ISMIP6-Projections-Antarctica

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Overview

NOTE: THIS PAGE IS UNDER CONSTRUCTION. WHEN IT IS FINISHED, THIS STATEMENT WILL BE REMOVED. LAST UPDATE: 13 FEB 2019

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 outputs become available, ISMIP6 will include simulations based on these new 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-day climate near Antarctica in agreement with observations (evaluated by model biases over the historical period), and 2) 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" values for the forcing 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 according to their importance. Groups are encouraged to work through the list presented below (see Table of Experiments), starting from the top, 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.

Antarctic exp design.png

Figure 1: Overview of the Antarctic experimental framework

Table of Experiments

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: MIROC_ESM_CHEM, NorESM1-M and CCSM4. This table is the minimum contribution expected from ISMIP6 models. Modeling groups that can run many simulations are encouraged to explore the ice sheet response using three additional CMIP5 AOGCMs. Groups that have their own methods for implementing ocean and atmosphere forcing, are encouraged to do the suite with "open" experiments (1-4). Models that perform the "open" experiments can use the parameterization of their choice to simulate atmospheric and oceanic forcings, but these parameterizations must use the given CMIP5 AOGCM outputs. All groups are expected to contribute to the "standard" experiments. Depending on the results of experiments 3 and 7, which consider RCP2.6, additional AOGCMs may be suggested with RCP2.6 for models that are able to do many simulations, but these would be a lower priority than the completing the set of experiments with the 6 AOGCMs for the RCP8.5 scenario. Once all the datasets are prepared for the additional AOGCMs, the next series experiments will be announced.

Note: As of Feb 13, Xylar has processed the Ocean dataset for the 3 models under RCP8.5 and Erika has processed the atmosphere dataset for the 3 models under RCP8.5. Masks for ice shelf fracture will be uploaded in the coming days.


Expt RCP AOGCM Std/open Forcing Param Fracture Note
0 N/A N/A Control N/A None Model drift evaluation
1 8.5 NorESM1-M Open Medium None Low atmospheric change and mid-to-high ocean warming
2 8.5 MIROC_ESM_CHEM Open Medium None High atmospheric changes and median ocean warming
3 2.6 NorESM1-M Open Medium None Low atmospheric change and mid-to-high ocean warming
4 8.5 CCSM4 Open Medium None Large atmospheric warming and variable regional ocean warming
5 8.5 NorESM1-M Standard Medium None Low atmospheric change and mid-to-high ocean warming
6 8.5 MIROC_ESM_CHEM Standard Medium None High atmospheric changes and median ocean warming
7 2.6 NorESM1-M Standard Medium None Low atmospheric change and mid-to-high ocean warming
8 8.5 CCSM4 Standard Medium None Large atmospheric warming and variable regional ocean warming
9 8.5 NorESM1-M Standard High None Forcing Uncertainty
10 8.5 NorESM1-M Standard Low None Forcing Uncertainty
11 8.5 CCSM4 Open Medium Yes Experiment with ice shelf hydrofracture
12 8.5 CCSM4 Standard Medium Yes Experiment with ice shelf hydrofracture

Initial state and experiment duration

The projections start on January 2015 and end in December 2100. The start date follows the CMIP6 protocol, while the cut off date is constrained by the availability of forcing. The "start ice sheet configuration" for the projections will in many cases be distinct from the "ice sheet initial state". Depending on the date that a modeler assigns to its "initial state", modelers will need to do a short historical run to bring their models to January 2015. Datasets for atmospheric and oceanic forcing are provided, based on the AOGCM used for the projections, but groups can choose their own datasets to reach the "start date".

Groups can reuse their initMIP initial state, or start from a new initial state, in which case they need to also perform the initMIP-Antarctica experiments.

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 the "reference period".

Control run and schemaric forcings

The control run ('ctrl') is needed to evaluate model drift. As in the initMIP-Antarctica setup (Seroussi et al., 2019), the control run is obtained by running the model forward without any anomaly forcing, such that whatever surface mass balance (SMB) and ocean forcing used in the initialization technique would continue unchanged. The other two initMIP experiments ('asmb' and 'abmb') also need to be repeated for each new model version.

Atmospheric forcing: SMB and temperature anomalies

ISMIP6 provides surface forcing datasets for the Antarctic ice sheet (AIS) based on CMIP AOGCM simulations. Two approaches are possible: using AOGCM output directly, or re-interpreting the GCM climates through higher-resolution regional climate models (RCMs). The later allows to capture narrow regions at the periphery of the ice sheet with large surface mass balance (SMB) gradients, which are not captured by CMIP5 AOGCMs, and is the technique used for the Greenland ice sheet. For the Antarctic CMIP5 based projections, RCMs are not used, so SMB anomalies based on AOGCM are directly applied.

For CMIP6, many of the AOGCMs 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.

For the ISMIP6 projections based on CMIP5 AOGCMs, the surface forcing consists of anomalies in SMB and surface temperature. 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. SMB and SMB anomalies are given in units of kg m-2 s-1 (water equivalent), and surface temperature in units of deg K. The following remarks refer mostly to SMB, but the same comments would generally apply to surface temperature as well.

