Why that Choice of Earth Radius

[Kevin Matthew Nuss]

Sorry, I also asked this in the github issues before I found the help forum.

It seems like an odd choice to use 6371229.0 m as the radius of the earth in MPAS. That number seems to be based on Hayford's 1910 estimate of the Earth. It's average radius is 6371229.31 m, by my calculation.

Here's a few sphere alternatives, all based on the more recent WGS84 ellipsoid:

average of the 3 radii: 6371008.7714 m
radius of sphere of equal area: 6371007.1809 m
radius of sphere of equal volume: 6371000.7900 m

These were taken from page 3-7 of the WGS84 definition at: 
http://earth-info.nga.mil/GandG/publications/tr8350.2/wgs84fin.pdf

Something based on mean sea level might also be good.

I might be missing something, of course, but thought I would speak up sooner rather than later. Either way, I am curious about the choice.

[Michael Duda]

Hi, Kevin.

Almost certainly, the radius 6371229 m was taken from the Jablonowski and Williamson test case (QJRMS, 2006), which was used early testing of the original MPAS hydrostatic atmosphere core. How this radius came to be used in that test case, though, I have no idea.

In any case, it seems like a fair statement that the choice of 6371229 m as Earth's radius is not among the largest sources of error in the MPAS-Atmosphere model: note, for example, that we have g = 9.80616 m/s^2, while the ISO standard value for Earth's gravity is 9.80665 m/s^2.

Best regards,

Michael

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