[CF-metadata] Example of forecast data

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 10 Jun 2003 09:24:08 +0100

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 - 02:24:08 BST