Re: reduced grid an pyNIO

Hi Jim,
PyNIO automatically expands reduced (quasi-regular in GRIB-speak)
grids using (by default) a cubic interpolation algorithm.
The code is from the ECMWF GRIBEX package, and the specific function
is qu2reg3.F. You can examine the source at this URL:

http://www.ecmwf.int/publications/manuals/libraries/gribex/
qu2reg3.F.html

The NIO library modifies this code slightly in order to dynamically
allocate some work arrays from the calling C code and to
adjust the error handling.

There is a GRIB-specific file option ('ThinnedGridInterpolation')
that allows you to switch the interpolation mode between 'cubic'
and 'linear'. This just adjusts the qu2reg3 KCODE argument. Note
that if there are any missing values in the data, then the 'linear'
method is forced.

I would be interested to know how the values differ. Dave Stepaniak,
from the Data Support Services here at NCAR has done some
fairly rigorous testing of the ECMWF code and concluded that the
qu2reg3 'cubic' method was significantly more accurate than the 'linear'
method and as good as another candidate method. That is why 'cubic'
is the default in PyNIO.
  -dave

On Nov 11, 2008, at 3:51 PM, James Boyle wrote:

> I have a grid file from ECMWF. The CDO ( Climate Data operators)
> say that it contains a reduced gaussian grid.
> I can expand the grid using the CDOs to a full gaussian grid.
>
> when I read a variable using pyNIO from the expanded full gaussian
> grid file and the original 'reduced?' grid, both variables are on
> the full grid.
> but the values are not the same
>
> What does pyNIO do with variables on a reduced grid? I fully
> expected to get an error when I read a reduced grid file.
>
> --Jim
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Received on Tue Nov 11 2008 - 16:32:29 MST