Change a tuning parameter in CAM

Exercise: Modify a parameter, dcs

In the tuning lecture, we talked about the parameter dcs: https://www.cesm.ucar.edu/sites/default/files/2025-07/2025cesmthannay.pdf

Create a case called b1850_dcs using the compset B1850 at f19_g17 resolution.

Locate where to change the parameter Dcs and change from the default value:
dcs = 200.D-6
to
dcs = 500.D-6

Make a 1-month run.

Click here for hints

• The trick is locating where to change dcs

• Compare this run to the first run you did today: b1850_high_freq.

• You can use ncdiff and ncview to look at the difference between the 2 runs. For instance:

ncdiff /glade/derecho/scratch/$USER/archive/b1850_dcs/atm/hist/b1850_dcs.cam.h0.0001-01.nc /glade/derecho/scratch/$USER/archive/b1850_high_freq/atm/hist/b1850_high_freq.cam.h0.0001-01.nc diff.nc

ncview diff.nc

• Analyze how this does this affect the LWCF ?

Click here for the solution

# Create a new case

Create a new case b1850_dcswith the command:

cd /glade/u/home/$USER/code/my_cesm_code/cime/scripts/
./create_newcase --case ~/cases/b1850_dcs --compset B1850 --res f19_g17 

# Setup

Invoke case.setup with the command:

cd ~/cases/b1850_dcs
./case.setup

# Customize namelists

Locate the variable to modify dcs. (Locating variables requires a learning curve. A good place to start is to look at the Variable namelist documentation.

The variable is: micro_mg_dcs.
By default this variable is set to: micro_mg_dcs = 200 microns You want to set it to: micro_mg_dcs = 500 microns

Edit the file user_nl_cam and add the lines:

micro_mg_dcs = 500.D-6 

# Set run length

Change the run length:

./xmlchange STOP_N=1,STOP_OPTION=nmonths

# Change the job queue and account number

If needed, change job queue and account number.
For instance, to run in the queue tutorial and the project number UESM0014, use the command:

./xmlchange JOB_QUEUE=tutorial,PROJECT=UESM0014 --force

# Build and submit

Build the model and submit your job:

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

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# Look at your solution

• When the run is completed, compare this run to the first run: b1850_high_freq.

• Create a file with the difference in LWCF between b1850_dcs and b1850_high_freq.

• You can use ncdiff and ncview to look at the difference between the 2 runs.

• For instance, use ncdiff to create a file with the difference between the 2 runs:

cd /glade/derecho/scratch/$USER/archive/b1850_dcs 
ncdiff /glade/derecho/scratch/$USER/archive/b1850_dcs/atm/hist/b1850_dcs.cam.h0.0001-01.nc /glade/derecho/scratch/$USER/archive/b1850_high_freq/atm/hist/b1850_high_freq.cam.h0.0001-01.nc diff.nc

• Use ncview to look at the file you created.

ncview diff.nc

#How does this affect the LWCF ?

Dcs = Threshold diameter to convert cloud ice particles to snow

Figure: Representation of Dcs.

We increased Dcs from 200 microns to 500 microns. We should have more ice cloud and the LWCF should be larger.

The field LWCF difference looks like

Figure: Difference of LWCF for micro_mg_dcs = 200.D-6->500.D-6

We can see that the change in Dcs affects the tropics where LWCF is large. However, 1-month run is too short to look at robust statistics.