Exercise 3: Change physics timestep

Exercise 3: Change physics timestep#

Exercise: Change physics timestep in CAM and CLM

Hybrid start a fully coupled run under pre-Industrial conditions with a resolution of ne16pg3_t201.

Use restart and initial files from the run b1850.initial at year 2. The restart files for this run are located in:

/glade/campaign/cesm/tutorial/tutorial_2026_archive/b1850.initial/rest/0002-01-01-00000

Change the physics timestep in the atmosphere and land to 1200 seconds (default is 1800). Run 5 days.

Note: Due to a small bug (one line of code) in the CESM3 tutorial tag, we must modify/copy one file in the code base to complete this exercise AND reset the wave time step to its original value (./xmlchange WAV_NCPL=48) after resetting the atmosphere timestep. You must copy this file before completing this exercise:

cp /glade/work/dbailey/cesm3_0_tutorial/components/cmeps/cime_config/runseq/runseq_general.py /glade/u/home/$USER/code/my_cesm_code/components/cmeps/cime_config/runseq/runseq_general.py
Hint!

The atmosphere physics timestep is determined by the base period of coupling NCPL_BASE_PERIOD and the coupling frequency ATM_NCPL.
Find out their default values using xmlquery.

Click here for the solution

  1. From the script directory, create your case:

    cd /glade/u/home/$USER/code/my_cesm_code/cime/scripts/
    ./create_newcase --case /glade/u/home/$USER/cases/b1850.timestep --res ne16pg3_t201 --compset B1850C_LTso --run-unsupported

  1. In your case directory, change run type and runtime variables:

    cd /glade/u/home/$USER/cases/b1850.timestep
    ./xmlchange RUN_TYPE=hybrid
    ./xmlchange RUN_REFCASE=b1850.initial,RUN_REFDATE=0002-01-01,GET_REFCASE=FALSE,JOB_WALLCLOCK_TIME=2:00:00
    ./xmlchange JOB_QUEUE=tutorial --force

  1. Change the physics timestep for CAM: (and WAV due to the bug)

    ./xmlchange ATM_NCPL=72
    ./xmlchange WAV_NCPL=48

(Why ATM_NCPL = 72? Do the math).

  1. Set up the case and position your initial and restart files in the $RUNDIR:

    ./case.setup
    cp /glade/campaign/cesm/tutorial/tutorial_2026_archive/b1850.initial/rest/0002-01-01-00000/*   /glade/derecho/scratch/$USER/b1850.timestep/run/

  1. Continue to build and submit:

    qcmd -- ./case.build
    ./case.submit

Remember that qcmd is used on Derecho only.