Hi Jonathan and Juerg,
I agree with what Jonathan has written below, and would like to add a
suggestion for treating the positive attribute:
To identify a vertical coordinate one looks for pressure units or the
positive attribute. The positive attribute was defined by COARDS in the
context of 1D coordinate variables. Since it has proven to be a useful way
to identify a generic vertical coordinate, I propose that we extend its
definition to cover the multi-dimensional variable case. The current
wording is
The direction of positive (i.e., the direction in which the coordinate
values are increasing), whether up or down, cannot in all cases be
inferred from the units. The direction of positive is useful for
applications displaying the data. For this reason the attribute positive
as defined in the COARDS standard is required if the vertical axis units
are not a valid unit of pressure (a determination which can be made using
the udunits routine, utScan) -- otherwise its inclusion is optional. The
positive attribute may have the value up or down (case insensitive).
I suggest we modify this to something like:
The direction of positive (i.e., the direction in which the coordinate
values along a vertical column are increasing), whether up or down,
cannot in all cases be inferred from the units. The direction of
positive is useful for applications displaying the data. For this reason
the attribute positive as defined in the COARDS standard is required if
the vertical coordinate units are not a valid unit of pressure (a
determination which can be made using the udunits routine, utScan) --
otherwise its inclusion is optional. The positive attribute may have the
value up or down (case insensitive). This attribute may be applied to
either coordinate variables or auxillary coordinate variables that
contain vertical coordinate values.
Brian
On Mon, Jul 28, 2003 at 05:03:21PM +0100, Jonathan Gregory wrote:
> Dear Juerg
>
> > But why the positive
> > attribute? Is the positive attribute not only applicable to 1-dimensional
> > coordinate variables?
> I think this is something we'll have to decide.
>
> I see what you are getting at with the generalised height coordinate: you
> regard Z as a function of model level, x and y, and see this as analogous
> to having pressure as a dimensional vertical coordinate being a function of
> dimensionless sigma, x and y (via ps), for instance.
>
> However, I still think that a better scheme is to name it in the coordinates
> attribute. It can be identified by its standard_name and units if not by a
> positive attribute. It isn't really a "function" of model level, x and y in
> the sense for which formula_terms was intended. There is no computation
> involved - it's just a look-up. I see an auxiliary coordinate variable
> Z(zlay,yc,xc) as analogous, for instance, to a 2D longitude variable
> longitude(yc,xc).
>
> Cheers
>
> Jonathan
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Received on Mon Jul 28 2003 - 14:20:11 BST
