[ferret_users] modulo longitude standards?

[Ryo Furue <furue@xxxxxxxxxx>]

Hi Ferret users,

I've come across a netCDF file whose longitude axis is marked
"modulo".  The dataset includes both values at x = -180 and x = 180.
I examined the dataset and confirmed that var[x=-180] == v[x=180] for
all latitudes.  But, Ferret doesn't recognize the longitude axis as
modulo.  I understand why not.

I looked at the CF conventions but didn't find specifications about
modulo longitude.  I can see that such a grid configuration as above
is sometimes useful.  Think about computing gradients in the
x-direction when your tool doesn't natively recognize the cyclicity of
longitude as when you program in Fortran (because writing
var(mod(i-1,N)+1, j, k) is tedious and you have to look up the Fortran
manual to see what mod does when i <= 0).

For this reason, I hesitate a bit to try to convince the creators of
the dataset to fix their longitude axis.  Is there a widely-followed
convention that deals with this issue for me to cite to convince them
with?  Or can Ferret be extended to handle this special case?

Regards,

Ryo