Hi Don:
Thanks for your ideas, I hadnt considered leaving "reference time" as
otherwise undefined. The downside would be that GRIB will then be
different from, say, netCDF-CF forecast model collections.
Examining the NCEP IDD feed, 13 out of 400K GRIB-2 records have this
feild set to 0, the rest to 1:
significanceOfReference
0: count = 13
1: count = 406196
Im not sure if these are miscoded or not. What do you think the
difference is between "Analysis" and "Start of Forecast" ?
John
On 11/22/2013 11:51 AM, Don Murray (NOAA Affiliate) wrote:
Hi John-
GEMPAK supports two times for each grid - a reference time and a valid
time. I believe that the reference time for GRIB data is the value
below, but either Michael (the Unidata GEMPAK developer) or the NCEP
GEMPAK developers could easily confirm that. Whatever they use seems to
work quite well for the US generated grids.
Perhaps instead of having the CDM grid API have a RunTimeAxis, it should
be a ReferenceTimeAxis. The values would be populated with the
reference time from the GRIB message.
For the grids on thredds.ucar.edu, what does this value in table 1.2
tend to be? Is it 1 (which I would interpret as "run time")?
Don
On 11/22/13 11:03 AM, John Caron 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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