[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

20010605: Grib files and DCGRIB



>From: "Mark J. Laufersweiler" <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200106051602.f55G2up02768

>
>I have been asked to produce graphics from the local in-house ARPS
>model here at OU. What I have available as output is the ARPS grib
>files which apparently do not follow the WMO standards (extra model
>params etc).
>
>Is there a way to use DCGRIB if I create new tables to mark the
>grid, variables etc ?
>
>I can get the code from APRS that creates the grib files.
>
>Also, I can also have as input, netcdf files that are to be used by
>the NWS Awips system, if that may be a better way to go.
>
>Any ideas, suggestions or comments would be appreciated.
>
>Thanks
>
>mjl
>
>-- 
>---------------------------------------------------------------------
>  Dr. Mark J. Laufersweiler     |
>  School of Meteorology         | If "con" is the opposite of "pro,"
>  University of Oklahoma        |   then what is the opposite of
>  address@hidden                |             progress?
>  (405) 325-6032                |
>  (405) 325-7689  (fax)         |
>


Mark,

If the grib standard is followed, dcgrib2 or nagrib should be able to decode 
the messages.

The grib parameter tables are stored in $GEMTBL/grid, or in your local 
directory.

Some things to know about grib:

1) parameter numbers 1-127 are WMO assigned. When dcgrib2 decodes a grib 
message,
   and the parameter number is in that range, it looks in the wmogribx.tbl file.
   The "x" is the grib table edition number used in PDS octet 4.

2) parameters 128-255 are reserved for local use. These are expected to be 
defined in
   XXXXgribx.tbl, where XXXX is the modeling center specified in PDS octet
   5. The modeling center is looked up in the cntrgrib1.tbl table.
   For example, if Octet 5 is "60", then cntrgrib1.tbl identifies this as
   "NCAR". Then, local parameters are searched for in ncargribx.tbl.
   For NOAAPORT grids, the center is typically 7, 8, 9, 74, or 98 for
   NCEP, NWS, IKMET and ECMWF respectively.

So, what you probably need to do is find out what model center your ARPS is 
using
(Table 0 of WMO388 lists those already assigned). Then, you would add that
center to the cntrgrib1.tbl file.

NAGRIB allows you to specify the GBTBLS for what coordinate, center and 
parameter
tables are used (to override the default). Dcgrib & dcgrib2 expect the
tables to match what is in the PDS block so there isn't any way to override
what tabels will be used.

The only other thing that could be a problem is if the data was using some
strange may projection which GEMPAK doesn't recognize. As long as the grid is
on a CED, Lambert, or polar stereographic projection, you are fine.
Once the models start using gaussian grids, or staggered grids, you
can start running into hassles since that information tends to be hard coded
into the decoder.


Steve Chiswell
Unidata User Support