Dear Jonathan,
Let us consider the following use cases:
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. This is a part of the description of x_wind
that is not clearly documented. (Will dig deeper in use case c)
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.
c) After writing down b, I start to wonder how I actually should
interpret all this. The most common use case for us is a curvilinear
model in local x,y coordinates. Hence we have the direction of the
curvilinear coordinates (i,j), the directions of the local coordinates
(x,y), and the direction in lon-lat coordinates. Which may all happen to
align, but won't do so in general. The eastward_wind and northward_wind
clearly refer to the lon-lat coordinates. However, there is ambiguity in
the definition of x_wind since it matches "the grid x-axis". I know that
for most classic CF applications it is assumed that a grid in non
lon-lat coordinates is actually a cartesian grid in some other
coordinate system, but this isn't the case for general models. Hence
(skipping all the meta-data for the moment) we end up with variables:
waterlevel(i,j) associated with local coordinates x(i,j) and y(i,j)
which may be transformed into lon(i,j) and lat(i,j) containing true
longitude and latitude. What I need to store is the velocity are the
velocity components in the i and j directions. 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?
Best regards,
Bert
--------
Jonathan Gregory wrote:
> Dear Mark
>
> I see your point of view, and when we discussed this on the phone it did not
> sound like a large issue to me. I agree with you that X/Y and lon/lat are
> related ideas in CF. However, it appears there are some concerns.
>
> As you say, as a data-producer, where the vector component is aligned to the
> data grid you have to use a different standard name according to whether the
> grid is lon-lat or not. However, there are other reasons why you have to be
> aware of this too. Specifically, to write a CF-compliant file, if the grid
> is not lon-lat you have to provide 2D lon and lat aux coord vars. If the grid
> is lon-lat you won't provide these. The program writing the data must be aware
> of the distinction, and can use it to decide which standard names to use.
>
> If the file is correctly written then, as a data-consumer, you can decide
> whether eastward/northward would be identical with x/y simply by inspecting the
> standard_names of the coordinate variables.
>
> Can similar tests be used with Grib conversion? I assume that Grib must provide
> sufficient metadata to indicate whether the grid is lon-lat.
>
> Best wishes
>
> Jonathan
> _______________________________________________
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> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
>
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Received on Wed Apr 25 2012 - 01:12:27 BST
