Near-modern CESM1.0 | Atmosphere

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

Changes to topography

If your scientific problem 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 must 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.

Topographic boundary conditions (PHIS, SGH, SGH30, LANDFRACT, LANDM_COSLAT) are defined in the file referenced by the  'topo_bnd' namelist variable in user_nl_cam.  SGH and SGH30 describe the surface roughness.

Smoothing topography

We recommend that you smooth your newly modified high resolution (10min) topography.  We provide a rudimentary smoothing program for this purpose:  smth9_topo.ncl.  The code allows you to set the number of smoothing passes (niter) you make over your initial topography, and produces a latitudinal cross section for comparison.

Definesurf [bnd_topo]

Definesurf is a fortran-based tool designed to create a new topography input file.  Near-modern paleo modelers use definesurf to map their newly modified high resolution topography onto the CESM grid resolution of their model simulations.    

Usage: definesurf -remap -t topofile -g gridfile -l outfile

FV 0.9x1.25 e.g.: 

definesurf -remap -t -g -l

FV 1.9x2.5 e.g.: 

definesurf -remap -t -g -l

If you have extended land over ocean points (e.g., over Hudson Bay), you will need to modify landm_coslat.ncl (use mod_landm_coslat.ncl).

Initial Conditions [cam.i]

Near-modern simulations can typically use the default pre-industrial atmosphere initialization.

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

You must assign appropriate values for the solar constant and for trace gas concentrations.

Note:  Orbital parameters (as opposed to the solar constant) are set in the coupler namelist for a fully coupled CESM1 experiment.


solar_constSolar constant
ch4vmrCH4 volume mixing ratio
n2ovmrN2O volume mixing ratio
f11vmrCFC11 volume mixing ratio
f12vmr1CFC12 volume mixing ration

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