RE: ncdigest V1 #660 (Writing NetCDF Files: Best Practices) (fwd)

• To: NetCDF mailing list <netcdfgroup@xxxxxxxxxxxxxxxx>
• Subject: RE: ncdigest V1 #660 (Writing NetCDF Files: Best Practices) (fwd)
• From: Timothy Hume <tim.hume@xxxxxxxxxxxxxx>
• Date: Thu, 5 Sep 2002 21:10:09 +0000 (UTC)

Hi,

For many purposes it is useful to have vector quantities stored as two (or
three)  variables, instead of a single two (or three) dimensional
variable. For example, in the case of wind, people often are interested in
the east-west component, and ignore the north-south component. It is
easier to read a complete variable containing the relevant component,
rather than extracting a subset of a two (or three) dimensional variable.

There is also the need to consider vector quantities where the different
components have different units. In the case of wind, the north-south and
east-west components often have the same units (m/s), but the vertical
component often has something like Pa/s for units. In this case it makes
sense to store the components as separate variables, since it is not
trivial to convert Pa/s to m/s (you either have to make assumptions about
a standard atmosphere, or have other information such as geopotential
height).

Tim Hume
Meteorological Service of New Zealand
Wellington
New Zealand

> The mention of vector quantities (e.g. wind) is an important topic which I
> feel needs further discussion.  There should be some standard netCDF
> conventions for vector values. Most wind files I have encountered have
> used separate variables for the two components (north and east), but
> it would seem more logical to use a single variable with a dimension
> (size 2 in this case) for this component of vector quantities. How about
> having a coordinate variable of type character with value "NE" indicating
> the directions?  Maybe there should be a special attribute for such
> a coordinate variable.