[CF-metadata] question about proper cell bounds for ocean_volume_transport_across_line

Jonathan Gregory j.m.gregory at reading.ac.uk
Thu Jan 17 06:39:20 MST 2019


Dear Matthias

> Recap: I have a time series of ocean volume transport (given in
> Sverdrup) across a line. The line has two end points, and the transport
> is computed over a specific depth range. I would like to represent this
> in a clever way in a netcdf file with CF conventions.
> 
> As far as variables go, I really only have two (happy to add more to
> e.g. contain the cell boundaries), and their standard_names are:
>    time
>    ocean_volume_transport_across_line
> 
> I know how to include bounds for the time such that I can show over how
> many days my data were averaged.

Yes, including a cell_methods of "mean" for time.

> I suppose I could figure out the depth range by including a single
> scalar for depth, with associated bounds, and having a "coordinates"
> attribute on my transport that points to the depth. Does this sound
> correct?

Yes, that would be right. The cell_methods for depth would be "sum".

> How about the two section endpoints? And when I have those, how would
> the user know which direction across that section is counted positive?

So far, the tranport across line has been used only with "named" sections. The
names of the sections haven't been standardised in CF, but there's a list of
them for CMIP6, defined by endpoints, in Griffies et al. 2016, Table J1,
doi:10.5194/gmd-9-3231-2016. The reason for using names is that the exact line
depends on the model, so matching the names facilitates model comparison. I
don't remember a discussion about how to specify them more quantitatively. I
think it would be reasonable to attach scalar latitude and longitude (like the
depth range) and use their bounds to specify the endpoints.

The standard name table doesn't mention the sign convention. We should have
defined that. Griffies et al. says it's positive for northwards and eastwards.
But what if the normal to the section is north-west or south-east? Do you have
any in those quandrants? We ought to generalise this.
 
Best wishes

Jonathan

> 
> Cheers, Matthias
> 
> 
> 
> On Tue, 2016-04-26 at 08:25 -0700, Matthias Lankhorst wrote:
> > Dear CF,
> > 
> > what is the proper way to define the shape and boundaries of an oceanic
> > section, for which I want to report the volume transport across that
> > section?
> > 
> > The use case is an irregular-shaped section like OSNAP
> > (http://www.o-snap.org/wp-content/uploads/2013/11/20160329_OSNAP_planeview.jpg), and the property to be reported is the seawater volume transport in a given depth range as a timeseries with this CF standard_name:
> > ocean_volume_transport_across_line
> > 
> > There needs to be some ancillary variable to say what the line
> > coordinates are, and I am not sure how to squeeze that into
> > "cell_bounds".
> > 
> > In addition, how about the lower limit vertically if this is the
> > seafloor? If the transports were to be everything below e.g. 1000 m,
> > would it be appropriate to state vertical cell bounds as 1000 to 5000 m,
> > even if the ocean is not 5000 m deep? I.e. is the user intelligent
> > enough to realize that "5000" really means "5000 m or the seafloor,
> > whichever is shallower"?
> > 
> > Thanks, Matthias
> > 
> > 
> 
> -- 
> _______________________________________
> 
>  Dr. Matthias Lankhorst
>  Scripps Institution of Oceanography
>  9500 Gilman Drive, Mail Code 0230
>  La Jolla, CA 92093-0230
>  USA
> 
>  Phone:  +1 858 822 5013
>  Fax:    +1 858 534 9820
>  E-Mail: mlankhorst at ucsd.edu
>  http://pordlabs.ucsd.edu/mlankhorst/
> 
> 
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