[CF-metadata] omega

[Karl Taylor]

Dear Jonathan,

I'd like to stick with my original vote which was Lagrangian tendency of 
air pressure, but what are we going to call sigma-dot?  If we end up 
calling it the "vertical velocity in sigma coordinates", then perhaps it 
would be more consistent to call omega the vertical velocity in pressure 
coordinates.  Maybe someone has a better idea.

Karl

Jonathan Gregory wrote:
> 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
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