Fixed-SST Aquaplanet

This document outlines the steps necessary to run CESM1 configured as an aquaplanet with prescribed (time invariant) sea-surface temperatures (SST). User-defined settings and code modifications will be reviewed. When CAM5 is the atmospheric physics package, there are also several options for aerosol, which are briefly described. Additional modifications, like the introduction of seasonality and sea-ice, are planned to be implemented in the near future but are not supported at this time. The most important part, though, is the recipe that allows the model to run in this configuration.

Assumptions

In setting up the following cases, we make some basic assumptions. First, the code that is being used as an example is CESM1.2.2, which is the latest public release as of writing. We have verified that these modifications work for this version of the code using the finite volume (FV) dynamical core; they should also work in similar versions and with the spectral element (SE) dynamical core. That being said, these modified cases have not been rigorously scrutinized, and there may be omissions in these instructions. If you find details that seem important, let us know and we will add them to this document. All of the following make use of the standard CESM script workflow, as covered in the CESM tutorial for example. We recommend checking out a copy of CESM1.2.2 into your personal workspace before proceeding further.

CAM4 Physics

Create the case from the CESM scripts directory
```
./create_newcase -case [CASEPATH]/cam4aquaFixSST -compset
FC4AQUAP -res f19_f19 -mach yellowstone
```
where CASEPATH is the location where the case will be created, i.e., $CASEROOT = [CASEPATH]/cam4aquaFixSST.
cd [CASEPATH]/cam4aquaFixSST
The user can change the prescribed (time invariant) SST distribution by modifying integer parameter sst_option in file
```
 $CESMROOT/models/atm/cam/src/utils/cam_aqua/ocn_comp.F90'
```
It is strongly recommended that a copy of that file first be placed into
```
$CASEROOT/SourceMods/src.aquap
```
prior to making modifications to it. Currently this is a hard-coded change, but for convenience we plan to make sst_option a namelist parameter for CESM2.
Change the use case, which contains a collection of important parameters settings:
```
./xmlchange -file env_run.xml -id CAM_NML_USE_CASE -val
 aquaplanet_cam4
```
Invoking "-compset FC4AQUAP" and setting "CAM_NML_USE_CASE" to aquaplanet_cam4 should set CAM_CONFIG_OPTS in $CASEROOT/env_build.xml to the correct entry:
```
-phys cam4 -ocn aquaplanet
```
It is recommended that the user check this.
Run setup:
```
'./cesm_setup'
```
Note that an arbitrary initial condition will currently be used. Discard first several months of simulation as "spin-up."
Build the case:
```
$CASENAME.build
```
Modify project numbers etc. and invoke the run script:
```
$CASENAME.submit
```

CAM5 Physics

To run prescribed-SST aquaplanet with CAM5 physics, additional minor adjustments are needed. Some options to use simplified version of CAM5 physics are listed in the command sequence example below (run on NCAR's Yellowstone machine on a 2-degree FV grid):

Move into the CESM scripts directory:
```
cd $CESMROOT/scripts'
```

Create the case:

./create_newcase -case [CASEROOT]/cam5aquaFixSST -compset
FC5AQUAP -res f19_f19 -mach yellowstone

where CASEPATH is the location where the case will be created, i.e.,

$CASEROOT =
[CASEPATH]/cam5aquaFixSST

Move to case directory:
```
cd [CASEROOT]/cam5aquaFixSST'
```
If desired, at this point the user can change the prescribed (time invariant) SST distribution by modifying integer parameter sst_option in file
```
$CESMROOT/models/atm/cam/src/utils/cam_aqua/ocn_comp.F90
```
It is strongly recommended that a copy of that file first be placed into
```
$CASEROOT/SourceMods/src.aquap
```
prior to making modifications to it. Currently this is a "hard-coded" change, but for convenience we plan to make sst_option a namelist parameter for CESM2.
OPTIONAL: Modify tracer_data.F90 and customize micro_mg1_5.F90 and locate in
```
$CASEROOT/SourceMods/src.cam
```
(See below.)
Run setup:
```
./cesm_setup'
```

Add the following to

$CASEROOT/user_nl_cpl

  orb_eccen = 0.
  orb_obliq = 0.
  orb_mvelp = 0.
  orb_mode  = 'fixed_parameters'

Add the following to

$CASEROOT/user_nl_cam

ncdata =
'/glade/u/home/benedict/ys/datain/ape.IC.2deg.0003-01-01-00000.nc'

Modify

$CASEROOT/env_build.xml

to reflect the desired CAM configuration changes:

./xmlchange -file env_build.xml id CAM_CONFIG_OPTS -val "-phys cam5 -ocn aquaplanet
-microphys mg1.5"

Builde the case:
```
$CASENAME.build
```

Modify runtime variables:

./xmlchange -file env_run.xml -id CAM_NML_USE_CASE -val aquaplanet_cam5'

Modify batch script, as desired, and submit job.

Aerosol

What aerosol should occupy the aquaplanet's atmosphere? It's arbitrary, but the decision can affect some aspects of the simulation. With CAM4 physics, there is a more well-defined convention, which is that aerosols do not interact with radiation; combine this with there being no interaction between aerosol and clouds in CAM4, and the result is a "no aerosol" aquaplanet. This is the recommendation for CFMIP aquaplanet experiments (Webb et al. 2016), and follows from the APE (Williamson et al. 2012). With CAM5, there are indirect effects of aerosol on clouds, and there is not a standard way to "turn off" aerosol effects.

As discussed in Medeiros et al. (2016), one option is to minimize aerosol effects by removing aerosol emissions and by using constant droplet and ice number concentrations in the microphysics. The emissions can be removed by removing the netCDF files that supply the emissions boundary conditions. The droplet and crystal number concentrations can be specified by modifying micro_mg1_5.F90.