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.

Definesurf [bnd_topo]

Definesurf is a fortran-based tool designed to create a new, smooth topography input file at the resolution of your model simulation.  Near-modern paleo modelers use definesurf to map modified (e.g., by ice sheets) high resolution topography onto the CESM grid resolution of their model simulations.   The fortran code also smooths the high resolution topography.  

Usage: definesurf -remap -t topofile -g gridfile -l landm_coslat.nc outfile

FV 0.9x1.25 e.g.: 

definesurf -remap -t topo_lgm21ka_10min.sm9.nc -g fv0.9x1.25.nc -l landm_coslat.nc topo_lgm21ka_remap.nc

FV 1.9x2.5 e.g.: 

definesurf -remap -t topo_lgm21ka_10min.sm9.nc -g fv_1.9x2.5.nc -l landm_coslat.nc topo_lgm21ka_remap.nc

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.

Variable         

Description
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.