[CF-metadata] identification of vector components

[Hedley, Mark]

There is a statement in the definition of many standard names which are used for vector component definitions, e.g.:

  x_wind
    alias: grid_eastward_wind
    "x" indicates a vector component along the grid x-axis, when this is not true longitude, positive with increasing x. Wind is defined as a two-dimensional (horizontal) air velocity vector, with no vertical component. (Vertical motion in the atmosphere has the standard name upward_air_velocity.)

I think that the statement 'when this is not true longitude' is problematic, particularly for software converting from other formats, where x indicates the grid i direction, independent of rotation or projection.  I do not think it is the place for standard_name to limit the use of the term 'x' to cases where the horizontal coordinate reference system is not 'true latitude longitude'

I propose that these terms be removed from all standard names which have 'x' or 'y' as a modifier.

This would enable all x-ward and y-ward definitions to be used, independent of the grid_mapping, as standard names.

eastward and northward remain useful modifiers as many models may choose to output eastward vector components where east is not the x direction for the model grid.

The work on vector containers in:
  https://cf-pcmdi.llnl.gov/trac/ticket/79
has indicated a good way forward for identifying vector components, and identifying that vectors are with respect to a grid_mapping.  I think this proposed change would interface nicely to the proposal in ticket 79

How would this proposal be viewed by the community?

mark