6. CICE Thickness Categories

The number of ice thickness categories affects ice model input files in three places:

  • $DNICECAT in the scripts
  • The source code module ice_domain_size.F90
  • The initial condition (restart) file in the input file directory

One must be very careful with changing the number of thickness categories as it impacts a number of places in the code. The number of ice thickness categories can be changed in $CASE/env_build.xml using the xml variable CICE_CONFIG_OPTS. One changes this by adding -ncat 5 to the variable CICE_CONFIG_OPTS. The default value is 5 categories. $DNICECAT is used to determine the CPP variable setting NICECAT in ice_domain_size.F90. More information on the CPP variables can be found here:

http://www.cesm.ucar.edu/models/cesm2/component_settings/cice_input.html

The information in the initial restart file is dependent on the number of ice thickness categories and the total number of layers in the ice distribution. An initial condition file exists only for the default case of 5 ice thickness categories, with four layers in each category. To create an initial condition file for a different number of categories or layers, these steps should be followed:

  • Set ncat to the desired number of categories in $CASE/env_build.xml.
  • Set the namelist variable dumpfreq = ’m’ in $CASE/user_nl_cice to print out restart files monthly.
  • Set the namelist variable ice_ic='default' in $CASE/user_nl_cice to use the initial conditions within the ice model.
  • Run the model to equilibrium.
  • The last restart file can be used as an initial condition file.
  • Change the name of the last restart file to iced.0001-01-01.$GRID.nc.
  • Copy the file into the input data directory or directly into the the executable directory.
  • There are a few restart files available in $DIN_LOC_ROOT/ice/cice.

$GRID is the name of the POP grid with resolution, $RES of 100x116 (gx3) and 320x384 (gx1) for low and medium resolution grids, respectively. Note that the date printed inside the binary restart file will not be the same as 0001-01-01. For coupled runs, $BASEDATE will be the starting o date and the date inside the file will not be used.

Note

To use one ice thickness category, the following changes will need to be made in the namelist and also adding -ncat 1 to CICE_CONFIG_OPTS.

, kitd          = 0
, kstrength     = 0

With these settings, the model will use the delta scheme instead of linear remapping and a strength parameterization based on open water area and mean ice thickness.