Near-modern CESM1.0 | Atmosphere

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

bnd_topo

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.

ncdata:  Initial state

Near-modern simulations can typically use 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 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.

TOOLS

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

Usage:

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

FV 0.9x1.25 e.g.:

% definesurf -remap -t topo_10min.nc -g fv0.9x1.25.nc -l landm_coslat.nc topo_remap.nc

FV 1.9x2.5 e.g.: 

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

 

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.