InitMIP-Antarctica

Overview

Earlier large-scale ice sheet experiments e.g. those run during the ice2sea and SeaRISE initiatives have shown that ice sheet initialisation can have a large effect on sea-level projections and gives rise to important uncertainties. Improving initialisation techniques is currently a field of active research, which makes it difficult to prescribe one technique as the method of choice for ISMIP6. Instead, we first propose a “Come as you are”- approach, which allows participants to contribute with their currently used model setup and initialisation technique for intercomparison (initMIP). This, we hope, allows getting modellers involved early in the ISMIP6 process and keeps the workload for participants as low as possible. Furthermore, the proposed schematic experiments may facilitate to document on-going model development. Starting early in the CMIP6 process implies relying on schematic forcing for the initiation experiments that is independent from CMIP6 AOGCM output, which will only become available later on. As accurately modeling Antarctica at continental scale remains challenging, we offer groups interested in participating but unable to run the entire Antarctic ice sheet to focus on specified basins instead. The initMIP-Antarctica is the first in a series of ISMIP6 ice sheet model intercomparison activities and is led by Helene Seroussi.

Goals

• Compare and evaluate the initialisation methods used in the ice sheet modelling community

• Estimate uncertainty associated with initialisation

• Get the ice sheet modelling community started with ISMIP6 activities

• Document on-going model development, as the simple experiments could be repeated with new model versions

Experimental setup

Experiments are for the large scale or regional Antarctic ice sheet and are designed to allow intercomparison between models of 1) the initial state itself and 2) the response in three schematic forward experiments:

1. init: Initialisation to present day with method of choice

2. Schematic forward experiments

2a. ctrl: Unforced control run (100 years)

2b. asmb: Prescribed schematic surface mass balance anomaly (100 years)

2c. abmb: Prescribed schematic basal melting anomaly under floating ice (100 years)

The three forward experiments serve to evaluate the initialisation in terms of model drift (2a, ctrl) and response to perturbations (2b, asmb and 2c, abmb). For 2a, the models are run forward without any anomaly forcing, such that whatever surface mass balance (SMB) was used in the initialisation technique would continue unchanged. The perturbation in 2b consists of a given surface mass balance anomaly, which has to be applied relative to the initial SMB inherent to the individual initialisation technique. The SMB anomaly in 2b (the same for each model) is schematic and should not be considered as a realistic projection. The perturbation in 2c consists of a given basal melting anomaly, which has to be applied relative to the initial basal melting inherent to the individual initialisation technique and should be applied only to floating ice at any given time of the simulation. The basal melting anomaly for floating ice in 2c is schematic and should not be considered as a realistic projection. The core experiment duration is set to 100 years.

Regional models

As accurately modeling the entire Antarctic ice sheet remains challenging, we offer groups the option to focus instead on modeling one or more of five regional basins:

• Amundsen Basin

• Amery Basin

• Wilkes Land

• Ronne-Filchner Basin

• Ross Ice Shelf Basin

Experiments and parameters for the regional models are similar to the continental scale models. Interested participants can submit results for any basin. Results should be reported on the same grid as continental scale models to simplify the analysis. Shapefiles with suggested extent of the regional models (starting from the observed ice divide position and consistent with the basins used in the IMBIE2 effort) are posted on the ISMIP6 ftp server (email ismip6@gmail.com to obtain the login information).

Requirements for the experiments

• Participants can and are encouraged to contribute with different models and/or initialisation methods.

• Participants can contribute with continental scale models and/or regional models for the regional basins chosen.

• Models have to be able to prescribe a given SMB anomaly.

• Models have to be able to prescribe a given basal melting anomaly under the floating ice only.

• No adjustment of SMB due to geometric changes in forward experiments (i.e. no elevation - SMB feedback is allowed)

• No bedrock adjustment in forward experiment

• The choice of model input data is unconstrained to allow participants the use of their preferred model setup without modification. Modelers without preferred data set choice can have a look at the ISMIP6 Datasets page for possible options.

• The specific year of initialisation (between 1950 and 2014) is equally unconstrained to allow the use of different observational data sets that may be tied to certain time periods.

