Dear Jonathan,
I agree that using the size-one axes is a sensible way to denote the
metadata and the four examples seem sensible.
I think the panacea is a metadata description (and software analysing it)
that could allow combinations of forecasts and analyses to be compared that
all represent the exact same time-step (in realtime). Such as:
20030101 12:00 analysis
20030101 00:00 analysis 12hr forecast
20030101 06:00 analysis 6hr forecast
I think the key will be how software can deal with it. Most of the packages
I am familiar with do not understand the concept of forecasts and they just
reduce a forecast step to a time-stamp and forget the relationship to an
analysis. I have e-mailed the cdat-discussion list to enquire whether cdat
has a way of dealing with forecasts.
Kind regards,
Ag
-------------------------------------------------------------
Ag Stephens Ph : +44 (0)1235 446220
Data Scientist, Fax: +44 (0)1235 446314
British Atmospheric Data Centre,
Rutherford Appleton Laboratory,
Chilton, Didcot, OX11 0QX, U.K.
Email: A.Stephens at rl.ac.uk Web:
http://badc.nerc.ac.uk
-------------------------------------------------------------
> -----Original Message-----
> From: Jonathan Gregory [mailto:j.m.gregory at reading.ac.uk]
> Sent: 10 June 2003 09:24
> To: Stephens, A (Ag) ; cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Example of forecast data
>
>
> Dear Ag
>
> We have had discussions about how to represent forecast time,
> but so far not
> put anything in the standard because we didn't have potential
> users. Evidently
> we have now!
>
> The methods earlier discussed all used the idea that forecast
> time would have
> a separate time dimension from analysis time. This seems to
> be a more flexible
> approach than giving a special interpretation to bounds. What
> do you think of
> the below? Your case would appear to be a simple kind of (i),
> with a size-one
> forecast time axis and a size-one analysis time axis. Using
> size-one axes is
> a convenient way of attaching metadata with units etc., as we
> do for example
> by having a size-one vertical axis to indicate a quantity at
> 1.5 m height.
>
> Best wishes
>
> Jonathan
>
>
> Four cases were identified:
>
> (i) A forecast run with multiple forecast periods (12 h, 24
> h, 36 h, etc.)
> from a single analysis. This can be done with a size-one
> analysis time axis
> and a separate validity time axis. If the reference time for
> the validity time
> units is the analysis time, the values of the validity time
> coordinates equal
> the forecast period.
>
> (ii) Multiple validity and analysis times, where all
> combinations exist. This
> can be done with 2 axes.
>
> (iii) Forecast runs with multiple forecast periods from
> various analyses,
> where all combinations exist. This is also a 2D case, where
> the analysis time
> is a time axis with a reference time, and the forecast period
> is plain elapsed
> time. You have to add the forecast period to the analysis
> time to get the
> validity time. The forecast period is a time offset.
>
> (iv) Multiple validity and analysis times, where not all
> cases exist, for
> instance forecasts out to a given validity time from
> successive analyses. This
> can be done by using a single index dimension, with validity
> and analysis time
> coordinates having this dimension pointed to by the
> coordinates attribute.
>
Received on Tue Jun 10 2003 - 08:35:44 BST