The SMB and SMB anomalies are provided on the ISMIP6 standard ice sheet grid for Antarctica. ISMs then horizontally interpolate the anomaly forcing from the standard grid to their native grids. Before applying SMB anomalies, ISMs need to be initialized by applying a baseline SMB (either a time series or a climatology). ISMIP6 provides SMB climatologies for the reference period (January 1995 to December 2014) from the same models used to provide the anomalies. ISMs can use these climatologies for spin-up, if desired, but are free to use their own preferred SMB forcing. Let SMB_ref(x,y) denote the SMB used to initialize the ISM. If a time-dependent SMB is used for spin-up, then SMB_ref(x,y) is the average over the reference period.

ISMIP6 provides yearly averaged surface mass balance anomalies, aSMB(x,y,t), along with its components (precipitation, evaporation and runoff). During the run, SMB is computed as:

SMB(x,y,t) = SMB_ref(x,y) + aSMB(x,y,t).

aSMB(x,y,t) is constant over the entire year and changes stepwise at the beginning of every year. To convert aSMB 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 datasets can be obtained via the ISMIP6 ftp server (email ismip6@gmail.com to obtain the login information) under: ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Projections/AIS/Atmosphere_Forcing

Files are provided for several resolutions (1 km, 2 km, 4 km, 8 km, 16 km, and 32 km). Modeling groups should use the resolution closest to their native grid to conservatively interpolate data to model (see Appendix 1, below).

Oceanic forcing: temperature, salinity, thermal forcing and melt rate parameterization

ISMIP6 provides datasets of extrapolated ocean "ambient" temperature (T), salinity (S) and thermal forcing (TF) from 1850-2100 that are appropriate for present and future ice-shelf cavities. These datasets originate from CMIP models and have been extrapolated under ice shelves, using rules that account for sills and troughs (Fig 2). The yearly anomalies are with respect to the reference climatology, which is also provided. The datasets are on the 8 km ISMIP6 Antarctic grid. More information on how the datasets were produced is available in the presentations and webinar that can be retrieved from: ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Projections/AIS/Ocean_Forcing or at https://github.com/xylar/ismip6-ocean-forcing

Ocean overview antarctica basin.png

Figure 2: Bathymetry and IMBIE2 basins (left) used in the sub-ice shelf extrapolation of ocean temperature (right).

ISMIP6 standard approach was developed by the Antarctic ocean focus group, and consist of two approaches for the parameterization of basal melt. The first approach, a non-local quadratic melting parameterization, is the preferred method for ISMIP6 simulations. However, an alternative (and easier to implement) takes the form of a local quadratic melting parameterization. Example codes for both parameterizations can be found in the "parameterizations" directory. In addition to the annual forcing datasets needed for use with the parameterizations, parameters needed to sample the uncertainty in the basal melt are also provided. The melt parameterizations, along with the motivation for the uncertainty parameter choices are described in ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Projections/AIS/Ocean_Forcing/parameterizations/melt_parameterization_ISMIP6.pdf

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.

Antarctic fracture

Surface melting can trigger ice shelf collapse (for example, the Larsen B ice shelf in the Antarctic Peninsula). This mechanism is separate from cliff-collapse, but is a precursor to cliff-collapse. Although the mechanisms for Larsen B-style ice shelf collapse are still poorly understood, ISMIP6 provides dataset for ice shelf collapse in the form of a time dependent mask. These datasets were derived from CMIP5 near surface air temperature (tas) following the method described in Trusel et al. (2015), which results in annual surface melt. For ISMIP6, Luke Trussel prepared the bias corrected annual surface melt, which were used to generate the masks. Ice shelves are assumed to collapse following a 10 year period with annual surface melt above 725 mm (Trusel et al., 2015). Some experiments require to model ice shelf collapse and the ISMIP6 masks provided should be used in this case. For the other experiments, ice shelf collapse should not be included. Models are free to decide on the appropriate method to simulate tributary glaciers' behavior following the collapse of ice shelves.

The datasets can be obtained from: ftp://cryoftp1.gsfc.nasa.gov/ISMIP6/Projections/AIS/Ice_Shelf_fracture

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 use one of the two ocean parameterizations (non-local or local) proposed

• 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

Seroussi, H., Nowicki, S., Simon, E., Abe Ouchi, A., Albrecht, T., Brondex, J., Cornford, S., Dumas, C., Gillet-Chaulet, F., Goelzer, H., Golledge, N. R., Gregory, J. M., Greve, R., Hoffman, M. J., Humbert, A., Huybrechts, P., Kleiner, T., Larour, E., Leguy, G., Lipscomb, W. H., Lowry, D., Mengel, M., Morlighem, M., Pattyn, F., Payne, A. J., Pollard, D., Price, S., Quiquet, A., Reerink, T., Reese, R., Rodehacke, C. B., Schlegel, N.-J., Shepherd, A., Sun, S., Sutter, J., Van Breedam, J., van de Wal, R. S. W., Winkelmann, R., and Zhang, T., initMIP-Antarctica: An ice sheet model initialization experiment of ISMIP6, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-271, in review, 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 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.

TO DO SOPHIE: DOUBLE CHECK CMIP REQUIREMENTS AND UPDATE EVERYTHING BELOW IF NECESSARY, ALSO CHECK FILE NAMES CONVENTIONS, INCLUDE GUIDANCE CMIP MODEL REGISTRATION

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

TABLE WILL BE MADE WHEN SUBMISSIONS ARE COMING IN

Contributors Model Group ID Group

Model Characteristics

TABLE WILL BE MADE WHEN SUBMISSIONS ARE COMING IN