ISMIP6-Projections-Greenland

Overview

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This page describes the experimental protocol for the ISMIP6 projections that target the upcoming IPCC AR6 assessment. Due to the delay in CMIP6 climate simulations, the initial set of ISMIP6 simulations are based on CMIP5 projections. As CMIP6 model output become available, ISMIP6 will include simulations based on these models.

The experimental framework was revised in September 2018 during an ISMIP6 workshop held in Sassenheim (NL). The protocol, summarized in Fig 1, allows for:

• Sampling CMIP scenarios: main focus is on the high emission RCP8.5, but ice sheet evolution in response to low emission RCP2.6 is also investigated.

• Sampling CMIP models: 6 AOGCMs have been selected from the CMIP5 model ensemble. The AOGCMs were identified based on the following steps: 1) present plausible climates near Greenland (evaluated by model biases over the historical period), 2) have the data at the temporal resolution needed for RCM downscaling, 3) sample a diversity of forcing (evaluated by differences in projections and code similarities).

• Sampling ice sheet model uncertainty: "standard" and "open" experiments. The "standard" experiments are based on parameterizations developed by the ocean and atmospheric focus groups, while "open" experiments utilize the parameterizations already in use by respective ice sheet models.

• Sampling forcing uncertainty: the standard experiments include "high", "mid" and "low" parameters.

• Experiment ranking: This experimental framework results in 12 to 72 experiments, and a control run. Not every ice sheet model will be able to carry out the full set of experiments. The experiments are therefore ranked, groups are encouraged to work shown the list and complete as many experiments as possible. This approach is based on Shannon et al. (2013): it ensures that all groups do a subset of identical experiments, while it also allows faster models to explore the experiment space more fully.

Figure 1: Overview of the Greenland experimental framework

Experiment Ranking

With the help of the atmosphere and ocean focus groups, six CMIP5 AOGCMs have been selected for ISMIP6 standalone ice sheet model projections. The table below lists the initial number of experiments based on the first three AOGCMs: <AOGCM1>, NorESM, MIROC5. This table is the minimum contribution expected from ISMIP6 models and should be worked on in order. Modeling groups that can run many simulations will be encouraged to explore the ice sheet response using three additional CMIP5 AOGCMs. For groups that already have their own methods for implementing ocean and atmosphere forcing in place, the suite of "open" experiments (1-4) allows for a direct start. The basic requirement for the "open" experiments is that the forcing is based on the given CMIP5 GCM and uses the given duration for the reference period and scenario (see below). All groups are expected to submit results for the "standard" experiments, using forcing methods suggested by ISMIP6. Depending on the results of experiments 3 and 7, which consider RCP2.6, additional AOGCMs may be suggested (exploring RCP2.6 in detail has low priority if the ISM response is very limited). Additional RCP2.6 experiments may be reserved for models that are able to do many simulations, but these would be of lower priority than completing the set with the 6 AOGCMs with RCP8.5.

Note: As of Jan 21, MAR has completed MIROC5 and NorESM for both RCP8.5 and RCP2.6. Greenland model selection team is actively working on final selection

Expt	RCP	AOGCM	Std/open	Forcing Unc	Note
0	N/A	N/A	Control	N/A	Model drift evaluation
1	8.5	<AOGCM1>	Open	Medium	Expected high SLR
2	8.5	NorESM	Open	Medium	Expected low SLR
3	2.6	<AOGCM1>	Open	Medium	Expected high SLR
4	8.5	MIROC5	Open	Medium	Expected mid SLR
5	8.5	<AOGCM1>	Standard	Medium	Expected high SLR
6	8.5	NorESM	Standard	Medium	Expected low SLR
7	2.6	<AOGCM1>	Standard	Medium	Expected high SLR
8	8.5	MIROC5	Standard	Medium	Expected mid SLR
9	8.5	<AOGCM1>	Standard	High	Forcing Uncertainty
10	8.5	<AOGCM1>	Standard	Low	Forcing Uncertainty

Initial state and experiment duration

Groups can reuse their initMIP initial state. If they chose to start from a new initial state, rerunning the initMIP-Greenland experiments ('ctrl', 'asmb') is required to place results with a new model version into context. To facilitate analysis of the sea-level projections resulting from the ISMIP6 suite of ice sheet model simulations, ISMIP6 uses January 1995 to December 2014 as reference period. The experiments start on January 2015 and end in December 2100. The cutoff date is constrained by the availability of forcing data.

