[CF-metadata] various "time" in CF

Antonio S. Cofiño antonio.cofino at unican.es
Wed Oct 18 09:17:27 MDT 2017


Hi Sebastien,

It maybe someone has a different approach, but this is my solution to your problem.

A variable with attribute standard_name="forecast_reference_time" means values for analysis times for a model simulation.


> forecast_reference_time: The forecast reference time in NWP is the "data time", the time of the analysis from which the forecast was made. It is not the time for which the forecast is valid; the standard name of time should be used for that time.

And forecast_period are the time offset/interval with respect the forecast_reference_time. This means that for a complete specification the forecast_period needs an associated forecast_reference_time.

In the case of forecast data from NWP or model simulations the forecast_reference_time and time_period are only required for completeness, the "time" it's complementary but no required.

In the case of observations/acquisition the standard_name="time" would be more appropriate because it means valid time (of the observation)

With respect to your use cases I have summarized to this ones:

 1. several forecasts in the same file with one analysistime

    double frt
       frt:standard_name = "forecast_reference_time" ;
       frt:units = "hours since 1999-01-01 00:00" ;

    double fp(fp)
       frt:standard_name = "forecast_period" ;
       frt:units = "hours" ;

    float height(fp,lat,lon);
       height:long_name = "geopotential height" ;
       height:standard_name = "geopotential_height" ;
       height:units = "m" ;
       height:coordinates = "frt" ;

    data:
       fp = 6., 12., 18., 24. ;
       frt = 0. ;


 2. several analyses in the same file with one forecast

    double frt(frt)
       frt:standard_name = "forecast_reference_time" ;
       frt:units = "hours since 1999-01-01 00:00" ;

    double fp
       frt:standard_name = "forecast_period" ;
       frt:units = "hours" ;

    float height(frt,lat,lon);
       height:long_name = "geopotential height" ;
       height:standard_name = "geopotential_height" ;
       height:units = "m" ;
       height:coordinates = "fp" ;

    data:
       fp = 6. ;
       frt = 0., 6., 12., 18.;


 3. several reforecasts/hindcasts in the same file

    double frt(frt)
       frt:standard_name = "forecast_reference_time" ;
       frt:units = "hours since 1999-01-01 00:00" ;

    double fp(fp)
       frt:standard_name = "forecast_period" ;
       frt:units = "hours" ;

    float height(fp,frt,lat,lon);
       height:long_name = "geopotential height" ;
       height:standard_name = "geopotential_height" ;
       height:units = "m" ;

    data:
       fp = 6., 12., 18., 24. ;
       frt = 0., 6., 12., 18. ;


 4. observations data assimilation/acquisition
    As I mention before I would use the standard_name="time" for the
    time coordinate.
 5. distinguish if the data variable is associated with an analysis time
    or a forecast reference time (some data variables do differ between
    analysis and forecast step 0!)
    To represent analysis time with NO forecast time the forecast_period
    coordinate would be omitted.

    double frt(frt)
       frt:standard_name = "forecast_reference_time" ;
       frt:units = "hours since 1999-01-01 00:00" ;

    float height(frt,lat,lon);
       height:long_name = "geopotential height" ;
       height:standard_name = "geopotential_height" ;
       height:units = "m" ;

    data:
       frt = 0., 6., 12., 18. ;




    And with forecast step 0 the forecast_period coordinate would be specified

    double frt(frt)
       frt:standard_name = "forecast_reference_time" ;
       frt:units = "hours since 1999-01-01 00:00" ;

    double fp
       frt:standard_name = "forecast_period" ;
       frt:units = "hours" ;

    float height(frt,lat,lon);
       height:long_name = "geopotential height" ;
       height:standard_name = "geopotential_height" ;
       height:units = "m" ;
       height:coordinates = "fp" ;

    data:
       fp = 0. ;
       frt = 0., 6., 12., 18.;


Hope this helps

Regards

Antonio


--
Antonio S. Cofiño
Grupo de Meteorología de Santander
Dep. de Matemática Aplicada y
         Ciencias de la Computación
Universidad de Cantabria
http://www.meteo.unican.es


On 18/10/17 16:03, Sebastien Villaume wrote:

> Hi all,
>
> I am trying to find a consistent way to describe Time in CF-netCDF, so far without success. :(
>
> The main reason is that there are only 3 standard names to describe Time (unless I missed something in the official CF standard name list):
>
>   - time
>   - forecast_reference_time
>   - forecast_period
>
>
> I would like to describe properly (and without relying at all on the file names):
>   - a forecast
>   - several forecasts in the same file
>   - an analysis
>   - several analyses in the same file
>   - a reforecast/hindcast
>   - several reforecasts/hindcasts in the same file
>   - observations
>   - data assimilation/acquisition
>   - distinguish if the data variable is associated with an analysis time or a forecast reference time (some data variables do differ between analysis and forecast step 0!)
>   - etc.
>
> of course, I could use the generic "time" standard name and put all the required informations in the "long_name" attribute but I don't want to do that: long_name is a free text attribute, I need controlled vocabulary which is exactly the purpose of the standard_name attribute.
>
> I also need to be able to reference these "kinds" of time coordinates in the cell_method attribute of my data variables to describe statistical processing like sums/accumulations, min/max, means, standard deviations, etc.
>
> The simplest solution that comes to my mind would be to add new standard names for these kinds of time coordinates but it seems to be such a common issue that I can't believe nobody proposed before. Hence, I am wondering if I am missing something here or if someone tried and it was not very well received/rejected...
>
> Is it worth spending time drafting a proposal to define new standard names for time coordinates?
>
> cheers,
> ____________________________________
>
> Dr. Sébastien Villaume
>
> M.A.R.S administrator
> DATA Governance facilitator
>
> ECMWF
> Shinfield Park,
> Reading RG2 9AX, UK
> +44 7825 521592
> sebastien.villaume at ecmwf.int
> ____________________________________
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> CF-metadata mailing list
> CF-metadata at cgd.ucar.edu
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