Prescribed SMB anomaly

The SMB anomaly can be obtained via the ISMIP6 ftp server (email ismip6@gmail.com to obtain the login information). Modeling groups should use the 1 km version to conservatively interpolate to their model native grid (see Appendix 1, below).

For experiment 2b, the amplitude of the SMB anomaly is to be implemented as a time dependent function, which increases step-wise every full year (it is therefore independent of the time step in the model):

SMB(t) = SMB_initialization + SMB_anomaly * (floor (t) / 40); for 0 < t < 40 in years

SMB(t) = SMB_initialization + SMB_anomaly * 1.0; for t > 40 years

where SMB_anomaly is the anomaly provided by ISMIP6 and SMB_initialization is the model specific SMB used for the initialization. The units of SMB_anomaly are (meter ice equivalent/year) with an assumed density of 910 kg/m^3 and 31556926 s/yr.

Prescribed basal melt anomaly under floating ice

The basal melt anomaly can be obtained via the ISMIP6 ftp server (email ismip6@gmail.com to obtain the login information) on a 1 km grid. Basal melt anomalies are uniform across a basin to accommodate all the possible initial grounding line positions. These anomalies are somewhat similar to the observed basal melt under floating ice, so they lead to a doubling of the ocean induced melting after 40 years for models with initial basal melting close to today's observations.

For experiment 2c, the amplitude of the basal melt anomaly is to be implemented as a time dependent function, similar to experiment 2b. The amplitude of the function applied increases step-wise every full year (it is therefore independent of the time step in the model):

basal_melt(t) = basal_melt_model + basal_melt_anomaly * (floor (t) / 40); for 0 < t < 40 in years

basal_melt(t) = basal_melt_model + basal_melt_anomaly * 1.0; for t > 40 years

where basal_melt_anomaly is the anomaly provided by ISMIP6 and basal_melt_model is the model specific basal_melt. The anomaly should only be applied on floating ice and should be applied on all floating ice, so that area newly ungrounded should include this anomaly on top of the basal melt applied for the ctrl run (e.g. depth dependant parameterization) as soon as the ice starts floating. The basal melt anomaly is defined over the entire Antarctic grid (see figure below), so newly ungrounded areas or ice front advances should also apply this anomaly as long as the ice is freely floating. The units of basal_melt_anomaly are (meter ice equivalent/year) with an assumed density of 910 kg/m^3 and 31556926 s/yr.

Unlike the SMB forcing, the basal melt anomaly is constant per regional basins, so conservative interpolation is not needed and the basal melt anomaly applied should simply equal the value prescribed for each basin. Several version of this anomaly with different grid resolutions (1 km, 2 km, 4 km, 8 km, 16 km and 32 km) are available on the ftp server.

Specific uncertainty analysis

At a later stage and informed by the diversity and similarities of participating models, ISMIP6 will suggest further experiments to explicitly address certain aspects of uncertainty in the initialisation. It is hoped that participating groups will contribute to these additional experiments, which apply specific perturbations to the initialisations. These would take the form of repeating the experiments with systematic perturbations of the initialization choices, for example:

– Boundary conditions and other datasets

– Parameters

– Model structure

– Methods and judgments, e.g. tolerance for data mismatch or drift

- ...

Appendix 1 – Output grid definition and interpolation

All 2D data is requested on a regular grid with the following description. Polar stereo-graphic projection with standard parallel at 71° S and a central meridian of 0° W on datum WGS84. The lower left corner is at (-3040000 m, -3040000 m) and the upper right at (3040000 m, 3040000 m). This is the same grid used to provide the SMB and basal melting anomaly forcings. The output should be submitted on a resolution adapted to the resolution of the model and can be 32km, 16 km, 8 km, 4 km, 2 km or 1 km. The data will be stored on this resolution for archiving and conservatively interpolated on a 8 km resolution for diagnostic processing by ISMIP6. Output should be provided with single precision.

If interpolation is required in order to transform the SMB forcing (1 km grid data) to your native grid, and transform your model variables to the initMIP output grid (32 km, 16 km, 8 km, 4 km, 2 km, 1 km), it is required that conservative interpolation is used. The motivation for using a common method for all models is to minimize model to model differences due to the choice of interpolation method.

