NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
Current content can be found at unidata.ucar.edu.
To learn about what's going on, see About the Archive Site.
NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
Current content can be found at unidata.ucar.edu.
To learn about what's going on, see About the Archive Site.
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 _______________________________________________ thredds mailing list thredds@xxxxxxxxxxxxxxxx For list information or to unsubscribe, visit: http://www.unidata.ucar.edu/mailing_lists/
thredds archives: