[CF-metadata] CF convention for vector quantities

From: Bert Jagers <Bert.Jagers>
Date: Mon, 20 Mar 2006 11:18:53 +0100

Dear Jonathan,

> > My program works the other way around. You select a quantity and then
you
> > press plot
>
> If your program offers the user a list of what is in the file, or of
stdnames
> available, why can't it produce a vector plot whenever either of the
components
> is selected? Given one of them, the program can find the other easily.

Yes, if the program has the right logic to find it. Because of the
variations in the standard names, those names did not seem to me as a
reliable means for automatic detection of vector quantities. Are there no
quantities containing earthward and northward that do not form a vector
quantity together (e.g. flux_through_grid_cell_face_in_x_direction and
_y_direction) ?

> > I don't think that I said anything about requiring files to define
> > vector quantities.
>
> Quite right, you didn't. I had incorrectly assumed you were talking about
> putting the component as a dimension, but actually you were talking about
> taking it out of the standard name and putting it in a separate attribute.
> However, I feel the same. Why should we make a special case for this?
There
> are lots of ways the standard name could be factorised and associations
that
> can be made between sets of standard names. These associations are
properties
> of how you use the data, I think, than intrinsically of the data, so they
are
> the responsibility of the data-reader rather than the data-writer. I don't
> see why the relationship between the components of a stress tensor (six of
> them if it is symmetric) is less fundamental than the one you point out.

I would indeed support any request to handle stress tensor components in the
same way as vector components (of course at surfaces you don't have all
stress components). However, I do not completely agree with you that these
associations are properties of the how you handle the data. When generically
interpolating data from one mesh to another, you should be careful to treat
vector quantities as such and not their components as independent scalars
(this may be allowed for eastward and northward components given the right
coordinate system, but isn't allowed for grid dependent x- and y-components
and magnitude & direction pairs. In my opinion, being a vector quantity is
most often a physical property not a postprocessing quantity (although there
are also purely user dependent vector quantities that have no physical
interpretation at all, e.g. (salinity,temperature), that may give useful
plots and animations but are not based on any physical vector
interpretation).

The easiest way forward for me is to avoid the discussion and to introduce a
configuration file that users can edit to support the vector quantities they
want to use. Yet, I still think there is a reason for having a CF convention
in order to be able to specify the vectorial relationship between variables.

> > By the way, are these eularian or lagrangian velocities to be
> > precise? I.e. is the Stokes drift of waves included?
>
> That precision hasn't been requested yet. If you have an application where
we
> need to distinguish, please propose new names. We can accommodate names
which
> are more or less discriminating, appropriate for different users.

I will check and propose new names.

By the way: in case the naming convention "change_in_" is introduced it
should probably require the user to specify its context (e.g. to what period
it relates) otherwise such quantities are useless without further outside
information.

Best regards,

Bert
Received on Mon Mar 20 2006 - 03:18:53 GMT