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initMIP: Focus on initialization

Earlier large-scale Greenland ice sheet (GrIS) experiments e.g. those run during 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. The initMIP is the first in a series of ISMIP6 ice sheet model inter comparison activities and the design of the experiment was led by Heiko Goelzer.

initMIP 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

initMIP Experimental setup

Experiments are for the large scale GrIS and are designed to allow intercomparison between models of 1) the initial state itself and 2) the response in two 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)

The two forward experiments serve to evaluate the initialisation in terms of model drift (2a, ctrl) and response to a large perturbation (2b, asmb). For 2a, the models are run forward without any anomaly forcing, such that whatever surface mass balance (SMB) was used in the initialization 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 core experiment duration is set to 100 years.

Requirements for the 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

• 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 wiki for possible options:

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

The SMB anomaly can be obtained via the ISMIP6 ftp server (email to obtain the log in information), and was prepared by Heiko Goelzer. Modeling groups should use the 1km version to conservatively interpolate to their model native grid (see Appendix 1, below). Files of lower resolution (5km, 10km, and 20km) are provided for groups using the standard output grid (Bamber et al., 2001) as “native grid”. For 2b, the SMB anomaly is to be implemented as a time dependent function, that takes the form of a linear function which increases stepwise every full year (it is therefore independent of the time step in the model):

SMB(t) = SMB_initialization + SMB_anomaly * (floor (t) / 40); for t in year from 0 to 40

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 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.

initMIP 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

- ...


Bamber, J. L., Layberry, R. L., and Gogineni, S.: A new ice thickness and bed data set for the Greenland ice sheet 1. Measurement, data reduction, and errors, J. Geophys. Res.-Atmos., 106, 33773–33780 (2001).

Bamber, J.L., Griggs, J.A., Hurkmans, R.T.W.L., Dowdeswell, J. A., Gogineni, S. P., Howat, I., Mouginot, J., Paden, J., Palmer, S., Rignot, E., and Steinhage, D.: A new bed elevation dataset for Greenland. Cryosphere 7, 499–510 (2013).

Appendix 1 – Standard output grid definition and interpolation

All 2D data is requested on a standard regular grid with the following description. 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. The output should be submitted on a resolution adapted to the resolution of the model and can be 20 km, 10 km, 5 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 grid (1km, same as Bamber et al. 2013) to your native grid, and transform your model variables to the initMIP output grid (20 km, 10 km, 5 km, 1 km, Bamber et al. 2001), 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 methods. Examples of tools that have been tested by ISMIP6 members (along with tips) are available on the Regridding page.

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

File name convention for 2D fields:


File name convention for scalar variables:


File name convention for readme file:



<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:

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, and email for the user name and latest password. Note sftp does not work!

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.

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

The variable requested in the table below serves 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).

Note that there has been a few tiny changes since the October document. These changes are so that the data request for initMIP is consistent with the ISMIP6 data request, the later changed under the direction of the WIP/CMIP6. The old standard names are kept in italic. Note that the binary masks are no longer requested, but instead we ask for area_fractions

Furthermore, we now distinguish between state variables (e.g. ice thickness, temperatures and velocities) and flux variables (e.g. SMB). State variables should be given as snapshot information at the end of one year (scalars) and five year periods (2D), while flux variables are to be averaged over the respective periods.

Last update: 01 Feb 2016 by Heiko

Variable Dim Variable Name Standard Name Units Comment
2D variables requested as snapshots, every five years, starting at t=0
Ice thickness x,y,t lithk land_ice_thickness m The thickness of the ice sheet
Surface elevation x,y,t orog surface_altitude m The altitude or surface elevation of the ice sheet
Bedrock elevation x,y,t topg bedrock_altitude m The bedrock topography (unchanged in forward exps.)
Geothermal heat flux x,y hfgeoubed upward_geothermal_heat_flux_at_ground_level (was upward_geothermal_heat_flux_at_bedrock) W m-2 Geothermal Heat flux (unchanged in forward exps.)
Surface mass balance flux x,y,t acabf land_ice_surface_specific_mass_balance_flux kg m-2 s-1 Surface Mass Balance flux
Basal mass balance flux x,y,t libmassbf land_ice_basal_specific_mass_balance_flux kg m-2 s-1 Basal mass balance flux
Ice thickness imbalance x,y,t dlithkdt tendency_of_land_ice_thickness m s-1 dHdt
Surface velocity in x x,y,t uvelsurf land_ice_surface_x_velocity m s-1 u-velocity at land ice surface
Surface velocity in y x,y,t vvelsurf land_ice_surface_y_velocity m s-1 v-velocity at land ice surface
Surface velocity in z x,y,t wvelsurf land_ice_surface_upward_velocity (was land_ice_surface_z_velocity) m s-1 w-velocity at land ice surface
Basal velocity in x x,y,t uvelbase land_ice_basal_x_velocity m s-1 u-velocity at land ice base
Basal velocity in y x,y,t vvelbase land_ice_basal_y_velocity m s-1 v-velocity at land ice base
Basal velocity in z x,y,t wvelbase land_ice_basal_upward_velocity (was land_ice_basal_z_velocity) m s-1 w-velocity at land ice base
Mean velocity in x x,y,t 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 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 litempsnic temperature_at_ground_level_in_snow_or_firn (was land_ice_temperature_at_snow_or_firn_base) K Ice temperature at surface
Basal temperature x,y,t litempbot land_ice_basal_temperature K Ice temperature at base
Basal drag x,y,t strbasemag magnitude_of_land_ice_basal_drag Pa Magnitude of basal drag
Calving flux x,y,t licalvf land_ice_specific_mass_flux_due_to_calving (was land_ice_calving_flux) 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 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 sfrgrf 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 sftflf floating_ice_sheet_area_fraction 1 Fraction of grid cell covered by ice sheet flowing over seawater
Grounded ice mask NO LONGER REQUESTED x,y,t gismask grounded_ice_sheet_binary_mask 1 The binary mask is set to 1 where grounded ice sheet, 0 elsewhere
Ice shelf masks NO LONGER REQUESTED x,y,t fismask floating_ice_shelf_binary_mask 1 The binary mask is set to 1 where ice shelves (land ice floating over sea), 0 elsewhere
Ice sheet mask NO LONGER REQUESTED x,y,t istmask ice_sheet_binary_mask 1 The binary mask is set to 1 where ice sheet (grounded ice sheet and ice shelves), 0 elsewhere
Scalar outputs (time average, yearly). If possible, the t=0 value should contain a one year time average of the last year of the initialization.
Total ice mass t lim land_ice_mass kg spatial integration, volume times density
Mass above floatation t limnsw land_ice_mass_not_displacing_sea_water kg spatial integration, volume times density
Grounded ice area t iareag grounded_land_ice_area m^2 spatial integration
Floating ice area t iareaf floating_ice_shelf_area (was floating_land_ice_area) m^2 spatial integration
Total SMB flux t tendacabf tendency_of_land_ice_mass_due_to_surface_mass_balance kg s-1 spatial integration
Total BMB flux t tendlibmassbf tendency_of_land_ice_mass_due_to_basal_mass_balance kg s-1 spatial integration
Total calving flux t tendlicalvf tendency_of_land_ice_mass_due_to_calving kg s-1 spatial integration