Ive had a look at your files and Im cc'ing Bob Simons for the ERDDAP
perspective and Wesley Ebisuzaki for wgrib POV.
The fields 10-0-87 10-0-8, and 10-0-9 in the file you sent me use level
type 241, which is in the "local table" section of table 4.5. For Center
7 we use NCEP local tables, and 241 is called "Ordered Sequence of Data".
GRIB is a collection of 2D records; the CDM tries to put the data back
into 3 (time), 4 (level) or 5 (ensemble) dimensional arrays. So all
records that have the same parameter and level type are placed in one
variable, with a level coordinate variable that distinguishes the
records. From that POV, it looks like the code is doing the right thing.
wgrib2 is an NCEP library that apparently is doing something special
with level type 241, apparently keeping each level in a seperate
variable. There's nothing in the GRIB spec (that I know of) that
indicates that, so theres no way for the CDM to do the same thing
without some special handling. Furthermore, theres nothing I can find in
NCEP documentation that would indicate this kind of special handling. I
guess that this is one of those "you just have to know" cases. Ive cc'ed
Wesley in case he wants to comment on that.
I assume that ERDDAP can handle the "Ordered Sequence of Data" level
coordinate, and you can see both records? If thats the case, Id be
inclined to say that its a different, but equally valid way of
presenting the GRIB data in netCDF.
On 6/17/2011 5:27 AM, Comiskey, Glenn wrote:
Have an issue with publishing GRIB v2 datasets via ERDDAP which my
current understanding is due to a way NetCDF-Java reads GRIB v2 files.
Had set-up a Tomcat v6.0.32/ERDDAP v1.28 server on Windows and run the
GenerateDatasetsXml.bat to generate the XML for inclusion into
datasets.xml. Specified option EDDGridFromNcFiles and pointed it to a
forcast GRIB v2 file with records WIND, WDIR, UGRD, VGRD, HTSGW,
PERPW, DIRPW, WVHGT, SWELL (1 in sequence), SWELL (2 in sequence),
WVPER, SWPER (1 in sequence), SWPER (2 in sequence), WVDIR, SWDIR (1
in sequence), SWDIR (2 in sequence) for every 3 hours 0 - 180 from
reference time. GenerateDatasetsXml.bat, which I understand uses
NetCDF-Java to read the file, recognised the dimensions time, lat, lon
and therefore reported data variables for WIND, WDIR, UGRD, VGRD,
HTSGW, PERPW, DIRPW, WVHGT, WVPER, WVDIR without any problem, but only
reported a single data variable for SWELL (1 in sequence), SWELL (2 in
sequence) and similarly for SWPER and SWDIR (which when accessing the
data via ERDDAP appeared to relate to the "2 in sequence" record),
i.e. it was only possible to access GRIB v2 records SWELL (2 in
sequence), SWPER (2 in sequence) and SWDIR (2 in sequence).
However, having set-up a Tomcat v6.0.32/ERDDAP v1.32 server on Windows
and run GenerateDatasetsXml.bat on the same GRIB v2 file and selected
option EDDGridFromNcFiles it now appears to recognise dimensions time,
ordered_sequence_of_data, lat, lon, and as a result only reports data
variables SWELL, SWPER and SWDIR due to the fact that they are the
only one that share the recognised dimesions - which is a requirement
Given my understanding of how GRIB v2 files are constructed, and how
that data is meant to be interpeted, I am expecting
GenerateDatasetsXml.bat to generate XML entries for three dimensions
(time, lat, lon) and data variables for all 16 records; with SWELL (1
in sequence) and SWELL (2 in sequence) being distinctly seperate data
variables as with SWPER and SWDIR. This is to say that for any given
location (lat, lon) and forcast time, or time period, then SWELL (1 in
sequence) and SWELL (2 in sequence) etc. should be reported.
Why is it that NetCDF-Java does not appear to be able to understand
that a GRIB v2 file can have records that have common dimensions and
distinctly seperate data variables that represent that same data type.
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