Name: Peter E.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 2001
Question:
Dear Dr.R.Cook.
I have read your archival info re "Jet Streams" and in my studies have
realized that this complex 3-D system (according to some authors)
BEGINS with the temp/press gradients at a low level, influencing
mid level winds moving either cyclonically or anticyclonically;add
the intrusion of high level winds and the Coriolis Effect; plus the
effect of the pressure gradient at the 300mb. level and the position
of the Polar Front == THE PATTERN for the Jet Stream.
1] in this simplification have I left out an important "factor"?
2]does the system BEGIN at the temp/press gradient at a LOW level?
3]or perhaps, there is a causality which begins with the jet stream
and works its way down to the surface?
I appreciate your time and effort in this section and would
be very grateful if you could enlighten me on the above questions.
Sincerely Peter E.
Replies:
Peter,
The strength of the polar jet stream is
largely determined by the strength of the
temperature and pressure gradients at the surface.
That is why the polar front is so important.
Strong temperature gradients (and pressure gradients
secondarily) at the surface are amplified to stronger
gradients with increasing altitude, thereby
resulting in large horizontal wind shear at about
300-500 mb (depending on the time of year and
latitude of the polar front) and strong winds (the
jet stream) at that level. The fastest winds of the jet stream
are restricted to just below the Tropopause, the boundary
between the Troposphere and Stratosphere. The stable
stratification of the Stratosphere prevents intrusion
of the jet stream into it. The high level winds and
pressure gradient at 300 mb are more affected by the polar
jet than the jet is by the high level winds and
pressure gradient at 300 mb.
This system is complex! A good place to see the
complex structure of the polar front, polar jet,
subtropical front, and subtropical jet (the latter two
existing only during the warm half of the year) is a
diagram (with a description of the jet stream before it)
at the Univ. of Oregon site at
zebu.uoregon.edu/~js/glossary/jet_streams.html
Another interesting site is one from Lyndon State College
showing the Northern Hemisphere jets. Pick "Northern
Hemisphere Jet Stream with MSL Pressure" at
apollo.lsc.vsc.edu/textonly/weather/hemis.html
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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