Control Run and schematic forcing

The control run ('ctrl') is needed to evaluate model drift. As in the initMIP setup, the control run is obtained by running the model forward without any anomaly forcing, such that whatever surface mass balance (SMB) and ocean forcing was used in the initialization technique would continue unchanged. The second initMIP experiment ('asmb') that also needs to be repeated for each new model version is forcing with a schematic SMB anomaly.

Atmospheric forcing: SMB and temperature anomalies

Introduction

General intro on forcing approach, why anomalies of SMB? 1. More robust forcing signal. 2. applicable to a large ensemble of models

Before applying SMB anomalies, ISMs will need to be initialized by applying a baseline SMB (either a time series or a climatology). ISMIP6 will provide SMB climatologies for the reference period (January 1995 to December 2014) from the same models computing the anomalies. ISMs can use these these climatologies for spin-up, if desired, but are free to use their own preferred SMB forcing.

Elevation feedbacks have been shown to be important in century-scale simulations. For example, Le clec’h et al. (2019) considered differences between no coupling (SMB independent of z), one-way coupling (i.e., correcting SMB outputs for ISM topography changes), and two-way coupling (allowing ice-sheet topographic changes to feed back on the RCM simulation.) Their results suggest that one-way coupling is sufficient to represent elevation feedbacks until the end of the 21st century. For large topographic changes on longer timescales, it might be necessary to incorporate two-way feedbacks, which are beyond the scope of the standalone ISM effort (but will be considered in the ISMIP6 coupled GCM-ISM experiments).

Provided forcing data sets

ISMIP6 will provide surface forcing datasets for the Greenland ice sheet (GrIS) based on CMIP global climate model (GCM) simulations. The GCM output is re-interpreted through higher-resolution regional climate models (RCMs). The later allows to capture narrow regions at the periphery of the Greenland ice sheet with large surface mass balance (SMB) gradients, which are not captured by CMIP5 GCMs.

For CMIP6, many of the GCMs that have indicated participation in ISMIP6 now use multiple elevation classes to downscale SMB to finer grid resolution. Once these models have completed the CMIP6 projections, our goal is to include additional ISMIP6 projections using SMB downscaled via elevation class. HG:Move to somewhere else?

For the ISMIP6 projections based on CMIP5 GCMs, the surface forcing datasets were prepared by Xavier Fettweis, using the MAR regional climate model. RCM downscaling can take account of modest future topography/extent changes, and thus cope with the fact that individual ISM runs may not use exactly the same geometry. All RCM runs use a fixed topography, but vertical SMB gradients for each grid cell are derived using the method described by Franco et al. (2012). At each location the vertical gradient in SMB is found by summing and averaging pairwise differences in the nearest neighbor cells. This vertical gradient is used to downscale the SMB (15 km) to a finer grid (1 km), allowing resolution of steep topography that is not represented accurately on the coarse grid. The same information will be used to parameterise the SMB-height feedback in the projections. In addition, MAR calculates a potential SMB term for areas that are outside of the observed ice sheet extent, allowing application to ISMs with ice lying outside the MAR ice-sheet mask. However, experience with initMIP has shown that large variations in ice-sheet extent can lead to considerable bias in the projections. We therefore propose a method (see below) for models with large difference from the observed ice sheet extent, to remap the SMB anomaly to the individual modelled ice sheet geometry.