A1.1 Regridding Tools and Tips

• An overview of the regridding process can be found on the Regridding page.
• Regridding_with_CDO contains tools and tips that have been used by ISMIP6 members.
• If you need help with conservative interpolation, please email ismip6-at-gmail.com.

Appendix 2 – Naming conventions, upload and model output data.

Please provide:

• one variable per file for all 2D fields (no need to provide coordinates)

• all variables in one file for the scalar variables

• a completed readme file

• single precision should be used for all output

A2.1 File name convention

File name convention for 2D fields:

<variable>_<IS>_<GROUP>_<MODEL>_<EXP>.nc

File name convention for scalar variables:

scalar_<IS>_<GROUP>_<MODEL>_<EXP>.nc

File name convention for readme file:

README_<IS>_<GROUP>_<MODEL>.doc

where

<variable> = netcdf variable name (e.g. lithk)

<IS> = ice sheet (AIS or GIS)

<GROUP> = group acronym (all upper case or numbers, no special characters)

<MODEL> = model acronym (all upper case or numbers, no special characters)

<EXP> = experiment name (init, ctrl, asmb or abmb)

For example, a file containing the scalar variables for the Antarctic ice sheet, submitted by group “JPL” with model “ISSM” for experiment “ctrl” would be called: scalar_AIS_JPL_ISSM_ctrl.nc

If JPL repeats the experiments with a different version of the model (for example, by changing the sliding law), it could be named ISSM2, and so forth.

A2.2 Uploading your model output

Please upload your model output on the FTP server cryoftp1.gsfc.nasa.gov, and email ismip6@gmail.com for the user name and latest password. Note sftp does not work!

After log in, go to the ISMIP6/initMIP/AIS/output directory via:

ftp> cd /ISMIP6/initMIP/AIS/output

and create a directory named <GROUP> with the following sub-directory structure:

initMIP/AIS/output/<GROUP>/<MODEL>/<EXP_RES>/

<EXP_RES> should include both the experiment name and the output grid used to simplify the processing (e.g., asmb_08). Only the directory name should include this resolution, unlike the output files.

Create additional <MODEL> directories when participating with more than one model or model version.

An example of model output files can be found in /ISMIP6/initMIP/AIS/output/JPL1/ISSM/.

A2.3 Model output variables and README file

The README file is an important contribution to the initMIP submission. It may be obtained here or requested by email to ismip6-at-gmail.com

The variables requested in the table below serve to evaluate and compare the different models and initialization techniques. Some of the variables may not be applicable for your model, in which case they are to be omitted (with explanation in the README file).

We distinguish between state variables (ST) (e.g. ice thickness, temperatures and velocities) and flux variables (FL) (e.g. SMB). State variables should be given as snapshot information at the end of one year (for scalars variables) and five year periods (for 2D variables), while flux variables are to be averaged over the respective periods. Please specify in your README file how your reported flux data has been averaged over time. Ideally, the standard would be go average over all native time steps.

Flux variables are defined positive when the process adds mass to the ice sheet and negative otherwise.

Time should be defined in seconds since the beginning of the run (e.g., units should be "seconds since 2007-01-01 00:00:00").

Example model output files can be found in /ISMIP6/initMIP/AIS/output/JPL1/ISSM/ on the ftp server.

