Re: 20051206: Gempak to grib conversion of CED type grid


You have left most of the fields in GDGRIB blank.

You have to specify the values of the PDS block to identify the grib bits that are needed to identify the model center, version,
etc which will be used to identify the grib.

Steve Chiswell
Unidata User Support

On Wed, 2005-12-07 at 11:20, Kevin Hill wrote:
Steve and others,

I apologize for not being very specific with my question! I am aware that
I need to use gdgrib to convert from Gempak to grib. For my work I am
taking Gempak grids and modifying them using a fortran program and then
trying to take those Gempak files and convert them back to grib. When
doing that, I used a gdgrib script, but when trying to use those grib
files in WRFSI I receive errors. Therefore, I was not sure if the error
was due to my alterations of the original grid using fortran or if it was
due to the GDGRIB conversion script I ran.

In order to test that, I tried to modify 1 grib file that WRFSI was able
to process. I took that grib file and converted it to Gempak using dcgrib.
Next I used GDGRIB to take one grid from that Gempak file and place it
back into the grib file. After doing this, WRFSI gave an error. Therefore,
I concluded that something was wrong with the script I was using to
convert from Gempak to grib.

The previous problems I have mentioned have been found when trying to
manipulate a grid that is a global 1 degree analysis (Cylindrical
Equidistant projection). I tried the previous procedure using a grid with
a mercator projection and I was able to have success. Therefore, I felt
that the problem with the GDGRIB conversion process must be due to the CED
projection type, and was wondering if something in GDGRIB needed to be set
differently due to the fact that the grid is CED.

Here is the script I used to take 1000 mb height out of a Gempak file and
write it back into the grib file.

gdgrib << endin
gdfile  = "gfs-avn-hi_3_20040911_0000_000.gem"
gfunc   = hght
gdattim = "040911/0000F000"
glevel  = 1000
gvcord  = pres
gbtbls  = wmogrib131.tbl
gbfile  = "gfs-avn-hi_3_20040911_0000_000.grb"
vercen pdsval precsn wmohdr cpyfil = "gfs-avn-hi_3_20040911_0000_000.gem" proj grdarea kxky r


Here, the gfs-avn-hi_3_20040911_0000_000.grb file is a grib file that I
downloaded from Nomads. The gfs-avn-hi_3_20040911_0000_000.gem file is a
Gempak file created form the gfs-avn-hi_3_20040911_0000_000.grb grib file
using dcgrib. All I am trying to do is take 1 grid from the Gempak file
and write it back into the grib file. This should lead to a grib file that
is identical to the original. However, after doing that, WRFSI is no
longer able to run.

Overall, it appears that converting the CED grib file to GEMPAK using
dcgrib and then converting back from GEMPAK to grib using gdbrib somehow
leads to a grib file that is different than the original and will no
longer be able to work in wrfsi. I had thought that doing this type of
conversion would yield a grib file that was identical to the original and
would be able to be processed by WRFSI. (Note that this process did work
when I did the same procedure using a grid that was a mercator projection

Does anyone have any ideas as to why the CED grid would experience this
problem? Since I am setting cpyfil to the gempak file created from the
original grib file using dcgrib I figured the necessary parameters would
be set correctly; or maybe I am wrong and something else needs to be set.

Thanks for any assistance!


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