Change a tuning parameter in CAM

Exercise: Modify a parameter, `micro_mg_dcs`

In the tuning lecture (https://www.cesm.ucar.edu/sites/default/files/2025-07/2025cesmthannay.pdf), we discussed the parameter Dcs, which controls the threshold diameter at which cloud ice particles are converted to snow. Changing this parameter affects the amount of cloud ice and therefore influences cloud radiative forcing.

For this exercise, we will use:

• Compset: B1850C_LTso

• Resolution: ne16pg3_t201 (Because the resolution ne16pg3_t201 not yet supported at this stage, use the option: --run-unsupported)

To simplify the analysis, we will also interpolate all CAM output to a regular 2° × 2° latitude–longitude grid.

Create a case called b1850_dcs.

Locate the namelist variable controlling Dcs and change it from its default value:

micro_mg_dcs = 450.D-6

to

micro_mg_dcs = 800.D-6

Run the model for one month.

Click here for hints

Where is the Dcs parameter?

The variable you are looking for is:

micro_mg_dcs

It can be modified directly in user_nl_cam.

How do I compile?

qcmd -- ./case.build

Compare this run with the interpolation exercise

Compare this run with the case you created in the previous exercise:

b1850_interp

Both cases produce output on a regular 2° × 2° latitude–longitude grid, making them easy to compare using NetCDF tools.

You can use ncdiff to compute differences:

ncdiff /glade/derecho/scratch/hannay/archive/b1850_dcs/atm/hist/b1850_dcs.cam.h0a.0001-01.nc \
/glade/derecho/scratch/$USER/archive/b1850_interp/atm/hist/b1850_interp.cam.h0a.0001-01.nc \
diff.nc

Then visualize the result:

ncview diff.nc

How does changing Dcs affect the LWCF field?

Click here for the solution

# create new case

cd /glade/u/home/$USER/code/my_cesm_code/cime/scripts/

./create_newcase \
  --case ~/cases/b1850_dcs \
  --compset B1850C_LTso \
  --res ne16pg3_t201 \
  --run-unsupported

# Setup

cd ~/cases/b1850_dcs
./case.setup

# Customize the CAM namelist

Edit user_nl_cam and add:

micro_mg_dcs = 800.D-6

interpolate_output = .true.
interpolate_nlat = 91
interpolate_nlon = 180

The first line changes the cloud microphysics parameter Dcs.

The remaining lines interpolate all CAM history output from the native spectral-element grid to a regular 2° × 2° latitude–longitude grid.

# Set the run length

./xmlchange STOP_N=1,STOP_OPTION=nmonths

# Change the job queue and project number

If needed:

./xmlchange JOB_QUEUE=tutorial,PROJECT=UESM0016 --force

Use the project number provided for the tutorial.

# Build and submit

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

# Analyze the results

When the run completes, compare it with the b1850_interp case.

Create a file containing the differences:

cd /glade/derecho/scratch/$USER/archive/b1850_dcs

ncdiff \
/glade/derecho/scratch/$USER/archive/b1850_dcs/atm/hist/b1850_dcs.cam.h0a.0001-01.nc \
/glade/derecho/scratch/$USER/archive/b1850_interp/atm/hist/b1850_interp.cam.h0a.0001-01.nc \
diff.nc

Visualize the differences:

 module load ncview
 ncview diff.nc

Display the LWCF field and examine how it changes.

# Discussion

Dcs is the threshold diameter at which cloud ice particles are converted to snow.

Increasing

micro_mg_dcs = 450.D-6

to

micro_mg_dcs = 800.D-6

allows cloud ice particles to grow larger before being converted to snow. As a result, more cloud ice tends to remain suspended in the atmosphere, which generally increases the Longwave Cloud Forcing (LWCF).

The largest changes are typically found in the tropical deep convective regions, where high ice clouds are abundant.

Keep in mind that this simulation is only one month long. Although you should see differences in the LWCF field, a one-month simulation is too short to draw statistically robust conclusions about the climate response.

#How does this affect the LWCF ?

Dcs = Threshold diameter to convert cloud ice particles to snow

Figure: Representation of Dcs.

We increased Dcs from 450 microns to 800 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 = 450.D-6->800.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.