Deep Time CESM1 | Atmosphere

As a deep time modeler, you are responsible for creating a new initial condition file (cam.i) which you will point to in the user_nml_cam namelist.  This is where you can also specify trace gas constituents and the solar constant appropriate for you time period. The CESM User Guide describes the user namelist functionality in CESM1.

run2create_cesm1_cami.csh

  • run2create_cesm1_cami.csh
  • create_cesm1_cami.ncl
  • inputs:  topobathy.nc, cami_skeleton_atm_resolution.nc
  • output:  cami_mycase.nc

We assume that for deep time periods, atmospheric initial conditions are unknown. Given this assumption, we attempt to provide a basic physical atmospheric state suitable for initialization that allows the atmospheric model to spin-up during the first few decades of integration.  The tools compute a cosine weighted equator-pole latitudinal temperature distribution based on user-specified polar and equator temperature estimates. Default values are set at 28oC (equator) and 12oC (pole). Surface pressure and the surface geopotential are computed based on the topography (topobathy.nc), and the zonal velocity is based on the thermal wind balance.

The c-shell script run2create_cesm1_cami.csh drives an NCL script called create_cesm1_cami.ncl which requires  topobathy.nc as well as a cami skeleton file at your atm/land resolution. The skeleton file is simply used as a template for the required cami variables names and will be overwritten by the  basic physical atmospheric state computed by create_cesm1_cami.ncl.  You can use any cesm1 cami file from a previous CAM run as a template for this process.

Edit the environmental variables in run2create_cesm1_cami.csh to your local environment. Details on this script can be found in the comments of the script itself. 

Other atmoshere forcing files:

  • Absorption/Emissivity:
  • The present day absorption/emissivity forcing dataset was built with wide constraints and is therefore flexible and can be used for paleoclimate cases.
    •   [CESM1.0 DEFAULT]
  • Ozone:
  • Typically, present day or pre-industrial ozone mixing ratio boundary forcing files are used for paleoclimate cases. Choice of dataset will depend on your control experiment.
    •   [CESM1.0 DEFAULT]
  • Aerosols:
  • Although it is necessary to input present day or pre-industrial aerosol boundary forcing files into the model, it is recommended that deep time paleoclimate researches use the namelist functionality to flag CAM to internally compute aerosol optical depths. 
    •   [CESM1 DEFAULT]
    •   [CESM1 DEFAULT]

 

Namelist adjustments for physical forcing:

Group A:  Solar constant and trace gases

Define appropriate values for the solar constant and for trace gas concentrations for your time period. Orbital parameters are set in the coupler.

Variable     Description
SCON Solar constant
CO2VM2 CO2 volume mixing ratio
CH4VMR CH4 volume mixing ratio
N20VMR N2O volume mixing ratio
F11VMR1 CFC11 volume mixing ratio
F12VMR1 CFC12 volume mixing ration

 1 For deep time paleo experiments, F11VMR and F12VMR should be set to 0.

Group B: Aerosol Optical Depth

CAM radiation code requires a boundary forcing dataset for aerosol mass mixing ratios and aerosol optical properties. Because this is unknown for many paleoclimate cases, we must flag the code to use the CCM3.6 method for computing aerosol optical depths, which does not require spatial knowledge for aerosol mixing ratios. This method will compute a uniform optical depth across all grid points. Group B parameters must all be specified in the namelist. 

A tauback value of 0.28 is equivalent to CCM3.6 default value of TAUVIS =0.14. The various scaling parameters must be set to zero to render the present day aerosol mixing ratio values in the code obsolete and allow the uniform optical depth to be used.

Default values for all namelist parameters can be found in the CAM3 User’s Guide.

VariableDescriptionDeep time
TAUBACK uniform optical depth 0.28
CARSCL carbon scaling 0.0
DUSTCL dust scaling 0.0
SSLTSCL sea salt scaling 0.0
SULSCL sulfate scaling 0.0