[CF-metadata] COARDS name for a time offset

From: Jonathan Gregory <j.m.gregory>
Date: Sat, 23 Dec 2006 15:47:23 +0000

Dear Ed

Thanks for the further information and example.

> I would like to specify
> a time offset such that for a certain wind pixel:
geographical indices j,i
> time[0] + sst_dtime[0,j,i] + this_offset = time of observation of
> the wind pixel.

I presume that this_offset is dimensioned [nj,ni] and that 0 is an example of a
time-index. From the example I understand that the point of this is to specify
2D time arrays (nj,ni) for SST, wind etc. rather than 3D (nt,nj,ni). This can
be done because the time-offset from time(t) of each variable at each location
is the same for all values of time index t.

I agree with you that there is no existing CF convention which would be able
to indicate this procedure. However, most CF features are optional anyway so
CF compliance is a fairly minimal requirement. I suspect that yours is quite a
specialised requirement, so maybe we can approach it with a combination of CF
conventions, and your own GHRSST conventions.

For instance, we could introduce a new standard name of time_offset, which has
time units but is for a time-difference rather than an absolute time. We have
a standard name of forecast_period with the same characteristics, but that
would not be appropriate here. You could give the sst_dtime variable this
standard name, and indicate it as an auxiliary coordinate variable of the SST
variable, by listing it in the coordinates attribute. You then require a GHRSST
convention that if a data variable has a 1D coordinate variable of time, and an
auxiliary coordinate variable with the time dimension and a standard_name of
time_offset, the absolute time of datum(t,j,i) is
time(t)+t_dependent_time_offset(t,j,i).

Then for each non-SST variables such as wind speed, you need another 2D
variable e.g. wind_dtime, again with standard_name of time_offset. You can
indicate both the sst_dtime and the wind_dtime as auxiliary coordinate vars
of the wind_speed variable, and extend the GHRSST convention to say that if
there are is also an auxiliary coordinate variable with standard_name of
time_offset but which does not have the time dimension, that should be added as
well, so that the absolute time of datum(t,j,i) is
time(t)+t_dependent_time_offset(t,j,i)+t_independent_time_offset(j,i)

I wonder why you store time(t) at all? t could just be an index dimension,
and the sst_dtime could be an absolute time(t,j,i) rather than an offset. That
would simplify this a bit.

Best wishes

Jonathan
Received on Sat Dec 23 2006 - 08:47:23 GMT