Variable	Dim	Type	Variable Name	Standard Name	Units	Comment
2D variables requested every five years, starting at t=0, snapshots for type ST and as five year average for type FL.
Ice thickness	x,y,t	ST	lithk	land_ice_thickness	m	The thickness of the ice sheet
Surface elevation	x,y,t	ST	orog	surface_altitude	m	The altitude or surface elevation of the ice sheet
Base elevation	x,y,t	ST	base	base_altitude	m	The altitude of the lower ice surface elevation of the ice sheet
Bedrock elevation	x,y,t	ST	topg	bedrock_altitude	m	The bedrock topography (unchanged in forward exps.)
Geothermal heat flux	x,y	C	hfgeoubed	upward_geothermal_heat_flux_at_ground_level	W m-2	Geothermal Heat flux (unchanged in forward exps.)
Surface mass balance flux	x,y,t	FL	acabf	land_ice_surface_specific_mass_balance_flux	kg m-2 s-1	Surface Mass Balance flux
Basal mass balance flux	x,y,t	FL	libmassbf	land_ice_basal_specific_mass_balance_flux	kg m-2 s-1	Basal mass balance flux
Ice thickness imbalance	x,y,t	FL	dlithkdt	tendency_of_land_ice_thickness	m s-1	dHdt
Surface velocity in x	x,y,t	ST	uvelsurf	land_ice_surface_x_velocity	m s-1	u-velocity at land ice surface
Surface velocity in y	x,y,t	ST	vvelsurf	land_ice_surface_y_velocity	m s-1	v-velocity at land ice surface
Surface velocity in z	x,y,t	ST	wvelsurf	land_ice_surface_upward_velocity	m s-1	w-velocity at land ice surface
Basal velocity in x	x,y,t	ST	uvelbase	land_ice_basal_x_velocity	m s-1	u-velocity at land ice base
Basal velocity in y	x,y,t	ST	vvelbase	land_ice_basal_y_velocity	m s-1	v-velocity at land ice base
Basal velocity in z	x,y,t	ST	wvelbase	land_ice_basal_upward_velocity	m s-1	w-velocity at land ice base
Mean velocity in x	x,y,t	ST	uvelmean	land_ice_vertical_mean_x_velocity	m s-1	The vertical mean land ice velocity is the average from the bedrock to the surface of the ice
Mean velocity in y	x,y,t	ST	vvelmean	land_ice_vertical_mean_y_velocity	m s-1	The vertical mean land ice velocity is the average from the bedrock to the surface of the ice
Surface temperature	x,y,t	ST	litempsnic	temperature_at_ground_level_in_snow_or_firn	K	Ice temperature at surface
Basal temperature	x,y,t	ST	litempbot	land_ice_basal_temperature	K	Ice temperature at base
Basal drag	x,y,t	ST	strbasemag	magnitude_of_land_ice_basal_drag	Pa	Magnitude of basal drag
Calving flux	x,y,t	FL	licalvf	land_ice_specific_mass_flux_due_to_calving	kg m-1 s-1	Loss of ice mass resulting from iceberg calving. Only for grid cells in contact with ocean
Grounding line flux	x,y,t	FL	ligroundf	land_ice_specific_mass_flux_due_at_grounding_line	kg m-1 s-1	Loss of grounded ice mass resulting at grounding line. Only for grid cells in contact with grounding line
Land ice area fraction	x,y,t	ST	sftgif	land_ice_area_fraction	1	Fraction of grid cell covered by land ice (ice sheet, ice shelf, ice cap, glacier)
Grounded ice sheet area fraction	x,y,t	ST	sftgrf	grounded_ice_sheet_area_fraction	1	Fraction of grid cell covered by grounded ice sheet, where grounded indicates that the quantity correspond to the ice sheet that flows over bedrock
Floating ice sheet area fraction	x,y,t	ST	sftflf	floating_ice_sheet_area_fraction	1	Fraction of grid cell covered by ice sheet flowing over seawater
Scalar outputs requested every full year, as snapshots for type ST as 1 year averages for type FL. The t=0 value should contain the data of the initialization.
Total ice mass	t	ST	lim	land_ice_mass	kg	spatial integration, volume times density
Mass above floatation	t	ST	limnsw	land_ice_mass_not_displacing_sea_water	kg	spatial integration, volume times density
Grounded ice area	t	ST	iareag	grounded_land_ice_area	m^2	spatial integration
Floating ice area	t	ST	iareaf	floating_ice_shelf_area	m^2	spatial integration
Total SMB flux	t	FL	tendacabf	tendency_of_land_ice_mass_due_to_surface_mass_balance	kg s-1	spatial integration
Total BMB flux	t	FL	tendlibmassbf	tendency_of_land_ice_mass_due_to_basal_mass_balance	kg s-1	spatial integration
Total calving flux	t	FL	tendlicalvf	tendency_of_land_ice_mass_due_to_calving	kg s-1	spatial integration
Total grounding line flux	t	FL	tendligroundf	tendency_of_grounded_ice_mass	kg s-1	spatial integration

Appendix 3 – Participating Models and Characteristics.

