Near-modern CESM1.0 | Atmosphere

The atmospheric model in CESM1 is the Community Atmosphere Model (CAM5). 

BND_TOPO

Topographic boundary conditions are defined in the file referenced by the  'topo_bnd' namelist variable in user_nl_cam.  The bnd_topo file can be produced using the fortran tool definesurf (see below).  

ncdata

Initial model state:  Near-modern and deep-time simulations can typically start from the default pre-industrial atmosphere initialization.

Changes to topography

If your science question requires a change to land surface elevation from present day (e.g., increasing the height of the Greenland Ice Sheet, lowering the Rocky Mountains, etc.) you need to create a new topography file for the atmosphere model that reflects these changes.   Surface topography in CESM is parameterized by the CAM variable PHIS (surface geo-potential [m2/s2]), which is defined by the relationship:

  • PHIS [m2/s2] = elevation[m] × gravity [m/s2]
  • PHIS ÷ 9.81 m/s2 = elevation [m]

Users are advised to create their new topography file by first creating an anomaly map of their time period relative to present day (ΔZ = topomy_TOPO_paleo– topomy_TOPO_present-day) and then adding this anomaly (ΔZ) to the CESM present-day base topography.

 

Definesurf

Definesurf is a fortran-based tool designed to create a new, smooth topography input file at the resolution of your model simulation that will be used for bnd_topo, defined above.  Variables output from definesurf include:  PHIS:  Surface topography in CESM is parameterized by the CAM variable PHIS (surface geo-potential [m2/s2]), which is defined by the relationship:  PHIS [m2/s2] = elevation[m] × gravity [m/s2].  SGH [m]:  standard deviation of surface elevation.  SGH30 [m]: a measure of surface roughness, estimated by definesurf from the variance of high resolution (10 min) input topography.  LANDFRACT:  ignored by CAM4 or CAM5; fractional land is defined by the coupler mapping files.  LANDM_COSLAT: CAM3/CAM4 scale factor for offshore aerosols; relict variable; no longer used in CAM5.  NOTE:  Definesurf adds the attribute: from_hires = "true" to all output variables, which used to force CESM to do additional spectral filtering at runtime.  This feature is no longer active in CESM1.0.

Near Modern Usage:  Near-modern paleo modelers can use definesurf to map topography modifications onto the CESM default grid.   The fortran code also smooths the high resolution topography.  Typical scientific applications might include incorporating ice sheets across N. America, or modifying the Greenland or Antarctic Ice Sheets.  If you are using CAM4, and have extended land over ocean points (e.g., over Hudson Bay), you will need to modify landm_coslat.ncl (use mod_landm_coslat.ncl).

Command line examples:

General Form: % definesurf -remap -t topofile -g gridfile -l landm_coslat.nc outfile.nc

Example 1:  FV 0.9x1.25 grid, Ross Ice Shelf turned ON

%  definesurf -remap -t topo_10min.nc -g fv_0.9x1.25.nc -l landm_coslat.nc topo_remap.nc

Example 2:  FV 1.9x2.5 grid, Ross Ice Shelf turned OFF

%  definesurf -remap -r -t topo_10min.nc -g fv_1.9x2.5.nc -l landm_coslat.nc topo_remap.nc

Deep Past Usage:  Deep-past paleo modelers can also use definesurf to map topography modifications onto the CESM default grid.  However, lacking the 10min high resolution topography and associated variance required by definesurf to create SGH and SGH30, we first create a downscaled 10min topography from whatever input scale topography is available.  Definesurf will produce SGH, but not SGH30, so we then approximate SGH30 using a global mean value of the present day SGH30/SGH ratio (0.16):  SGH30DeepTime = SGHDeepTime * 0.16

Step 1:  create 10min downscaled topography:  mk_definesurf_input_paleo.ncl
Step 2:  run definesurf with Ross Ice Shelf turned OFF:
% definesurf -remap -r -t topo_10min.nc -g fv0.9x1.25.nc -l landm_coslat.nc topo_remap.nc
Step 3:  add SGH30:  add_SGH30_paleo.ncl  (NOTE:  LANDM_COSLAT will not be written to the output file unless you set "write_landm_coslat = True" in add_SGH30_paleo.ncl).

 

CAM Namelist options [user_nml_cam]

This section describes the namelist parameters that control aspects of your physical boundary forcing.

A:  Solar constant and trace gases

Note:  Orbital parameters are set in the coupler namelist for a fully coupled CESM1 experiment.

Variable         

Description
solar_constSolar constant
co2vmrCO2 volume mixing ratio
ch4vmrCH4 volume mixing ratio
n2ovmrN2O volume mixing ratio
f11vmr1CFC11 volume mixing ratio
f12vmr1CFC12 volume mixing ration

1 For Pre-Industrial paleo experiments, F11VMR and F12VMR should be set to 0.

 

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]