Change the initial conditions

Note, these instructions are only valid for the 2018 CESM2 release. If you wish to run the Held-Suarez configuration prior to the CESM2 release, please contact Isla Simpson (

The dry dynamical core can be run out of the box at three different resolutions. When using the FHS94 compset at these resolutions the initial conditions are set-up automatically. But users may wish to start from different initial conditions for a variety of reasons. NCL scripts that can be used to generate an initial condition file in the correct format can be downloaded here.

These routines can be used to generate the initial conditions for both the DABI test case and the Held-Saurez configuration. The main script is makeic.ncl and this makes use of various other functions and sub-routines to set up constants, resolutions and functional forms of the initial conditions. The option icase=1 can be used to set-up the 250K isothermal initial conditions for Held-Suarez. This script can be modified to alter the initial conditions or generate new initial conditions at different horizontal or vertical resolutions.

Alternatively, users who prefer to generate initial condition netcdf files using their language of choice can see the required format by setting up one of the default configurations and looking at the format of the default initial condition file that is used. This is the path to which the variable ncdata is set within $CASEDIR/Buildconf/camconf/atm_in.

In order to run with a user defined initial condition file, Step 1 of the Held-Suarez instructions should first be followed to set up a case in the directory $CASEDIR. Then at the configure stage (step 2), ncdata should be set to the location of the new initial condition file by placing the following in user_nl_cam


where myfilepath points toward the new initial condition file