Hello,

This is my first post.

I was trying to understand the coupling between the conditions at the surface condition and aloft. So far, I got to the following:
- An imbedded short wave disturbs the upper airflow creating cold and warm air advections and intensifying the trough/ridge
- The polar jet stream creates areas of convergence aloft (supplying air to the surface high pressure) and divergence aloft (removing air from the surface cyclone).

Now, I had a look at the creation of these short waves, I found that they usually originate from terrain disturbance (barotropic instability) or from baroclinic instability (where air gets squeezed between frontal zones and the tropopause).

How does it actually work because it seems that surface influences aloft and aloft influences surface, but which one starts first?

Many thanks,

Stephane

Hello Stephane.
Your understanding is fine. But the answer to your question is not
straightforward. It is often not possible to ascertain at what level in the
atmosphere a disturbance actually begins. The atmosphere is a complex system
containing many feedback paths, but in my opinion, it can be made a little
more understandable if considered from the 'top down'. The pressure pattern
and winds (and therefor the troughs/ridges, highs/lows) at any level are
determined by the pressure pattern/winds at some higher level, plus the
thermal pattern in between. Very often disturbances in the flow in the upper
troposphere/lower stratosphere produce a dynamic coupling which causes
descent and warming in the lower stratosphere and vice versa, which will
cause troughs and ridges in the upper troposphere to amplify or decay.
Disturbances in the flow can propagate downstream, and influence the
downstream development in a way that reflects their initial development,
which may in turn have been influenced by earlier upwind changes. By that I
mean that a short-wave upper trough, with its attendant overlying couplet of
warming and cooling, can find itself, by dint of moving downstream, in an
environment conducive to rapid amplification through positive feedback
between the flow strength and shape (vorticity) and the overlying thermal
field. But changes in vorticity experienced by the flow are accompanied by
stretching/contraction of vertical columns in the upper troposphere and
lower stratosphere, and the effect of this on the thermal field there is
most marked in the lower stratosphere because of the stable temperature
lapse rate that generally exists there. So, it is a case of feedback that
determines whether the vorticical stretching and contraction will be able to
produce amplification or damping of a disturbance, and the development
itself will influence the vorticity pattern and the way it changes.
Perhaps this doesn't make it any clearer for you, but hopefully may help.

--
Bernard Burton
Wokingham, Berkshire, UK.

Weather satellite images at:
www.woksat.info/wwp.html

"FrenchScotPilot" wrote in
message ...

Hello,

This is my first post.

I was trying to understand the coupling between the conditions at the
surface condition and aloft. So far, I got to the following:
- An imbedded short wave disturbs the upper airflow creating cold and
warm air advections and intensifying the trough/ridge
- The polar jet stream creates areas of convergence aloft (supplying air
to the surface high pressure) and divergence aloft (removing air from
the surface cyclone).

Now, I had a look at the creation of these short waves, I found that
they usually originate from terrain disturbance (barotropic instability)
or from baroclinic instability (where air gets squeezed between frontal
zones and the tropopause).

How does it actually work because it seems that surface influences aloft
and aloft influences surface, but which one starts first?

Many thanks,

Stephane




--
FrenchScotPilot

Thanks Bernard for taking the time to answer me in such details. It indeed sounds like a complex matter. I'll have a look at your link, it looks very interesting.

For information, I managed to compile a succinct explanation:

A strong IPV anomaly at upper levels has a strong cyclonic circulation associated with it. A weaker extension of this circulation extends down to the surface. When the anomaly moves over a baroclinic zone the lower level circulation induces a wave in the thermal field, the wave crest forming a positive temperature anomaly. This new center establishes its own cyclonic circulation, the upward extension of which can eventually reinforce the flow about the upper centre. Thus a process a mutual reinforcement (positive feedback) is established.

Ref: Structure and Evolution of Baroclinic Waves and Fronts - Isentropic Potential Vorticity approach

http://www.comet.ucar.edu/class/aes_...cs/PVintro.pdf

Stephane