The atmospheric forcings consist of annual anomalies of SMB and surface temperature, along with a fields to represent the dependence of SMB and surface temperature on elevation (dSMBdz, dTdz). SMB is needed by ISMs to compute mass changes at the surface, and surface temperature (i.e., the ice temperature at the base of the snow, as distinct from the 2-m air temperature or skin temperature) is used by many ISMs as an upper boundary condition in the ice temperature calculations.

The SMB anomaly aSMB is given in units [kg m-2 s-1] in yearly values, one year per file. It should be applied constant over a full year and step change at the beginning of a new year. To convert to units [m yr-1] typically used in an ice sheet model, multiply the netcdf variable by 31556926 s/yr, 1/1000 m3/kg and by the density ratio rhow/rhoi:

aSMB [m yr-1] = aSMB [kg m-2 s-1] * 31556926 / 1000 * (1000/rhoi), where rhoi is your specific ice density (typically 917.0 or similar).

The SMB and its anomalies will be provided on the ISMIP6 1 km standard ice sheet grid for Greenland. ISMs will then horizontally interpolate the anomaly forcing conservatively from the standard grid to their native grids.

The SMB change with surface elevation dSMBdz is given in units [kg m-2 s-1 m-1] in yearly values, one year per file. To parameterise the SMB-height feedback, the SMB has to be corrected by dSMBdz * h-h_ref, updated every full year, where h and h_ref are the modelled surface elevation the initial modelled ice sheet surface elevation, respectively.

The ISMIP6 standard grid has a horizontal resolution of 1 km and uses the projection EPSG:3413. Note that this grid is different than that used in initMIP-Greenland. We have changed to the new grid because it's projection is used by many observational datasets that are key input for ice sheet models.

Implementation

Let SMB_ref(x,y) denote the SMB used to initialize the ISM, and let h_ref(x,y) denote the ice sheet surface elevation at the end of the initialization. If a time-dependent SMB is used for spin-up, then SMB_ref(x,y) is the average over the reference period.

Here we propose two different methods for implementation of atmospheric forcing, depending on how close the ice sheet mask at the end of the initialization is to the observed ice sheet mask, the one assumed by the RCM.

Method 1: when the ice sheet mask is similar to the observed

This will typically be the case for ice sheet models that use data assimilation in their initialization, but could be the case for other modelling approaches. For those models, ISMIP6 provides aSMB(x,y,t) at h_rcm(x,y), along with dSMBdz(x,y,t). Here, aSMB is the time-dependent SMB anomaly in a changing climate, computed in an RCM with fixed surface topography h_rcm, and dSMBdz(x,y,t) is the time-dependent vertical gradient of SMB. aSMB and dSMBdz will be provided on an annual basis and should be updated every full year.

Given aSMB(x,y,t) and dSMBdz(x,y,t) on the standard grid, the ISM will horizontally interpolate these fields to its local grid. Then during runtime, the SMB at a given time and location is computed as

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

where h(x,y,t) is the time-dependent surface elevation. ISMs will likely need to implement code changes to handle the lapse-rate correction. The models will not need h_rcm(x,y) to compute SMB, but it is provided for reference. Since dSMBdz is computed at h_rcm and is not given as a function of z, this approach may be inaccurate if h_ref is significantly different from h_rcm.

The datasets can be obtained via the ISMIP6 ftp server (email ismip6@gmail.com to obtain the login information).

A Test data set is available for MAR3.9-MIROC5-RCP85 at ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Atmosphere_Forcing/Greenland/

Method 2: when the ice sheet mask is very different from the observed

This will typically be the case for ice sheet models that use a glacial-interglacial spinup in their initialization, but could also be the case e.g. for other models that fully relax to a suboptimal SMB. For those models, ISMIP6 will generate a time-dependent SMB anomaly, aSMB(x,y,t) and dSMBdz(x,y,t) that are applied as described above. However, the main difference is that the forcing files are specific for the geometry of your modelled initial state.

In order to make the forcing applicable for different ice sheet geometries, we first translate a given SMB anomaly field as a function of absolute location, to a function of surface elevation for 25 regional drainage basins. This step exploits the strong elevation dependence of aSMB. We can then remap aSMB (and dSMBdz) to different modelled geometries. This preserves the overall aSMB patterns and reduces unphysical biases. The procedure to generate the remapped aSMB and dSMBdz is described in Goelzer et al. (2019, in prep).

