[CF-metadata] omega

[Bryan Lawrence]

Hi Folks

I don't think the use of the word Lagrangian is appropriate ... in nearly all 
cases, we are measuring omega in Eulerian models ... we should reserve the 
word Lagrangian for the case where the data is truely Lagrangian (as it might 
for example be in balloon measurements, or in trajectory studies).

The problem here lies in the fact that we have a bunch of different coordinate 
systems floating around, and we're having trouble using words that need to 
refer to coordinate systems - but 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?

Bryan

On Monday 09 February 2004 22:58, Jonathan Gregory wrote:
> Dear All
>
> Last year we replaced "omega" with the more explicit
> vertical_air_velocity_expressed_as_tendency_of_pressure
> but this is rather clumsy.
>
> It has been suggested we should instead have
> vertical_velocity_in_pressure_coordinates
> Speaking for myself, I feel that this is not so good, although more
> familiar, because I think the sign convention is not obvious, and I am
> worried it might be mistaken for vertical velocity on pressure levels.
>
> Mike Blackburn has pointed out that omega isn't strictly vertical velocity,
> since pressure surfaces are not necessarily horizontal. Omega is actually
> the rate of change of pressure of an air parcel. This suggests a new
> possibility: langrangian_tendency_of_air_pressure
> for omega. What do you think? I like this because it is accurate and
> shorter than the others. We could define lagrangian_tendency in the
> guidelines. The usual tendency is Eulerian. Plain tendency_of_air_pressure,
> which is already a standard name, means the partial derivative wrt time
> e.g. of air pressure measured at a synoptic station, whereas
> lagrangian_tendency means the total derivative i.e. following a parcel.
>
> The same argument would apply to vertical air velocity in sigma
> coordinates.
>
> Cheers
>
> Jonathan
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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