On Sat, 10 Jan 2004 08:58:11 -0000, PJB wrote in


Thanks Paul, I'm OK on the low level baroclinic zone. Could you just
amplify
the last sentence. What is a 'shortwave Upper Trough' is this just a
'trough' of low pressure, but at altitude and not necessarily at the
surface?

Yes Phil a shortwave is a trough of low pressure at a certain height (say
300mb) it is called a shortwave becuase it is much smaller than a typical
"long wave" upper trough which stretch for thousands of miles (such as when
we are in an omega block - that is the longwave pattern) a shortwave is a
much smaller feature, but it can provide the required forcing to deepen a
low very quickly.

It's also worth noting these "short waves" typically move faster [1] than
the long waves in which they are embedded. The latter can almost be
thought of as quasi-stationary in many situations, with the shorter ones
moving through them. There is a slight analogy with ocean waves [2] where
the long waves move at greater speed than the short waves.

[1] The speed of wave movement "c" is given by the Rossby Equation:

c = U - Lβ²/4π²

Where U is wind speed within the wave, β is ∂f/∂y a measure of how fast the
coriolis effect (parameter) is changing with latitude and L is latitude.
This formula shows the importance of wavelength. When it exceeds a certain
figure, c goes negative and the wave retrogresses - goes back to the west.

[2] The major difference between the two types of wave is that motion
within an atmospheric wave is largely horizontal, whilst that of an ocean
wave is vertical.

Apoligies if the symbols don't show up correctly. This is my first attempt
at using the Windows Character Map.

--
Mike 55.13°N 6.69°W Coleraine posted to uk.sci.weather 10/01/2004 09:43:26 UTC
My aurora images here http://www.mtullett.plus.com/29a-oct and
http://www.mtullett.plus.com/20-nov/