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