Deep Time Climate Simulations | Atmosphere

As a deep time modeler, you are responsible for creating a new initial condition file (cami) and using the CAM namelist functionality to specify trace gas constituents and the solar constant appropriate for you time period. Instruction on how to change the namelist parameters is discussed in Section 6.2.2.  For further details on, please refer to the CESM User Guides: http://www2.cesm.ucar.edu/

cami_create_ccsm.csh

  • ccsm_cami_create_ccsm3.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 tool used below computes a cosine weighted equator-pole latitudinal temperature distribution based on the user-specified polar and equator temperature estimates. Default values for these are currently 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 cami_create_ccsm.csh drives an NCL script called ccsm_cami_create_ccsm3.ncl.  You will need your topobathy.nc file as well as a cami master (skeleton) file for your resolution. The scripts and master file are provided in setup_tools.tar.   The master file is simply used as a placeholder for initial variables names and will overwrite all initial data with the basic physical state computed by the NCL program. Therefore, if a master (skeleton) file for your resolution is not provided in setup_tools.tar, you can use any cami file from the CCSM3 distribution or from a previous CAM run.

 You will need to edit the environmental variables in the script to values appropriate for your case. 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.
    • abs_ems_factors_fastvx.c030508.nc  [CCSM3.0 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.
    • pcmdio3.r8.64x1_L60_clim_c970515.nc  [CCSM3.0 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. 
    •  AerosolOptics_c040105.nc    [CCSM3.0 DEFAULT]
    •  AerosolMass_V_48x96_clim_c031029.nc [CCSM3.0 DEFAULT]

 

Namelist adjustments for physical forcing:

Group A:  Solar constant and trace gases

You will need to determine appropriate values for the solar constant and for trace gas concentrations. Note that orbital parameters are set in the coupler for a fully coupled CESM1 experiment, and not in CAM. This is different from CAM stand-alone or CAM-SOM runs.

 

VariableDescription
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