To obtain the datasets, modelers simply need to provide ISMIP6 with their modelled initial surface elevation h_ref(x,y) and ice mask sftgif(x,y). ISMIP6 will then compute aSMB for you. Please send an email to ismip6-at-gmail.com when you are ready to upload your initial state (h_ref, sftgif).

The decision on which forcing method to use depends on the expected biases inherent to both approaches. Given the initial state (h_ref and sftgif) we can estimate the biases with a simple integration of the SMB anomaly for a static case (i.e. no ice dynamics). If you are not sure which forcing strategy is best suited for you model, please contact us by email to ismip6-at-gmail.com.

Oceanic forcing: Calving and frontal melt

ISMIP6 provides dataset of runoff and ocean thermal forcing for models that have their own methods for implementing oceanic induced retreat. In addition, modeling groups are encouraged to participate with the ISMIP6 Standard approach described below. The later is a simple retreat intended to be easily implemented by the majority of ISM taking part in ISMIP6. In addition, for models that wish to implement a more complex oceanic forcing, the ISMIP6 Greenland ocean focus group has developed a second methodology, described in ISMIP6 high resolution ocean melt rate approach.

ISMIP6 Standard approach imposes an empirically-derived, sector averaged retreat (Fig 2) as a function of climate forcing. This method was developed for ISMIP6 as a result of the ocean forcing focus group and is described in greater details in Slater et al. (in prep), and in the webinar: ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Ocean_Forcing/Greenland/Webinar_2018_11_08

Fig 2: Example of the empirically-derived retreat scenarios for the 7 sectors of the Greenland ice sheet, obtained with MIROC5, RCP8.5. TO DO: MODIFY FIG so that it shows the 7 ISMIP6 Basins

As described in the webinar, retreat will be imposed when the ice sheet geometry and ice front retreat scenario indicates that the land_ice_area_fraction_retreat mask is ice free for a given year. (Note the name was chosen so that it is closely related to the standard name land_ice_area_fraction -corresponding to variable name sftgif- in the ISMIP6 data request. For ease of communication, we use the longer standard name). Implementation for a specific ISM (also illustrated in Fig 3) requires the following steps:

1. Identify ice prone to outlet glacier retreat, by interpolating initial ice mask conservatively to 1 km ISMIP6 diagnostic grid
 to obtain a mask for ice fraction:

land_ice_area_fraction(x,y) = [0.0, …, 1.0] with 0.0 for ice free and 1.0 for ice covered

2. Calculate the distance to nearest ocean grid cell, or "distance_map", based on model ice fraction and observed mask of ice with potential ocean contact -> distance_map(x,y)

3. Calculate land ice fraction retreat based on distance map and retreat scenario: 


land_ice_area_fraction_retreat(x,y,t) = [0.0, …, 1.0] with 0.0 for ice free and 1.0 for ice covered

4. Interpolate land_ice_area_fraction_retreat mask from diagnostic grid to model grid (conservatively)

5. Apply retreat in forward experiments (sub-grid implementation may be required for models with coarse resolutions to allow for partial retreat):

if land_ice_area_fraction_retreat(x,y,t) = 0.0, apply full retreat

if 0.0 < land_ice_area_fraction_retreat(x,y,t) < 1.0 apply partial retreat

Fig 3: Illustration of the steps required for the implementation of the oceanic retreat.

ISMIP6 will generate the land_ice_area_fraction_retreat masks (steps 2-3) for each model. As retreat is provided as a series of ice fraction masks, ice sheet models with coarse resolution should use a sub-grid approach. A suggestion is to apply an land_ice_area_fraction_retreat that is relative to the reference thickness. Models may have a different strategy for this sub-grid implementation.

question: do we want to give an eq for sub-grid implementation?"

