[CF-metadata] Example of forecast data

[Stephens, A (Ag)]

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.
>