To simulate changes in Earth orbital position, the model calculates the eccentricity, obliquity and precession based on Berger et al., 1993. Quaternary: The model sets these parameters automatically using a variable called orb_iyear, which is set in the coupler namelist (user_nl_cpl) and is defined in years before 1950. Deep Time: Since we have no definitive estimate of orbital variations beyond ~1Ma BP, eccentricity, obliquity, and the moving vernal equinox must be defined explicitly for pre-Quaternary experiments. The CESM default orbital year for the B1850C5CN pre-industrial compset is 1850.
Quaternary orbital settings
AD: (e.g., 1800, 1900, 1950, 1990, 2000, etc.) are expressed in calendar years.
BC: (e.g., 10ka, 130ka, 506ka) are expressed in years relative to 1950:
506ka BP: orb_iyear = (1950 - 506,000) = -504050
4ka BP: orb_iyear = (1950 – 4000) = -2050
Coupler Namelist (user_nl_cpl)
Example for the Last Glacial Maximum (21ka BP)
orb_iyear = -19050
orb_mode = 'fixed_year'
cesm1.1 -- http://www.cesm.ucar.edu/models/cesm1.1/cesm/doc/modelnl/nl_drv.html
cesm1.2 -- http://www.cesm.ucar.edu/models/cesm1.2/cesm/doc/modelnl/nl_drv.html
orbital model setting. this sets how the orbital mode will be configured. "fixed_year" uses the orb_iyear and other orb inputs are ignored. In this mode, the orbital parameters are constant and based on the year. "variable_year" uses the orb_iyear and orb_iyear_align. In this mode, the orbital parameters vary as the model year advances and the model year orb_iyear_align has the equivalent orbital year of orb_iyear. "fixed_parameters" uses the orb_eccen, orb_mvelp, and orb_obliq to set the orbital parameters which then remain constant through the model integration. set by ORBITAL_MODE in env_run.xml. [fixed_year, variable_year, fixed_parameters] default='fixed_year'.
NOTE: orb_mode may cause early CESM tags to crash. If so, remove orb_mode from cpl.buildnml.csh and resubmit.
year of orbit, used when orb_mode is fixed_year or variable_year. set by ORBITAL_YEAR in env_run.xml. default=unset
model year associated with orb_iyear when orb_mode is variable_year. set by ORBITAL_YEAR_ALIGN in env_run.xml. default=unset
eccentricity of orbit, used when orb_mode is fixed_parameters. default=unset
location of vernal equinox in longitude degrees, used when orb_mode is fixed_parameters. default=unset
obliquity of orbit in degrees, used when orb_mode is fixed_parameters. default=unset