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The atmospheric model in CESM1 is the Community Atmosphere Model (CAM5).
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:
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.
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 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 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).
Near-modern simulations can typically use the default pre-industrial atmosphere initialization.
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.
|ch4vmr||CH4 volume mixing ratio|
|n2ovmr||N2O volume mixing ratio|
|f11vmr||CFC11 volume mixing ratio|
|f12vmr1||CFC12 volume mixing ration|
1 For Pre-Industrial paleo experiments, F11VMR and F12VMR should be set to 0.