Fwd: Re: [CF-metadata] Example of forecast data

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 16 Jun 2003 10:33:27 +0100

Dear Brian and Ag

> On Fri, Jun 13, 2003 at 12:28:00PM +0100, Jonathan Gregory wrote:
> > I would like
> > to propose a new standard name of forecast_validity_time to use instead of
> > plain time in this scheme.
>
> My initial reaction to this is not favorable. This is using the time axis
> to express a property that belongs to the data variable. In other words,
> the fact that the variable may contain forecast data is a property of the
> data, not the time axis. A standard name already exists that allows us to
> distinquish the valid time of the data from a reference time. It seems to
> me that that's sufficient.

If we have two auxiliary coordinate variables, one for forecast reference time
and one for validity time, they have a similar status, and it seems natural to
me to give them standard names of forecast_reference_time and
forecast_validity_time. Both of these are more specific "varieties" of time,
and being specific about both of them is more informative to the user. Why
should one of them have the standard name of time when the other one doesn't?

> > Can't we use simpler schemes?
>
> I assume that what we're discussing is what to recommend (via examples) in
> the document.

That's right. The scheme with an index dimension and auxiliary coordinate
variables is a sensible use of an existing CF convention, and seems the best
way to deal with forecasts having various combinations of forecast period
and analysis time. My question is whether we should recommend it in simpler
cases, where there is only one analysis time, or one validity time, involved.

> > For Ag's case of a set of analyses I think I would use a single time axis with
> > standard name forecast_reference_time instead of time, as it's more informative.
>
> This was my initial thought as well. But on further reflection, whether
> data represents a forecast or an analysis is really a property of the data
> itself and not of the time axis. So I would say that this data should use
> a plain time axis which represents it's valid time, and that the fact that
> it's analysis data should be more precisely indicated by the "source"
> attribute which could include details of how the analysis was produced.
> That said, using forecast_reference_time to identify the time axis is
> certainly legal and would most likely be correctly interpreted by someone
> using the data.

Yes, I wondered about that argument too. In the context of other data which
is concerned with forecasts, using the standard name of the time axis might
seem more natural. In connection with forecasts, I think you can regard analysis
and validity time as two kinds of time dimension. In that case, using the
standard name to distinguish them is analogous to using it to distinguish
vertical levels of pressure and sigma, say. Software which identifies a time
axis by units or by the axis attribute would not have a problem with it. I
think it's too restrictive to require a time axis to have a standard name of
time. Since standard names are standardised, and the source attribute is not,
using the standard name may also be more helpful when comparing data from
different institutions.

Best wishes

Jonathan
Received on Mon Jun 16 2003 - 03:33:27 BST