name         Arvid G.
status       other
age          60s

I am fascinated by jet streams, and have read
http://146.139.100.40/webpages/askasci/wea00/wea00065.htm


Questions -   1.    Please describe the mechanisms of the jetstream terms
divergence and convergence, and the consequences of these.

2.    I understand that the jetstream is driven by the effects of the sun.
Please explain how.

Arvid,

The jetstream is not really driven by the sun.  The jetstream
forms at breaks in the tropopause, in other words, where the
tropopause changes height dramatically.  Over North America
there can be as little as one tropopause break (and therefore
one jet) or three breaks (thus three jets). Sometimes two jets
can split from one or combine into one.  The northernmost jet
is called the polarfront jet (it is usually located over the
northern states), the southernmost one is called the
subtropical jet (the latter usually hangs around the southern
border of the United States).  Sometimes there is another jet
inbetween.  The height of each jet (and the break in the
tropopause height) depends on latitude.  The subtropical jet is
higher (often much higher, 10-16 km) than the polarfront jet
(7-12 km).

Divergence and convergence are terms used to describe how air is
flowing into or out of a weather system of any kind and is not
specific to jetstream analyses.  Convergence implies that air is
flowing into the system and therefore upwards at the center (cooling
as it rises), as in a low pressure system.  Divergence implies
that air is sinking (and therefore warming) as air flows out
of the system (and the system expands) as in a high pressure area.
Jetstreams normally result in convergence beneath them.  They are
often involved in the genesis of weak low pressure systems at the
surface, especially in the wintertime.  The low pressure system
in the central part of the country today (Dec. 18, 2000) is a good
example of this.  The low pressure system normally is found just
north and west of a jet that is oriented northeast-southwest and
bulges to the north, as today.  I often say to people that the
low pressure area rides the jetstream bubble, west of this bulge.
Compare the upper level map and the surface map on Intellicast
weather to see this.

Another important aspect of the jetstream is the turbulence at
the edge (clear air turbulence, or CAT) that results from the
wind shear between the high jetstream speeds inside and the slower
wind speeds outside it.  This has damaged many aircraft and
shaken up many thousands of passengers as commercial airlines
use it to decrease travel times.

David R. Cook
meteorologist
Argonne National Laboratory
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This page originally published at:  http://www.newton.dep.anl.gov/askasci/wea00/wea00067.htm