Dear Tim
So for the case of multiple analysis and forecast times, where all combinations
exist, you have two multivalued time dimensions, one for forecast reference
time (= base time, analysis time, initialisation time) and one for forecast
period (lead time). The former has units of "UNITS since TIME" and the latter
is a plain time-unit. Is that right?
In
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2006/001008.html we
proposed instead to deal with this 2D case by introducing an index dimension
and collapsing it into 1D, with the forecast reference time and the forecast
period being 1D auxiliary coord vars. That obscures the 2D structure, which is
a loss. The reason for proposing it was that it reduced the number of different
structures that would have to be used. The 2D structure can only be used if
all 2D combinations exist.
In
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2007/001470.html, the
method that Doug uses, 2D is also collapsed into 1D, but instead of introducing
a plain index dimension, the 1D coordinate is validity time (forecast reference
time + forecast period). That is a better idea, I think, because it's a useful
coordinate variable rather than just an index. Again forecast reference time
and forecast period are 1D auxiliary coord vars.
I agree that the CF standard should have examples for each of the recommended
structures. That would need a proposal on trac and maybe John was offering to
make one.
What do you think about using Doug's/Paco's collapse of 2D into 1D? To get all
the 24-h forecasts, for instance, you'd have to search the 1D auxiliary coord
var of forecast period and use it to select particular indices of the time
dimension. That is a bit more awkward, but the structure has an advantage in
generality.
Best wishes
Jonathan
Received on Fri Jan 09 2009 - 01:34:41 GMT
