[gradsusr] Creating PDEF file from CMIP5 areacello_fx file.

[Muhammad Yunus Ahmad Mazuki]

Hi,

This is about reading CMIP5 ocean realm output with GrADS. First of all, I
can easily interpolate the model original grid to normal grid using cdo. To
achieve this, I must have the model data variable (usually tos, sic and
such) and the model data areacello_fx file. As they are downloaded
separately, I just use cdo merge to merge both file. After which I use

cdo genbil,linear0p5.nc infile weight.nc
cdo remap,linear0p5.nc,weight.nc infile ouputfile

Using ESMF_RegridWeightGen instead of cdo genbil also gives similar result.
After that I create descriptor file and open it with xdfopen

dset ^File.nc
dtype netcdf
OPTIONS 365_day_calendar
*tdef time 1956 linear 01JAN1850 1mo *Occasionally you need this option.
xdef x 720 linear -279.75 0.5
ydef y 360 linear -89.75 0.5

If I didn't merge arecello_fx file with the variable file, the
interpolation output grid will be wrong. So there is no problem in
interpolating CMIP5 ocean realm data to be read with GrADS. The downside is
the need to interpolate (which comes with its own methods and  problem) and
the increase in the file size depending on the interpolation target size.

As I understand the areacello_fx file adhere to some GRIDSPEC
specification. If that's the case, then it should be possible to create a
pdef based on the areacello_fx file. This is the part where I hope someone
or GrADS team would be able to develop such tool or intergrate it into
GrADS itself. Hopefully CMIP6 data would be more user-friendly.

Yunus.
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