Hello everyone,

I am doing simulations for maximum & minimum precession in cesm 1.0.4 with fixed orbital parameters.

Orb_mvelp is defined as "The location of vernal equinox in longitude degrees", but location

of vernal equinox wrt to what is not clear. I assumed it is wrt perihelion. But this gave me results opposite

of what it should be.

So could someone please clarify what "orb_mvelp" is?

—

Thanks & Regards,

Chetankumar

Hi Chetankumar,

orb_mvelp is the longitude of the perihelion. However, the definition of longitude of the perihelion can be confusing, and I think it's actually written incorrectly in the documentation that you quoted. The easiest way to think of it is the angle (going with the orbit of the earth) between the earth at NH AUTUMNAL equinox and the earth at perihelion. Here's some values:

0 degrees: Perihelion occurs at NH autumnal equinox

90 degrees: Perihelion occurs at NH winter solstice

180 degrees: Perihelion occurs at NH vernal equinox

270 degrees: Perihelion occurs at NH summer solstice

The current longitude of perihelion is ~103 degrees (see, for example: http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html). This is consistent with the values given above, since 90 degrees is the NH winter solstice and 180 is the NH vernal equinox (perihelion currently occurs around January 3, shortly after the NH winter solstice but still a ways from the NH vernal equinox). You can verifiy this number yourself in the CESM. If you find a cpl.log.* file of a preindustrial simulation, you'll find that the value of "Long of perh(deg)" is ~102.7.

Confusion arises because longitude of the perihelion is sometimes conflated with "longitude of the perigee", which is 180 degrees different. This is probably what you did. Appendix B of Berger et al. 1993 discusses this, with a helpful figure to visualize the angles: http://onlinelibrary.wiley.com/doi/10.1029/93JD00222/full . The current value of "longitude of the perigee" is ~283 degrees (180 degrees different from 103), but this value is sometimes called longitude of the perihelion. Here's an example of a website using this definition: http://data.giss.nasa.gov/ar5/srorbpar.html.

Because of this confusion, if you're running precession experiments with a new model, always double-check exactly what they mean by "longitude of the perihelion". But for CESM, you can refer to the values I gave at the top of this post.

Best,

Michael

Thank you very much Michael & Clay. The definition I was referring to was from here http://www.cesm.ucar.edu/models/cesm1.2/cesm/doc/modelnl/nl_drv.html

After making the appropriate corrections I am now getting the expected results.

Thanks & Regards,

Chetankumar

Hello

I actually had a similar question:

I was trying to understand how the moving vernal equinox to perihelion is calculated by CESM and other papers from which I need orbital parameter values from.

CESM computes its orbital parameters from the algorithm method of Berger, 1978 (1Ma years.)

The Berger 1978 paper calculates the earth sun distance relative from the earth and 180 is added to this moving vernal equinox longitude of perihelion? è which is the CESM method: 180 degrees is added to mvelp since observations are made from the earth and the sun is considered (wrongly for the algorithm) to go around the earth.

Is this longitude of perihelion is measured counterclockwise then?

I retrieved more information on CESM orbital calculations from here: http://www.cesm.ucar.edu/models/cesm1.0/cesm/cesmBbrowser/html_code/csm_share/shr_orb_mod.F90.html