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

• To: John Caron <caron@xxxxxxxxxxxxxxxx>
• Subject: Re: [thredds] Help with GRIB encoding - what is "reference time of data" ??
• From: Eizi TOYODA <toyoda@xxxxxxxxxxxxxx>
• Date: Tue, 26 Nov 2013 18:23:16 +0900

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
http://www.google.com/profiles/toyoda.eizi


On Sat, Nov 23, 2013 at 3:03 AM, John Caron <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
>
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