[mpas-developers] proposed additions to global_scvt

[Michael Duda]

Hi, All.

I'd like to propose two small improvements to the global_scvt code.

The first change is to always write out the final generating points
(i.e., what is currently written) plus a set of refinement points to the
locs.dat.out files. Since the grid_gen program will only read as many
points from the locs.dat file as are specified by the np variable in the
namelist, this change should not disrupt current workflows, but should
enable us to avoid a separate program for refining a grid: when iterations 
converge with the current number of generating points, we can copy 
locs.dat.out to locs.dat as usual, and simply set np to 4*np-6 in
the namelist.input file.

The second change adds code to estimate the number of generating points
required to reach a target fine-grid resolution based on the density
function encoded in src/module_scvt.F. The only change from the user's
perspective is the addition of a new variable, min_dx, in the namelist
that specifies the desired minimum resolution; the code will
automatically compute an estimate for the cell count using the following
method:

   area_per_sample = 4.0 * pi * 6370000**2.0 / n_samples
   nhexs = 0.0
   do i=1,n_samples
      call random_point(p)
      d = density_for_point(p)
      dl = min_dx / (d ** 0.25)
      hex_area = sqrt(3.0) / 2.0 * dl**2.0
      nhexs = nhexs + area_per_sample / hex_area
   end do
   write(0,*) 'Estimated # hexs:', nint(nhexs)

The routine random_point returns uniformly-distributed points on the
sphere. Although the method assumes planar hexagons (rather than
spherical hexagons), the discrepancy becomes smaller as the number of
hexagons increases.

I've placed a checked-out version of the code (on which you can run 
'svn status' and 'svn diff') at
http://www.mmm.ucar.edu/people/duda/files/global_scvt_co.tar.gz.

Any comments or suggestions would be appreciated; if there are no
objections, I'd like to commit the changes to the repository trunk.

Thanks,
Michael