Hi Folks
Ok, trying to be really clear ...
If we have s=(x,y,z), where s is displacement in cartesian coordinates, then
we all understand u =(Dx/Dt,Dy/Dt,Dz/Dt)=the velocity vector.
There is no fundamental difference should we change coordinates, so that
s = (x,y,p), and u=(Dx/Dt,Dy/Dt,Dp/Dt) is still the velocity vector
(unarguably I would argue :-)
Traditionally we call the third component the vertical component, and we say
things like the vertical velocity, where what we mean is the vertical
component of the velocity vector. This convention exists independent of
meteorology.
Introducing the word Lagrangian carries no new information, and implies,
wrongly, that there is something special about Dp/Dt in comparison to
any other velocity component in any coordinate system.
I'm sorry to be so picky about this, but as I said in private email to
Jonathan, I think it's important to be systematic and consistent where
possible with other disciplines (ie physics and mathematics, neither of whom
would see the need for the word Lagrangian here).
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.
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.
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).
OK, I'm going to butt out now :-). Do what you will. You will note that I
don't make a recommendation. I think you all know what I think, and I'm free
to put my own name in the files as well as any other name anyway :-)
Bryan
On Wednesday 11 February 2004 20:10, Brian Eaton wrote:
> Hi Jonathan and Bryan,
>
> I agree with Jonathan that "lagrangian_tendency_of_air_pressure" seems like
> a good description for the total, substantive, or material (take your pick)
> derivative Dp/Dt.
>
> Bryan wrote:
> > are partly Lagrangian, but when we calculate them in a model using
> > differencing schemes that require us to evaluate the derivatives by using
> > Eulerian coordinates, they're Eulerian quantities.
>
> I don't understand this statement. There are relationships that allow one
> to compute material derivatives (following a material parcel) from the
> partial derivatives obtained from a fixed (Eulerian) coordinate system.
> Are you suggesting that a material derivative is an Eulerian quantity? I
> think of it as a Lagrangian quantity.
>
> I'm actually comfortable with either Jonathan's suggestion or with Bryan's
> preference of "vertical_velocity_in_pressure_coordinates" as that is a
> commonly used description of omega. But I don't understand the argument
> against using the term Lagrangian. My preference (currently) is
> lagrangian_tendency_of_air_pressure.
>
> Brian
>
> On Wed, Feb 11, 2004 at 05:38:09PM +0000, Jonathan Gregory wrote:
> > Dear Bryan
> >
> > I don't really understand, I'm afraid. Perhaps I'm using wrong
> > terminology. What more accurate term for D/Dt could we use instead of
> > Lagrangian tendency? If we can choose a term for it, then it seems a
> > clear and simple distinction to me that can be made between omega=Dp/Dt
> > on the one hand and partial dp/dt on the other, which we have already
> > called tendency_of_air_pressure.
> >
> > > (see also Dtheta/Dt expressed as and called an "isentropic 'vertical
> > > velocity'" where the inner quotes are David's, in AHL, Middle
> > > Atmosphere Dynamics, p138).
> >
> > I think the inner quotes imply that he's calling it a vertical velocity
> > as a kind of useful analogy, not because Dtheta/Dt is really a velocity
> > in the popularly understood sense of the term. It is instead "standing
> > in" for a vertical velocity.
> >
> > > I really think we are trying to put too much information in one box
> > > (the standard name). I think standard bloat is a real issue to be
> > > careful of. Don't we have a description field for the extended
> > > explanation?
> >
> > All we are trying to do is choose a name for omega! If we can't agree on
> > a reasonably short and unambiguous phrase for it, then I do think we
> > should just call it omega instead and explain what it is in the extended
> > description.
> >
> > Best wishes
> >
> > Jonathan
> > _______________________________________________
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> > CF-metadata at cgd.ucar.edu
> > http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
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--
Bryan Lawrence, Head NCAS/British Atmospheric Data Centre
web: www.badc.nerc.ac.uk phone: +44 1235 445012
CLRC: Rutherford Appleton Laboratory, Chilton, Didcot, OX110QX, UK
Received on Thu Feb 12 2004 - 00:26:19 GMT
