1.6.2. Running a single point using global data

subset_data enables you to run the model using global datasets, but just picking a single point from those datasets and operating on it. It can be a very quick way to do fast simulations and get a quick turnaround.

1.6.2.1. Subset the data

For single-point cases, you need to subset a surface dataset and (optionally) DATM data. The Python script to subset this data can be found in the CTSM repository at tools/site_and_regional/subset_data.

Note that you will need to have a python environment set up that includes the packages scipy, xarray, and numpy. If you have conda or miniconda installed, you can create a conda environment for this and other CTSM python tools using the script py_env_create at the top level of your CTSM checkout.

To subset surface data and climate forcings (DATM) for a single point, use the command:

tools/site_and_regional/subset_data point \
   --lat $my_lat --lon $my_lon --site $my_site_name \
   --create-surface --create-datm \
   --datm-syr $my_start_year --datm-eyr $my_end_year \
   --create-user-mods --outdir $my_output_dir

  • $my_lat: latitude of point, must be between -90 and 90 degrees. E.g., Boulder, CO, USA: 40.

  • $my_lon: longitude of point, must be between 0 and 360 degrees. E.g., Boulder, CO, USA: 55.

  • $my_site_name: name of site, used for file naming

  • $my_start_year: start year for DATM data to subset, default between 1901 and 2014

  • $my_end_year: end year for DATM data to subset, default between 1901 and 2014; the default CRUJRA2024 DATM data ends in 2023, while the old default GSWP3 ends in 2015; see note below about switching the default DATM data

  • $my_output_dir: output directory to place the subset data and user_mods directory. This should be something specific to just your data for $my_site_name.

You can also have the script subset land-use data. See the help (tools/site_and_regional/subset_data --help) for all argument options.

Note that this script defaults to subsetting specific surface, domain, and land-use files and the CRUJRA2024 DATM data, and can currently only be run as-is on Derecho. If you’re not on Derecho, use the ``–inputdata-dir`` to specify where the top level of your CESM input data is. Also, to subset GSWP3 instead of CRUJRA2024 DATM data, you currently need to hardwire datm_type = “datm_gswp3” (instead of the default “datm_crujra”) in python/ctsm/subset_data.py.

The --create-user-mods command tells the script to set up a user mods directory in your specified $my_output_dir and to specify the required PTS_LAT and PTS_LON settings. You can then use this user mods directory to set up your CTSM case, as described below.

1.6.2.2. Create the case

You can use the user mods directory set up in the previous subset data step to tell CIME/CTSM where your subset files are located.

cime/scripts/create_newcase --case $my_case_name --res CLM_USRDAT \
   --compset $compset --run-unsupported \
   --user-mods-dirs $my_output_dir/user_mods

  • $my_case_name: the path of the case directory you want to create

  • $compset: the compset you would like to use (for example, I2000Clm60Bgc)

  • Note the use of $my_output_dir/user_mods which is the user_mods/ directory that the subset data script set up within your specified $my_output_dir.

Note that ./case.setup on Derecho will automatically set queue to develop and walltime to one hour. You might need a longer walltime, but the maximum walltime for develop is one hour. To change it to two hours on Derecho:

./xmlchange --subgroup case.run JOB_QUEUE=main,JOB_WALLCLOCK_TIME=2:00:00