[CF-metadata] omega

From: Jonathan Gregory <j.m.gregory>
Date: Thu, 12 Feb 2004 08:02:42 +0000

Dear Bryan

Thanks for your clearly expressed views. I think we have the same objectives
of correctness and consistency, but different preferences in how to achieve
them.

Holton "Intro to dynamical meteorology" introduces omega on p59 like this:
"Here omega \equiv Dp/Dt (usually called the 'omega' vertical motion) is the
pressure change following the motion, which plays the same role in the
isobaric coordinate system that w \equiv Dz/Dt plays in height coordinates."
I read that to imply that pressure change following the motion is the basic
definition, and the quantity is conveniently regarded as analogous to the
vertical component of velocity. Hence it would be correct to describe omega
either way. We have to choose whichever is least capable of misinterpretation.

> Sign convention? Well, I take the point there is the possibility of lazy
> people getting it wrong, but if we take this argument to it's illogical
> confusion, then we should never have names of books, simply books, because
> lazy people never bother reading the book ... they only read the names.

If we argued the other way to its extreme, we would give all books the same
name because we expect people to read them to find out what they are. Either
conclusion is absurd. We have to make a practical compromise of minimising
the possibility of mistakes where the overhead of doing so is not too great.

> Sign: Flux is a different animal, because we have to confront different
> conventions, but velocity is velocity, and has a direction and sign by
> definition.

I don't think so. Just think of the "east" component of wind: is it eastward
or easterly? This is a frequent sign confusion, so we similarly have to make
clear which one we mean in the standard name.

> Confusion between w(x,y,p) and omega(x,y,p) is not possible if anyone reads
> the description, and uses the appropriate units (or looks at the alias
> omega).

Confusion is reduced even among those who do not read the description if we
make the name itself less capable of misinterpretation. Ironically an
advantage of a cryptic name like "omega" is that it forces the user to look
up the description, rather than misinterpreting a more descriptive name!

Views from more people would be very helpful to resolve this. Should we call
it omega, vertical velocity expressed as tendency of pressure (its current
clumsy name), vertical velocity in pressure coordinates, Lagrangian tendency
of pressure, or something else?

Cheers

Jonathan
Received on Thu Feb 12 2004 - 01:02:42 GMT