Re: [netcdf-java] [thredds] Help with GRIB encoding - what is "reference time of data" ??

  • To: TOYODA Eizi <toyoda@xxxxxxxxxxxxxx>
  • Subject: Re: [netcdf-java] [thredds] Help with GRIB encoding - what is "reference time of data" ??
  • From: Benno Blumenthal <benno@xxxxxxxxxxxxxxxx>
  • Date: Wed, 27 Nov 2013 12:33:19 -0500
I have been looking at grib2, and was hoping I could get some help on some
lower-level metadata questions.

1) wmo code tables have version numbers which are also in the data records
-- is it sufficient to use a code table that has version number greater
than or equal to the number in the record, or do we have to match the
version number in the record?  WMO is on version 12, my test data of
interest is version 1, I would much rather have a single
latest-and-greatest that keep all twelve-and-counting versions.

2) there are xml versions of the wmo code tables, which can be converted
into something more programmatically useful.  Is there a standard set of
converted tables, i.e. XML with clean discipline, category, version coding?
   Can the same be said of local tables, e.g. NCEP for starters?

3) my understanding so far is that to identify a scientific parameter
(a.k.a standard_name) in grib we need wmo table version or local table
version, data source (if local table), discipline, category, parameter
number, and where (and if) we find this info depends on the PDS template
NCEP maps this to short names, though WMO does not. And you are presumably
mapping this to standard_names as well.   Are these mappings available,
i.e. in XML format?

On Wed, Nov 27, 2013 at 5:05 AM, TOYODA Eizi <toyoda@xxxxxxxxxxxxxx> wrote:

> Hi John,
> Yes JMA still issues GRIB edition 1 for:
> - bulletins sent to GTS since long time ago, and
> - the reanalysis project JRA-25 and JRA-55 (coming).
> In the reanalysis there are some products like monthly average, for which
> multiple model runs are used in a single message.  In that case my
> understanding is that the reference time is set to the beginning of the
> time
> range.
> Situations are similar in GRIB2.  I'm hoping the ECMWF reanalysis will
> start
> using GRIB2 sometime.  I'm afraid I don't know actual data using "Verifying
> time of forecast" (CT1.2:2).
> The reason why GRIB distinguishes "Analysis" time (CT1.2:0) and "Start of
> forecast" (CT1.2:1) might be rather historic nowadays.  It has a root in
> the
> difference between data, i.e. analysis and initialized analysis (sometimes
> called data for ft=0).
> Analysis is a best estimate of the state of the atmosphere, made from
> observation and past numerical forecast.  In the past NWP centers used
> methods called nudging or optimal interpolation (the latter is same as what
> geographers call kriging).  The bad thing is the result may contain
> inbalance between wind and pressure, which causes unnatural gravity wave in
> the forecast.  So people had to remove such inbalance after analysis.  That
> is the "initialization" heavily mentioned in GRIB edition 1.
> Recent NWP systems use variational assimilation, which has less such
> problem.  So the analysis simply becomes the initial field without much
> changes.  I don't think somebody still issues "initialized analysis"
> separated from "(uninitialized) analysis".  In GRIB edition 2, the concept
> remains in a footnote in the Code table 1.4, but usually the analysis and
> forecast are put in a single GRIB message for Forecast products (CT1.4:1) .
> In short, people no longer cares.
> Best,
> Eizi
> ----- Original Message ----- From: "John Caron" <caron@xxxxxxxxxxxxxxxx>
> To: "Eizi TOYODA" <toyoda@xxxxxxxxxxxxxx>
> Cc: "THREDDS community" <thredds@xxxxxxxxxxxxxxxx>; "Java NetCDF" <
> netcdf-java@xxxxxxxxxxxxxxxx>
> Sent: Wednesday, November 27, 2013 12:09 AM
> Subject: Re: [thredds] Help with GRIB encoding - what is "reference time
> of data" ??
>  Hi Eizi:
>> Thanks for clarifying that terminology, I see that my understanding has
>> been a bit fuzzy. I think I do indeed mean "Start of forecast", not the
>> actual time the model was run.
>> Does your agency still use GRIB-1? If so, do you ever code anything other
>> than the "Start of forecast" in the reference time ?
>> If you use GRIB-2, do you set "Significance of reference time" equal to 1
>> = "Start of forecast" ?
>> Finally, what in your opinion is the meaning of the other "Significance
>> of reference time" codes, esp how does "Analysis" differ from "Start of
>> forecast" ?
>> thanks,
>> John
>> On 11/26/2013 2:23 AM, Eizi TOYODA wrote:
>>> Hi John,
>>> I'm not sure what do you mean by "run time".
>>> If you want the date/time at which the forecast model started, I don't
>>> think GRIB contains it.
>>> If you mean the initial time of forecast model, that is "Start of
>>> forecast" and it is really common to use this as reference time.
>>> Best,
>>> --
>>> Eizi TOYODA: Japan Meteorological Agency
>>> Associate member of WMO/CBS/OPAG-ISS/IPET-DRMM
>>> Best Regards,
>>> --
>>> Eiji (aka Eizi) TOYODA
>>> On Sat, Nov 23, 2013 at 3:03 AM, John Caron <caron@xxxxxxxxxxxxxxxx
>>> <mailto:caron@xxxxxxxxxxxxxxxx>> wrote:
>>>     The question is whether the CDM can assume that GRIB "Reference time
>>>     of data" is the "run time" of a forecast model.
>>>     In GRIB-1 docs, in the PDS there is:
>>>     "Reference time of data – date and time of start of averaging or
>>>     accumulation period"
>>>     In GRIB-2 in Identification section, there is:
>>>     12 Significance of reference time (see Code table 1.2)
>>>     Reference time of data:
>>>     13–14 Year (4 digits)
>>>     15 Month
>>>     16 Day
>>>     17 Hour
>>>     18 Minute
>>>     19 Second
>>>     And Code table 1.2 has the following:
>>>     Code Table Code table 1.2 - Significance of reference time (1.2)
>>>          0: Analysis
>>>          1: Start of forecast
>>>          2: Verifying time of forecast
>>>          3: Observation time
>>>         -1: Reserved
>>>         -1: Reserved for local use
>>>        255: Missing
>>>     None of this obviously refers to "run time", although I suspect
>>>     that's how many centers use it. However, it appears that when you
>>>     want to define a time interval, say "average of the temperature,
>>>     starting 12 hours and ending 24 hours from reference, you may use
>>>     the reference time to define the start of that interval. In which
>>>     case, its not the runtime. Im hoping thats not the case, that
>>>     reference time is the same as the run time for forecast models.
>>>     So if you know how to interpret these for any or all datasets,
>>>     please send me a note, or post to this group. Please pass this
>>>     question on to anyone who might be willing to contribute.
>>>     Thanks!
>>>     John
>>>     _______________________________________________
>>>     thredds mailing list
>>>     thredds@xxxxxxxxxxxxxxxx <mailto:thredds@xxxxxxxxxxxxxxxx>
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Dr. M. Benno Blumenthal          benno@xxxxxxxxxxxxxxxx
International Research Institute for climate and society
The Earth Institute at Columbia University
Lamont Campus, Palisades NY 10964-8000   (845) 680-4450
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