Hi Don:
The CDM now has an "auxiliary coordinate" that gives the reference time
(aka run time) for each valid time in a Best dataset. We are working with
the IDV group to help them show that info to the user.
Currently we are looking at the presence of analysis vs forecast GRIB
records in the NCEP datasets to see what makes sense there. As usual, its
not really clear what different groups do when they generate GRIB, so we
are trying to be careful not to assume too much.
All this work is being done on the 4.6 development branch.
Thanks for your comments.
John
On Wed, Feb 11, 2015 at 2:54 PM, Don Murray (NOAA Affiliate) <
don.murray@xxxxxxxx> wrote:
> Hi Ryan-
>
> I think this sounds like a workable solution. I assume that the Complete
> is essentially how the Best works now. Given the problems it has, I'm not
> sure if it's needed. An Analysis dataset would be a good addition.
>
> And of course, it would be good to be able to query any timestep for the
> model run time so the forecast hour can be computed.
>
> Will this be an update to 4.5 or will we have to wait for 4.6?
>
> Thanks for attacking this problem.
>
> Don
>
>
> On 2/11/15 11:24 AM, Ryan May wrote:
>
>> Greetings!
>>
>> The recent upgrades by NCEP to the time range of the operational GFS
>> half degree output has raised an issue with the Best timeseries dataset
>> on Grib Collections in TDS. Best is designed to take all the forecast
>> times available in the Grib Collection and, for each one, use the GRIB
>> record that has the smallest forecast offset (i.e. the forecast closest
>> to the start of its model run).
>>
>> The problem occurs where, for example, NCEP puts out GFS runs every 6
>> hours, but at the later parts of the forecast, the time interval within
>> a single run's output is 12 hours. The result is that even with no
>> missing data, eventually the Best time series has output every 6 hours
>> by interleaving two different forecasts. Given that forecasts this far
>> into the future can vary widely from run to run, this can yield very
>> surprising and confusing results. Note that this issue is not limited to
>> the GFS, but for any model collection where time steps in the output
>> become larger than the time between individual model runs.
>>
>> There is also a second issue regarding a model run having multiple
>> fields with a 0 hour forecast offset. It turns out that (at least
>> ideally) NCEP will put out one version of the 0 hour forecast with the
>> analysis flag set, denoting the field as the one to use to initialize
>> the model. Then a second field is put out as 0 hour forecast (without
>> the analysis flag) that actually corresponds to a time after a single
>> model integration step. This non-analysis 0 hour forecast contains the
>> full collection of parameters that are available throughout the
>> forecast, whereas the analysis only contains the fields that result from
>> data assimilation. Currently, the Best time series will contain
>> whichever version of the 0 hour forecast that comes first in the file.
>>
>> To address the first issue and make the data easier to use, we propose
>> to modify Best such that when a set of forecast runs is combined, the
>> forecasts included from a run will be consecutive in time. This implies
>> that any missing forecasts within a run will not be filled by a previous
>> run; however, an older forecast can be used to fill in gaps when a
>> collection is missing entire run(s). This behavior ensures that the Best
>> time series will no longer alternate repeatedly between different
>> forecast solutions, but rather only jump once from one set of forecasts
>> (model run) to another.
>>
>> To address the second issue, we propose an additional modification to
>> Best such that only 0 hour Forecasts are used to make up Best, and not
>> Analysis fields. Additionally, we propose to add two more virtual
>> datasets: Analysis and Complete. Analysis will contain the collection of
>> all analysis fields from the grib collection. Complete behaves just as
>> Best currently does, containing every possible time, using the smallest
>> forecast offset. In this case, the forecasts included from a run will
>> not be guaranteed to be be consecutive in time, as they could be mixed
>> between various runs as long as the smallest forecast offset condition
>> is met.Â
>>
>> We are seeking comments on these solutions to see if they represent a
>> useful set of features given the problems outlined above.
>>
>> Thanks!
>>
>> The TDS development team
>>
>>
>> _______________________________________________
>> thredds mailing list
>> thredds@xxxxxxxxxxxxxxxx
>> For list information or to unsubscribe, visit:
>> http://www.unidata.ucar.edu/mailing_lists/
>>
>>
> --
> Don Murray
> NOAA/ESRL/PSD and CU-CIRES
> 303-497-3596
> http://www.esrl.noaa.gov/psd/people/don.murray/
>
> _______________________________________________
> thredds mailing list
> thredds@xxxxxxxxxxxxxxxx
> For list information or to unsubscribe, visit:
> http://www.unidata.ucar.edu/mailing_lists/
>