To obtain the land_ice_area_fraction_retreat masks appropriate for your model, please submit your land_ice_area_fraction mask (step 1) to ISMIP6 on the 1km ISMIP6 grid (EPSG:3413) as a netcdf file called sftgif.nc. This file should be generated by conservative regridding. If your native grid is regular and on EPSG:3413, please provide your original modelled ice mask with x,y information instead. Upload your file to the directory ISMIP6/Ocean_Forcing/Greenland/Retreat_Implementation/MODELFILES/MODELNAME, where MODELNAME is the name of your model, and let us know via email that your file is uploaded.

ISMIP6 open approach is used sample a larger variety of oceanic forcing parameterizations, as it remains an active field of research. Models are free to continue applying the ocean forcing parameterization they used during the model initialization or their preferred method, but should still rely on the ocean forcing datasets provided by ISMIP6 to simulate future ocean conditions.

ISMIP6 high resolution ocean melt rate allows ice sheet models to specify terminus retreat for each individual marine-terminating outlet glacier, thus allowing glaciers to retreat at different rates. This is accomplished by specifying a glacier terminus melt rate, calculated as a function of subglacial discharge (approximated as surface runoff from each glacier catchment) and ocean thermal forcing (Fig. 4). In addition to the melt rate, a calving rate must be specified to obtain the total frontal ablation rate at each terminus. The high resolution ocean melt rate is considered an "open approach". Additional details can be found in the webinar and the updated presentation slides: ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Ocean_Forcing/Greenland/Webinar_2018_11_08

Fig 4: High-resolution melt-rate approach flow chart. Ocean thermal forcing and runoff fields are provided to the ice sheet modelers by the ISMIP6 ocean forcing working group. All parameters in the recommended melt-rate parameterization (Xu et al., 2013; Rignot et al., 2016) are provided, as well. Each ice sheet model must implement the approach and specify their own calving parameterization to calculate a frontal ablation rate for each outlet glacier.

Requirements for the standard experiments

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

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

• Models have to be able to prescribe a given ice front retreat

• Adjustment of SMB due to geometric changes in forward experiments is encouraged.

• Bedrock adjustment in forward experiment is allowed.

• 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.

• To allow for analysis, any modeling choice needs to be well documented. A README file needs to be submitted along the outputs as an integral part of the contribution to the ISMIP6. It may be obtained here (need to update the readme file) or requested by email to ismip6-at-gmail.com.

Requirements for the open experiments

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

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

• Models can choose the ocean parameterization of their choice but this parameterization should use the ocean forcing provided

• Adjustment of SMB due to geometric changes in forward experiments is encouraged.

• Bedrock adjustment in forward experiment is allowed.

• 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.

• To allow for analysis, any modeling choice needs to be well documented. A README file needs to be submitted along the outputs as an integral part of the contribution to the ISMIP6. It may be obtained here (need to update the readme file) or requested by email to ismip6-at-gmail.com.

References

Franco, B., Fettweis, X., Lang, C., and Erpicum, M.: Impact of spatial resolution on the modelling of the Greenland ice sheet surface mass balance between 1990–2010, using the regional climate model MAR, The Cryosphere, 6, 695-711, https://doi.org/10.5194/tc-6-695-2012, 2012.

Goelzer, H., Nowicki, S., Edwards, T., Beckley, M., Abe-Ouchi, A., Aschwanden, A., Calov, R., Gagliardini, O., Gillet-Chaulet, F., Golledge, N. R., Gregory, J., Greve, R., Humbert, A., Huybrechts, P., Kennedy, J. H., Larour, E., Lipscomb, W. H., Le clec'h, S., Lee, V., Morlighem, M., Pattyn, F., Payne, A. J., Rodehacke, C., Rückamp, M., Saito, F., Schlegel, N., Seroussi, H., Shepherd, A., Sun, S., van de Wal, R., and Ziemen, F. A. (2018). Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison, The Cryosphere, 12, 1433-1460, doi:10.5194/tc-12-1433-2018.

