and contains variables Greenland projections are required to provide. (Add base,ligroundf to this list for Antarctic projections.)

To save all the time series supported by PISM, omit the -ts_vars option:

pismr -ts_times TIMES -ts_file ts.nc

To save all variables requested by ISMIP6, use -ts_vars ismip6:

pismr -ts_times TIMES -ts_file ts.nc -ts_vars ismip6

Table 30 ISMIP6 variables¶
Variable	Units	Description
`lithk(x,y,t)`	m	Ice thickness
`orog(x,y,t)`	m	Surface elevation
`base(x,y,t)`	m	Base elevation
`topg(x,y,t)`	m	Bedrock elevation
`hfgeoubed(x,y)`	W m-2	Geothermal heat flux
`acabf(x,y,t)`	kg m-2 s-1	Surface mass balance flux
`libmassbfgr(x,y,t)`	kg m-2 s-1	Basal mass balance flux beneath grounded ice
`libmassbffl(x,y,t)`	kg m-2 s-1	Basal mass balance flux beneath floating ice
`dlithkdt(x,y,t)`	m s-1	Ice thickness imbalance
`xvelsurf(x,y,t)`	m s-1	Surface velocity in x
`yvelsurf(x,y,t)`	m s-1	Surface velocity in y
`zvelsurf(x,y,t)`	m s-1	Surface velocity in z
`xvelbase(x,y,t)`	m s-1	Basal velocity in x
`yvelbase(x,y,t)`	m s-1	Basal velocity in y
`zvelbase(x,y,t)`	m s-1	Basal velocity in z
`xvelmean(x,y,t)`	m s-1	Mean velocity in x
`yvelmean(x,y,t)`	m s-1	Mean velocity in y
`litemptop(x,y,t)`	K	Surface temperature
`litempbotgr(x,y,t)`	K	Basal temperature beneath grounded ice sheet
`litempbotfl(x,y,t)`	K	Basal temperature beneath floating ice shelf
`strbasemag(x,y,t)`	Pa	Basal drag
`licalvf(x,y,t)`	kg m-2 s-1	Calving flux
`lifmassbf(x,y,t)`	kg m-2 s-1	Ice front melt and calving flux
`ligroundf(x,y,t)`	kg m-2 s-1	Grounding line flux
`sftgif(x,y,t)`	1	Land ice area fraction
`sftgrf(x,y,t)`	1	Grounded ice sheet area fraction
`sftflf(x,y,t)`	1	Floating ice sheet area fraction
`lim(t)`	kg	Total ice mass
`limnsw(t)`	kg	Mass above floatation
`iareagr(t)`	m^2	Grounded ice area
`iareafl(t)`	m^2	Floating ice area
`tendacabf(t)`	kg s-1	Total SMB flux
`tendlibmassbf(t)`	kg s-1	Total BMB flux
`tendlibmassbffl(t)`	kg s-1	Total BMB flux beneath floating ice
`tendlicalvf(t)`	kg s-1	Total calving flux
`tendlifmassbf(t)`	kg s-1	Total calving and ice front melting flux
`tendligroundf(t)`	kg s-1	Total grounding line flux