Antarctica Standalone Ice Sheet Modeling

Contributors	Model	Group ID	Group
Nick Golledge	PISM	ARC	Antarctic Research Centre, Victoria University of Wellington, NZ
Thomas Kleiner, Johannes Sutter, Angelika Humbert	PISM	AWI	Alfred Wegener Institute for Polar and Marine Research, DE /University of Bremen, DE
Stephen Cornford	BISICLESPRELIM	CPOM	University of Bristol, Centre for Polar Observation and Modelling, UK
Christian Rodehacke	PISM0	DMI	Danish Meteorological Institute, Arctic and Climate, DK
Fabien Gillet-Chaulet	ELMER	IGE	Laboratoire de Glaciologie et Géophysique de l'Environnement, FR
Ralf Greve	SICOPOLIS	ILTS	Institute of Low Temperature Science, Hokkaido University, Sapporo, JP
Heiko Goelzer, Roderik van de Wal, Thomas Reerink	IMAUICE64	IMAU	Utrecht University, Institute for Marine and Atmospheric Research (IMAU), Utrecht, NL
Nicole Schlegel, Helene Seroussi	ISSM	JPL	NASA Jet Propulsion Laboratory, Pasadena, USA
Stephen Price, Matthew Hoffman, Tong Zhang	MALI	LANL	Los Alamos National Laboratory, Los Alamos, USA
Aurélien Quiquet, Christophe Dumas	GRISLI	LSCE	Laboratoire des Sciences du Climat et de l’Environnement,Université Paris-Saclay, France
William Lipscomb, Gunter Leguy	CISM	NCAR	National Center for Atmospheric Research
Torsten Albrecht	PISM3PAL	PIK	Potsdam Institute for Climate Impact Research, DE
David Pollard	EQNOMEC,GLNOMEC	PSU	Pennsylvania State University EMS Earth and Environmental Systems Institute, Pennsylvania, USA
Helene Seroussi, Mathieu Morlighem	ISSM	UCIJPL	NASA Jet Propulsion Laboratory, Pasadena, USA / University of California Irvine, Irvine, USA
Sainan Sun and Frank Pattyn	FETISH	ULB	Laboratoire de Glaciologie, Université Libre de Bruxelles, Brussels, BE
Jonas Van Breedam, Philippe Huybrechts	AISMPALEO	VUB	Vrije Universiteit Brussel, Brussels, BE

Model Characteristics

	ARC PISM1 PISM2	AWI PISM1Eq PISM1Pal PISM2Eq PISM2Pal	CPOM BISICLES PRELIM	ILTS SICOPOLIS	IMAU IMAUICE64	JPL1 ISSM	PSU EQNO MEC	PSU GLNO MEC	UCIJPL ISSM	ULB FETISH	VUB AISMPALEO
Numerics	FD	FD	FV	FD	FD	FE	FD	FD	FE	FD	FD
Ice Flow	HYB	HYB	SSA	SIA/ SSA	SSA	SSA	HYB	HYB	HO	HYB	SIA/ SSA
Initialization	SP	SP	DA	SP	SP	DA	DA+ SP	SP	DA	DA+ SP	SP
Initial Year	2000	2000	2010	1990	2005	2007	2007	2007	2007	2000	2000
Initial SMB	RA2	RA2.3	None	Arth.	RA2.3	RA2	PDD	PDD	RA2	MAR	PDD
Basal Sliding	PL	NP	CL	WS	VS	WS	WS	WS	WS	WS	WS
Initial Grid (km)	16	16	1-8	8	64	1-50	16	16	3-50	32	20

Numerical method: FD= Finite difference, FE= Finite element, FV= Adaptive mesh refinement

Ice flow: SIA= Shallow ice approximation, SSA= Shallow shelf approximation, HO= Higher order, HYB= Hybrid SIA-SSA

Initialization: DA= Data Assimilation, SP= Spin up

Initial SMB: RA2= RACMO2.1, RA2.3= RACMO2.3, PDD= Positive Degree Day Model, MAR= MAR

Basal sliding: PL=Pseudo-plastic, NP=Nearly Plastic, VS= Viscous Sliding, WS= Weertman Sliding