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

  • To: "John Caron" <caron@xxxxxxxxxxxxxxxx>
  • Subject: Re: [netcdf-java] [thredds] Help with GRIB encoding - what is "reference time of data" ??
  • From: "TOYODA Eizi" <toyoda@xxxxxxxxxxxxxxxx>
  • Date: Wed, 27 Nov 2013 19:03:16 +0900
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.

----- 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" ?


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.

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.



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