CESM2-CMIP5 Forcing Experiment Design

Forcing Dataset Description

The forcing is a hybrid combination of the CESM1 and CESM2 forcing:

• The ozone, stratospheric aerosols, nitrogen deposition, land surface datasets (offline land BGC spin-up) and the land use change is similar to the CESM1 forcing
• The aerosol emission is comparable with the CESM1 forcing but it includes extra datasets to be compatible with MAM4
• The dust prognostic follow the CESM2 forcing

Namelist comparison

A comparison of the namelists for CESM1, CESM2 and CEMS2-CMIP5 forcing is available on Github

## Download the release-cesm2.1.3 tag
git clone -b release-cesm2.1.3 https://github.com/ESCOMP/CESM.git my_cesm_sandbox
cd my_cesm_sandbox

# Edit the file Externals.cfg to modify the clm tag
# In the [clm] section, replace the line:
tag = release-clm5.0.30
# with
tag = branch_tags/cmip5.n03_release-clm5.0.30

## Check out the externals 
./manage_externals/checkout_externals 

CESM Tag and Case Setup

The model code and case setups for the CESM2-CMIP5 forcing experiments are available at:
https://github.com/NCAR/CESM2-CMIP5

Tuning

Changing the forcing in CESM2 led to a slight radiative imbalance at the top of the model (RESTOM) in the pre-industrial control. To achieve a radiative balance (and prevent temperature drifts over time) in the pre-industrial control, we adjusted a tuning parameter:

clubb_gamma_coef = 0.318

Then, we used that value of the clubb_gamma_coef parameter for the historicals and RCP8.5 runs.

As reference, the default value in CESM2 is:

clubb_gamma_coef = 0.308