[CF-metadata] CF convention for vector quantities

From: Jonathan Gregory <j.m.gregory>
Date: Sun, 19 Mar 2006 19:40:14 +0000

Dear Bert

> "eastward_sea_water_velocity" and "northward_sea_water_velocity"
etc.

Unlike you, I think the association between pairs of quantities like this is
something which should reside in your application that processes the data, and
not in the data itself. This is for various reasons including

(1) The components are generally regarded as different variables in models and
observational datasets. They are not always present in the same file. We can't
therefore introduce a general sea_water_velocity with the components as a
dimension. This might sometimes work, but much data already exists which is not
written in this way, and data would continue to be written with components
separated anyway. Hence your software could not depend on the association being
done for you in the file. You'd always need the ability to make the
association yourself. If you have that ability, you can always use it.

(2) The variables are systematically named already. You can derive an northward
from a eastward name easily, for instance, with s/eastward/northward/. One
solution might be for your program to expect the user to specify the vector
quantity by the name of either of its components rather than generically.

(3) Although the association between horizontal components of vectors is a
common association that has to be made, it's by no means the only one, so I'd
argue it's not appropriate to make it a special case. Why should we not also
want to include the vertical component of a vector too? Should we require
stresses always to be supplied as 6- or 9-component tensors? Should we require
that large_scale and convective precipitation be stored together?

> two other issues are related to this question:
>
> A) Distinction between components in east and north direction and components
> in the two grid directions: in which direction are the two components
> defined? It seems that "sea_water_x_velocity" and "sea_water_y_velocity" are
> used for components in the grid direction although in local coordinate
> systems x and y are generally associated with the two main coordinate
> directions whereas the grid may be curvilinear. Most likely
> "direction_of_sea_water_velocity" and "sea_water_speed" should form together
> a vector quantity. Central question: which types of vector components are
> needed?

You're right, a vector might be represented as (eastward,northward), (x,y) or
(direction,speed). All of these are acceptable and might be in a file together.
I don't think that presents a difficulty - do you?

> B) Staggering of data. The ROMS and Delft3D applications that we are looking
> at work both with staggered numerical schemes. So, in fact the two velocity
> components are provided at two different location sets. Before we can deal
> with these vector quantities correctly we need some standardization on
> staggered data. I know there have been some preliminary discussions on this
> matter in the context of CF2, but what is a realistic time schedule for
> these discussions to continue and settle?

There are two timescales: (1) a few months for deciding the future governance
arrangements of CF. (2) finding time to do the thinking and make a complete
proposal. That depends on people volunteering to do it.

Best wishes

Jonathan
Received on Sun Mar 19 2006 - 12:40:14 GMT