IMBIE SMB Model Intercomparison
This page describes the experimental protocol for the IMBIE SMB model inter-comparison for the IMBIE 2023 assessment. This model inter-comparison aims at (i) informing how the spread in SMB estimates contributes to the (dis)agreement between satellite-based estimates of ice sheet mass balance and (ii) understanding the sources of variations between existing SMB models to reduce uncertainties in current SMB modelled estimates.
This exercise is coordinated by Dr Charles Amory (charles.amory@univ-grenoble-alpes.fr).
Experimental Protocol
Here, we outline the experimental protocol that will be followed to analyse and compare the different SMB submissions received:
Use of a common grid for intercomparing SMB models
The different SMB estimates will be interpolated from the native grid on which they were calculated to a common output grid. We will be using the grid defined in ISMIP6 as a common grid. Several different grid resolutions (1 km, 2 km, 4 km, 8 km, 16 km, and 32 km) are available and we will pick the finest grid resolution possible to avoid degrading the quality of the SMB submissions received as horizontal resolution is likely to play an important role in differences between SMB models in mountainous regions such as the Antarctic Peninsula.
Interpolation method
To perform the interpolation, we will use a conservative interpolation method. We will be regridding the input SMB datasets with CDO, similarly to what has been done in ISMIP6. To perform this regridding, we ask participants to submit an input grid description file along with their SMB estimates (see below for a full description of the input grid description file required).
SMB ensemble comparison
We will generate the mean and standard deviation of the SMB ensemble and will assess differences between SMB models by examining differences across different regions using Rignot basin delineations and differences between total SMB and its components.
Participation Conditions
To participate in the IMBIE SMB model inter-comparison, participants are required to contribute a SMB dataset derived from a model that (i) assimilates meteorological observations, (ii) performs continuous runs, and (iii) includes a treatment of snow, surface melting, and refreezing processes using the standard format defined below.
Standard Data format
We are requesting SMB participants to submit:
- a grid description file
- a gridded time-series of SMB and its components at annual resolution (or at finer temporal resolution if available).
We have defined standard file formats to simplify data collection and post-processing. Please only submit SMB model estimates and grid description files using this format. Data submitted using other formats cannot be accepted. The file format is netCDF.
Please make sure that your files include the following global attributes, dimensions, and variables:
Grid description file
For your model grid description file, please include the latitude and longitude of the centre and four corners of the grid cells (required to perform the conservative regridding procedure).
| Variable | Dimension | Variable name | Standard_name | Long_name | Units | Comment |
| Latitude – centre | (x,y) | lat | latitude | grid centre latitude | degrees_north | additional attribute
bounds = lat_bnds |
| Longitude – centre | (x,y) | lon | longitude | grid centre longitude | degrees_east | additional attribute
bounds = lon_bnds |
| Latitude – corner | (x,y,nv4) | lat_bnds | – | grid corner latitude | degrees_north | Mind the orientation of the bounds depending on your coordinate system |
| Longitude – corner | (x,y,nv4) | lon_bnds | – | grid corner longitude | degrees_east | Mind the orientation of the bounds depending on your coordinate system |
| dummy variable | (x,y) | dummy | – | dummy variable | 1 | required for regridding |
Please name your grid description file as follows:
- For Antarctica: grid_model_AIS_nc
- For Greenland: grid_model_GrIS_nc
where model is the name of your model and reso is the horizontal resolution of your model in meters.
For example, for submitting a grid description file for outputs from MAR in Antarctica run at 35000 m, the corresponding grid file will be named: grid_MAR_AIS_35000m.nc
You can find an example of a model grid description file here: grid_MAR_AIS_35000m.nc
SMB and its components
Please provide the ice mask (islands excluded) as an ice fraction between 0 and 1, and SMB and its components over land only (with ocean masked) expressed in kg m-2 t-1. SMB components must include precipitation (ideally decomposed into snowfall and rainfall), sublimation and runoff. When appropriate, additional SMB components such as drifting snow sublimation or snow erosion should be provided in the same format.
| Variable | Dimension | Variable name | Standard name | Units | Comment |
| Time | (t) | time | time | months since 1900-01-15 12:00:00
or years since 1900-06-15 12:00:00 |
Please see our recommendations below |
| SMB | (x,y,t) | smb | surface mass balance | kg m-2 t-1 | Make sure that your netcdf files include the attributes _FillValue and missing_values |
| Snowfall | (x,y,t) | sf | snowfall | kg m-2 t-1 | Make sure that your netcdf files include the attributes _FillValue and missing_values |
| Rainfall | (x,y,t) | rf | rainfall | kg m-2 t-1 | Make sure that your netcdf files include the attributes _FillValue and missing_values |
| Surface sublimation | (x,y,t) | su | sublimation | kg m-2 t-1 | Make sure that your netcdf files include the attributes _FillValue and missing_values |
| Runoff | (x,y,t) | ru | runoff | kg m-2 t-1 | Make sure that your netcdf files include the attributes _FillValue and missing_values |
| Ice mask | (x,y) | ice | ice mask | – | Make sure that your netcdf files include the attributes _FillValue and missing_values |
You can find an example of a SMB file here: SMB_MAR_testcase
Recommendations and useful links for preparing your submission
To define your time variable
The participants can submit monthly or annual SMB estimates. We recommend to use a standard calendar and set the reference time to 1900.
For monthly estimates, we define the time units as the number of months since January 1900. You can use the following command to define your time vector:
cdo setreftime,1900-01-15,12:00:00,1month -settaxis,YYYY-MM-15,12:00:00,month -setcalendar,standard infile.nc outfile.nc
where YYYY-MM-15 is the starting date of your monthly time-series. This will give you the corresponding time as the number of months since January 1900 (for instance time = 0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, etc for a monthly time-series starting from January 1900). Note that we register to the middle of the month (15th) for any monthly estimate.
For annual estimates, we define the time units as the number of years since January 1900. You can use the following command to define your time vector:
cdo setreftime,1900-06-15,12:00:00,1year -settaxis,YYYY-06-15,12:00:00,month -setcalendar,standard infile.nc outfile.nc
where YYYY-06-15 is the starting date of your annual time-series. This will give you the corresponding time as the number of years since January 1900 (for instance time = 0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, etc for an annual time-series starting from 1900). Note that we register to the middle of the year (June 15th) for any annual estimate.
CDO regridding
We recommend that participants perform a regridding test of their native dataset to the ISMIP6 grid to ensure that the input grid description file they provide is accurate and sufficient to perform the regridding. The output grid description file can be downloaded here.
Using your input GDF file and the prescribed output GDF file, the CDO regridding can be performed using the following command line:
cdo -v remapycon,gdf_outfile.nc -setgrid,gdf_infile.nc infile.nc outfile_ycon.nc
More information on the CDO regridding procedure can be found on the CliC Wiki page.
Example files
Please refer back to the example files provided to ensure that your contribution can be included and do not hesitate to contact us if you have any questions. Please also follow the metadata information in these example files.
The example files to help you prepare your submission can be found at the following links:
- Input grid description file: grid_ISMIP6_AIS_10000m.nc
- Output grid description file: grid_MAR_AIS_35000m.nc
- SMB file: SMB_MAR_testcase
To test the regridding procedure with those example files, you can use the following command line:
cdo remapycon, grid_ISMIP6_AIS_10000m.nc -setgrid,grid_MAR_AIS_35000m.nc SMB_MAR_testcase.nc outfile_ycon.nc