[mpas-developers] attributes in grid generation code

[Michael Duda]

Hi, All.

Toward the goal of being able to distinguish between grids in the
plane and grids on the sphere in MPAS, I'm proposing that we add
two attributes to the grid.nc file, and make changes to our grid
generation codes (global_scvt, periodic_hex, and periodic_quad)
that will add these attributes to future grid.nc files. The two
attributes are

   on_a_sphere -- a character string, either 'YES' or 'NO'
   sphere_radius -- the radius of the sphere

I'd prefer on_a_sphere to be a logical, but it appears that having
a logical global attribute in netCDF is not possible (which isn't
to say that global attributes must be illogical!).

The code changes to module_write_netcdf.F in the three grid
generation codes mentioned above would essentially look like

95a96,97
>       real (kind=8) :: sphere_radius
>       character (len=16) :: on_a_sphere
104a107,109
>       on_a_sphere = 'YES             '
>       sphere_radius = 1.0
> 
110a116,117
>       nferr = nf_put_att_text(wr_ncid, NF_GLOBAL, 'on_a_sphere', 16, on_a_sphere)
>       nferr = nf_put_att_double(wr_ncid, NF_GLOBAL, 'sphere_radius', NF_DOUBLE, 1, sphere_radius)

with YES being replaced by NO and 1.0 being replaced by 0.0 in the
case of the doubly-periodic codes.

Although 'YES' requires only three characters and 'NO' requires
only two, I wanted both to be the same length, and I figured
making the attribute be 16 characters rather than three would
allow for some potential flexibility in future without adding an
unreasonable amount of storage. For example, if not on the sphere,
we might in future find it useful to be able to set on_a_sphere to
something like 'NO (X-Y PLANE)'. Who knows.

If there are no suggestions for improvements (or requests to see
the changes as they appear in the module_write_netcdf.F files),
I'll commit these changes. I can also update all of the existing
grid.nc files available from
http://www.mmm.ucar.edu/people/duda/files/mpas/ as well as provide 
a utility program to update any other grid.nc file with the 
new attributes.

Cheers,
Michael