[CF-metadata] identification of vector components

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 25 Apr 2012 18:21:11 +0100

Dear Bert

> a) A rectangular model in some UTM coordinates (or possibly a local
> derivative of that) in which x for all practical purposes measures
> distance east and y distance north. If we take the term "true
> longitude" in the definition of x_wind loosely, then we would have
> to write "eastward_wind" instead of "x_wind". However, if true
> longitude literally means only a coordinate with standard name
> "longitude" then we are allowed to use x_wind.

I don't know about the UTM in detail, but since it is a projection I guess
you would put x_wind. However this use-case points out that what I wrote to
Mark wasn't quite right. I suppose that if x was zonal distance and y was
meridional distance (not in degrees), the components would be eastward and
northward (even though x and y are not longitude and latitude).

> b) Our model users can create curvilinear grids in lon-lat
> coordinates. The numerical model doesn't know if the grid that the
> users created happens to be a rectangular grid in lon-lat
> coordinates. In 99% of the cases the grid will be curvilinear and
> hence it does not align with true longitude and thus we should use
> x_wind for the velocity component (see also case c). However, if the
> curvilinear grid that the user provides happens to be a rectangular
> grid then suddenly the program is not allowed to write x_wind
> anymore, and has to write eastward_wind. This is awkward for the
> model to check and therefore I would also support the request to
> allow x_wind to mean wind in the direction of the "x" axes of the
> model.

But, as I wrote to Mark, the model does have to know what it is doing, because
if the grid does not align with lon and lat you must write 2D lon and lat
aux coord vars, whereas if the grid is lon and lat you would not write those
aux coord vars (since the coord vars are lon and lat). The same distinction
can be used to decide which standard names to use.

> Based on the description "the grid x-axis" I
> assume that x_wind may indicate the velocity in i direction in which
> case we need to define an explicit coordinate variable i(i) with
> attribute axis="X". Is this correct, or is x_wind intended to be the
> wind velocity in the local x coordinate direction and not one of the
> grid directions?

I don't know if this is correct, because I am not sure what the distinction
is between the i- and x-directions - sorry. Could you explain a bit more,
please?

While I recognise the point of Mark's proposal, I also agree with the
argument that this may not be helpful to users of the data, who are the
"customers" of CF. It is important to know if you are dealing with lon/lat
or x/y components, simply because quite a lot of software is based on the
former and cannot deal with the latter. So, to be safe, such an application
would ignore data which said it was x/y, because it wouldn't know if was
actually lon/lat. It's much easier for the data-writer to tell the data-user
that the components are lon/lat than for the data-user to infer this from
the coordinates.

Best wishes

Jonathan
Received on Wed Apr 25 2012 - 11:21:11 BST