[CF-metadata] Return periods

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 9 Sep 2014 11:27:32 +0100

Dear Dan

Yes, I see what you mean regarding the aux coord, and it's a neat idea, but
it doesn't seem quite right to me. Aux coords are alternative or additional
information. The lat(x,y) and lon(x,y) coordinates provide an alternative way
to locate the point (x,y), in a different coordinate system. The precipitation
probability, however, would determine the precipitation entirely. There isn't
any coordinate information which would give you the precipitation amount. That
is why I don't think the probability can be an aux coord. Does that make sense?

> You are right regarding the calculation - we are using a statistical model of the relationship between monthly rainfall and return period that was developed many years ago by a colleague from an analysis of 60 years of historical data. The model uses values of the coefficients of variation and skewness to describe the distribution of monthly rainfall (assumed to be log-normal). To capture how the shape of the distribution varies with location we have pre-calculated values of these coefficients available at each point on a 5 km grid.

Right. So it is reasonable to describe it as a conversion of precipitation
amount to probability, I think.

> If a new standard name is required then I'm happy to take your advice on a suitable choice.

It would be useful to know if anyone else reading this has a view on my
suggestion of precipitation_amount_converted_to_cumulative_probability.

> What is still not clear to me is how I maintain a clear link between the two fields without storing some of the information twice. Is it simply a case of storing two variables in the same NetCDF file (so that they share coordinates)?

If they are in the same file, indeed it is obvious if the fields have the same
spatiotemporal coordinates, because they share the coord vars, as you say. If
they are in different files, the data-user has to check whether the coords are
the same. There is no convention which would allow one to be sure about that
without checking. CF does not rely on variable names, for instance. This is a
very common situation, in fact. For instance, in the CMIP archives each
quantity is in a separate file, and the data variables in many files typically
have the same spatiotemporal coordinates, but analysis software cannot be sure
of that without checking.

Best wishes

Jonathan
Received on Tue Sep 09 2014 - 04:27:32 BST