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 [109]:

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

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


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