[CF-metadata] omega

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 11 Feb 2004 11:26:27 +0000

Dear Bryan

The fact that the model is Eulerian doesn't mean you can't calculate
Lagrangian derivatives. Omega is Dp/Dt, not partial dp/dt. I think you'd
expect Dp/Dt calculated in an Eulerian model to be comparable with Dp/Dt
following the parcels in a Lagrangian model. It's the same physical quantity,
though differently computed, so should have the same standard name, I'd say.
We shouldn't confuse the quantity itself with the method used to obtain it.

> velocity is simply rate of change of
> displacement with time. In these coordinates, displacement is measured in
> pressure, and so the words "vertical velocity in pressure coordinates" would
> seem to cover it. The relationship of the coordinate system to the
> horizontal is irrelevant isn't it?

I agree that "vertical velocity in pressure coordinates" is a helpful way to
introduce what the quantity is used for in a discussion of atmospheric
dynamics, but I'm less happy with it as a choice of a standard name because
(a) Pa s-1 is not the normal use of the word "velocity". Of course we may
have other "kinds" of velocity such as "angular_velocity", but the phrase
"vertical velocity" means m s-1 to me.
(b) It's not vertical, so this is an inaccurate name.
(c) It doesn't say what its sign convention is.
(d) It could be understood as meaning w(x,y,p) i.e. that "in pressure
coordinates" distinguished w(x,y,p) from w(x,y,z). That would not be
something we would include in a standard name, but I think it's a possible
misinterpretation we should avoid.

What if we just call it omega instead. :-) (joke - I think)

Best wishes

Jonathan
Received on Wed Feb 11 2004 - 04:26:27 GMT