Goelzer, H. et al. (2019) Remapping of Greenland ice sheet surface mass balance anomalies for large ensemble sea-level change projections, in prep.

Le clec'h, S., Charbit, S., Quiquet, A., Fettweis, X., Dumas, C., Kageyama, M., Wyard, C., and Ritz, C.: Assessment of the Greenland ice sheet–atmosphere feedbacks for the next century with a regional atmospheric model coupled to an ice sheet model, The Cryosphere, 13, 373-395, https://doi.org/10.5194/tc-13-373-2019, 2019.

Shannon, S.R., Payne A.J., Bartholomew I.D., Van Den Broeke M.R., Edwards T.L., Fettweis X., Gagliardini O., Gillet-Chaulet F., Goelzer H., Hoffman M.J., Huybrechts P. (2013) Enhanced basal lubrication and the contribution of the Greenland ice sheet to future sea-level rise, Proceedings of the National Academy of Sciences, 110(35):14156-61.

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 70° N and a central meridian of 45° W (315° E) on datum WGS84 (EPSG3413 projection). The lower left cell center is at (-720000m,-3450000m) with nx=1681 and ny=2881 cells in x and y-direction at full km positions (xmin = -720 km, xmax = +960 km, ymin = -3450 km, ymax = -570 km). The output should be submitted on a resolution adapted to the resolution of the model and can be 20 km, 10 km, 5 km, 2 km or 1 km. The data will be conservatively interpolated to 1 km resolution for archiving and 5 km resolution for diagnostic processing by ISMIP6.

If interpolation is required in order to transform the SMB forcing to your native grid, and transform your model variables to the ISMIP6 output grid (20 km, 10 km, 5 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.

Note: The previously requested regular grid was in polar stereo-graphic projection with standard parallel at 71° N and a central meridian of 39° W (321° E) on datum WGS84. The lower left corner is at (-800000 m, -3400000 m) and the upper right at (700000 m, -600000 m). This is the same grid (Bamber et al., 2001) used to provide the SMB anomaly forcing previously. This grid was changed to the EPSG3413 projection described above.

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

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

Please provide:

• one variable per file for all 2D fields

• all variables in one file for the scalar variables

• a completed readme file

A2.1 File name convention

TO DO: REVISE EXP NAME (LOOK AT CMIP6 GUIDANCE, INCLUDING HOW TO REGISTER MODEL) AND CHANGE INFO 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 or asmb)

For example, a file containing the scalar variables for the Greenland ice sheet, submitted by group “JPL” with model “ISSM” for experiment “ctrl” would be called: scalar_GIS_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!

TO DO: CREATE DIRECTORY FOR UPLOAD AND CHANGE INFO

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

ftp> cd /ISMIP6/initMIP/output

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

initMIP output/ <GROUP>/ <MODEL>/ init/ ctrl/ asmb/

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/output/ISMIP6/REF.

A2.3 Model output variables and README file

The README file is an important contribution to the ISMIP6 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). Also, specify missing values in your netcdf file where needed, and fields should be undefined outside of the ice mask.

We distinguish between state variables (e.g. ice thickness, temperatures and velocities) and flux variables (e.g. SMB). Flux variables are defined as positive when the process adds mass to the ice sheet and negative otherwise. Note the different treatment for state variables (snapshots) and fluxes (time average). The standard should be averaging over all native time steps for yearly scalar output and for 5 year periods for 2D fields. Please specify how your reported flux data has been averaged over time in the README file.

Example model output files can be found in /ISMIP6/initMIP/output/ISMIP6/REF 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
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 (for areas covered by ice only)
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 (for areas covered by ice only)
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-2 s-1	Loss of ice mass resulting from iceberg calving. Only for grid cells in contact with ocean
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

Appendix 3 – Participating Models and Characteristics

Greenland Standalone Ice Sheet Modeling

Contributors

Model

Group ID

Group

Model Characteristics

Model ID	Numerics	Ice Flow	Initialization	Initial Year	Initial SMB	Velocity	Bed	Surface	GHF	Res min	Res max