From: Subject: Thursday Weather Topic in Greater Depth Date: Thu, 5 Nov 2009 10:55:06 -0800 MIME-Version: 1.0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/learn/r_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Thursday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Thursday, 5 = November=20 2009

JET STREAKS

Often imbedded in upper tropospheric jet streams are elongated = regions of=20 very strong winds called "jet streaks". This term is used because of the = streaky=20 appearance of these regions on upper-air weather charts. These streaks = may be=20 several hundred miles long, but only a hundred miles wide. Winds within = these=20 streaks may reach 200 mph. Meteorologists have found that these regions = help=20 explain how surface weather systems can develop and be maintained.

During the early part of the 20th century, meteorologists had = developed=20 elaborate models that attempted to explain why mid latitude low-pressure = systems=20 developed along frontal boundaries. Most of their observational data was = from=20 surface weather reports, since sparse upper-air measurements were = available to=20 altitudes that were usually below 20,000 feet. However, during the late = 1920's,=20 a British meteorologist (Henry Dines) proposed a theoretical model that=20 suggested a link between surface wind flow and upper tropospheric wind = flow. The=20 horizontal convergence of surface wind flow into the surface = low-pressure=20 systems could be maintained if a compensating horizontal divergence in = the upper=20 troposphere existed. Likewise, near surface divergence of the wind flow = from a=20 surface high-pressure cell could be maintained if upper tropospheric = convergence=20 were sufficient to provide the compensation. Vertical cross sections of = a=20 version of this theoretical model appear in Figures 8.21 and 8.22 of = your=20 Weather Studies textbook.

Following the discovery of the jet stream near the end of World War = II and=20 the accumulation of a sufficient data set of upper tropospheric wind=20 observations, the proposed theoretical model was verified. = Meteorologists found=20 that quite often surface low-pressure systems in the midlatitudes = developed=20 below regions where an upper tropospheric jet streak was detected. These = observations provided the needed compensating mechanisms for the = development and=20 maintenance of surface weather features.

Air parcels that move along with the upper tropospheric winds in the = jet=20 stream will accelerate when they enter the rear of a jet streak and then = decelerate when leaving the front end of the jet streak. As a result of = this=20 acceleration and deceleration, regions of horizontal divergence and = convergence=20 develop within this jet streak as the air winds its way through the = streak.=20 Figure 9.22 of your text provides a graphical depiction of the = distribution of=20 such features in the core of the jet streak. Vertical circulation = regimes=20 develop that link these upper level features with the surface pressure = patterns.=20 Specifically, a region of upper level horizontal convergence of air = produces=20 sinking motion of the air in the atmospheric column below the jet = streak,=20 contributing to the surface divergence of air from a surface = high-pressure cell.=20 Conversely, rising motion from a surface low pressure cell would appear = below a=20 region of upper level divergence. This model extends the hand twist = model that=20 you have applied to the surface weather features in order to explain why = fair=20 skies and little precipitation typically occur with surface highs, while = clouds=20 and precipitation tend to appear with lows.

Often, if you look at a surface weather analysis and find a distinct = surface=20 low pressure feature - such as a midlatitude storm system, you may be = able to=20 see a jet streak above that location on an upper level chart, such as = the 300 mb=20 chart that can be accessed from the DataStreme Atmosphere = website.

Return to the Thursday = Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =