Extratropical storms

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Extra-tropical storms

• Polar front lows
• Wind, rains and floods
• Strong Localised winds and rain
• Thunderstorms
• Tornados

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Polar-front  lows (40o N and S of equator)

• low pressure cells develop along the polar front
  with diameters 2000km.
• travel easterly
• location depends on the "Rosby waves" in polar
  front

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Global Atmospheric Circulation
Atmospheric circulation global circulation
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Jet stream
http//vortex.weather.brockport.edu/sweinbec/clas
s/25_JetStream.swf
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Jet Streams along the Polar front
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Summer weather and the jet stream
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Cold and Warm fronts
http//vortex.weather.brockport.edu/sweinbec/clas
s/05_CnWfronts.swf
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12pm today
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The Columbus Day Storm - the strongest
non-tropical wind storm on record to hit the
continental United States. Oct. 12, 1962. The
storm killed 46 people, injured hundreds and
interrupted power to several million households.
Windstorms produce strong winds to 60 mph and
cause property damage. About once every decade -
storms with powerful winds of 70 mph last an
average of three to six hours .
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Some regions affected

• Eastern seaboard of USA
• North sea region
• Middle ages saw cataclysmic storms - huge social
  impact - dramatic effect on coastline of Northern
  Europe
• All Saints Day Flood 1-6 November 1570 400,000
  people killed in Western Europe
• Erosion of Heligoland - 60km to 25km between 800
  and 1300AD - Gulf of the Zeider Zee formed by
  Lucia storm of 14 Dec 1287

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1953-Great Storm

• January 31 and February 1, 1953, a storm surge
  and high tide left a trail of death and disaster
  in the worst flooding in Britain this century.
• Floods claimed 307 lives, devastated 200,000
  acres of farmland, swept cattle, horses, sheep
  and poultry to their deaths and made 21,000
  people homeless. Over 100 more lives were lost at
  sea, and 1800 were lost in Holland.

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Localised Strong Winds

• Thunderstorms
• Supercells
• Tornadoes

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Formation of Thunderstorms

• Globally 1800 per hour
• Development depends on intensification of local
  instabilities in the atmosphere.
• Large parent cloud forms as air parcels heated
  near the ground expand, become less dense and
  rise through the more dense cooler surrounding air

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Temperature structure of the atmosphere
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The rate at which the temperature falls with
height in the troposphere is known as the lapse
rate
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Cooling on expansion
Heated air expands and becomes less dense, it
also cools as it does work on the surroundings.
Tp2
Tp2lt
Tp1
Tp1
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• Three possibilities depending on the lapse rate
  (rate of change of atmospheric temp with height)
• Stable atmosphere
• Neutral atmosphere
• Unstable atmosphere

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Atmospheric temperature
If Tp2 lt Ta2 the air parcel sinks again stable
atmosphere If Tp2 gt Ta2 the air parcel rises
further unstable atmosphere If Tp2 Ta2 the
air parcel does not move neutral atmosphere
Tp2
Ta2
Tp1
Ta1
Lapse rate
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Latent Heat 2,260 kJkg-1

• When water vapour condenses 2,260kJ kg-1 are
  released into the atmosphere - this heat greatly
  enhances the convective instability

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Cumulonimbus
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Single Cell Thunderstorms

• Typically do not produce severe weather
• low danger to public
• moderate to high danger to aviation
• Usually last for 20-30 minutes
• Seem quite random (perhaps because of our lack of
  understanding) in the production of brief severe
  events such as downbursts, hail, some heavy
  rainfall, and occasional weak tornadoes. 

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Within the cloud's lower regions condensing snow
and rain gradually outweigh the updrafts and drag
air down as they begin to fall
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The Storm Breaks
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Single-cell storm hazards hail
This hail fall occurred in Altus, Oklahoma in
1982 and was accompanied by several tornadoes.
Hail causes more monetary loss than any other
type of thunderstorm-spawned severe weather. 
Annually, the United States alone suffers about
one billion dollars in crop damage from hail.
China -May, 1986 -100 people were killed, 9,000
injured, and 35,000 homes destroyed by an intense
hailstorm.
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Hail Hazard
A hailstone of about this size killed a man in
northern Ft. Worth, in the same thunderstorm
which produced the downtown tornado in March
2000.
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Lightning Hazard
On average, lightning kills more people per year
than tornadoes. Strikes can occur many miles from
the base of a thunderstorm, even without rain --
and even with the sun shining!
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Weak Tornado from single-cell thunderstorm
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Multicell storms

• Squall lines consist of a line of storms with a
  continuous, well developed gust front at the
  leading edge of the line. An approaching
  multicell squall line often appears as a dark
  bank of clouds covering the western horizon.

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Squall line
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Squall line
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Hazards of Squall lines

• Severe weather near the updraft/downdraft
  interface at the storm's leading edge.
• Downburst winds
• Hail as large as golf balls
• Gustnadoes.
• Flash floods occasionally occur when the squall
  line decelerates or even becomes stationary

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Multicell Cluster Storms
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Supercell Thunderstorms
Thunderstorms with deep rotating updrafts. 
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Supercell storm
height
temp

• Low-level temperature inversion in the atmosphere
  can block weak convection, but a strong cumulous
  cloud can breach the inversion.
• The warm, moist air trapped below the inversion
  surges through the hole.
• Dry air above the inversion cools the edges of
  the upthrusting air by evaporation increasing the
  temperature difference between the central
  upthrust and the surroundings increasing the
  upthrust.
• Strong high altitude winds can suck up the lower
  air.

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• Most intense over tropical Australia - named
  stratospheric fountains - as the instability is
  so strong that it can penetrate the tropopause
  and inject air from the normally isolated
  troposphere into the stratosphere.

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Tornadoes
http//redrock.ncsa.uiuc.edu/AOS/imax.html
http//www.spc.noaa.gov/faq/tornado/The Basics
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Definition
According to the Glossary of Meteorology (AMS
2000), a tornado is "a violently rotating column
of air, pendant from a cumuliform cloud or
underneath a cumuliform cloud, and often (but not
always) visible as a funnel cloud." In order for
a vortex to be classified as a tornado, it must
be in contact with the ground and the cloud base.

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Tornado animation
http//vortex.weather.brockport.edu/sweinbec/clas
s/20_nTwinds.swf
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• The Bridge Creek-Moore-Oklahoma City-Midwest
  City, OK, tornado of 3 May 1999 currently ranks
  as the most destructive tornado in history with
  over 1 billion in damage, both in absolute
  amount and normalized to 1999 dollars. This made
  3 May 1999 the costliest tornado day on record as
  well.

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Tornado (without funnel)
The dust cloud and cloud base above it were
rotating, indicating a continuous cloud-to-ground
vortex (tornado). The lack of a visible funnel
can be related to several processes. Most likely,
the pressure drop and lift in the tornado vortex
was too weak to cool and condense a visible
funnel and/or the air below cloud base was too
dry.
http//www.spc.noaa.gov/faq/tornado/The Basics
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Supercell thunder storms
The most destructive and deadly tornadoes occur
from supercells -- which are rotating
thunderstorms with a well-defined circulation
called a mesocyclone.
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Funnel cloud
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Forecasting Tornadoes
Look for the development of temperature and wind
flow patterns in the atmosphere which can cause
enough moisture, instability, lift, and wind
shear for tornadic thunderstorms.
A large variety of weather patterns can lead to
tornadoes and often, similar patterns may
produce no severe weather at all.
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Doppler Radar Doppler radar can see not only the
precipitation in a thunderstorm (through its
ability to reflect microwave energy, or
reflectivity), but motion of the precipitation
along the radar beam. In other words, it can
measure how fast rain or hail is moving toward or
away from the radar.
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Many tornadoes contain smaller, rapidly spinning
whirls known as subvortices, or suction vortices
usually last less than a minute each.
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Satellite Tornado
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Location of Tornado in a Storm
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Tornado Alley
In the mid-west USA thunderstorm development is
linked to the path of the polar jet stream
attached to the polar front. In spring and summer
temperature differences of over 200C can exist
between warm humid Gulf air and outbreaks of cold
polar air. This leads to the formation of
supercells and tornados. Southern states in
Winter as far North as great lakes in Summer

• Warm humid tropical air flowing North
• Cold mass of dry air moving South from Canada at
  about 80km/h
• High altitude jet stream from NW at 250km/h
• Meeting of these three winds set up wind shear
  which spins the thundercloud

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Jet stream position North America
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Destruction

• Lifting force at funnel wall. Large objects
  200-300 tonnes lifted and moved tens of metres.
  Houses and train carriages carried several
  hundred metres. Can suck up millions of tonnes of
  water - draining rivers.
• Most destruction caused by the sudden change of
  pressure across the funnel wall. Air inside a
  building affected by a Tornado is at a much
  higher pressure and cannot escape rapidly enough
  causing an explosion. The sudden change in
  pressure can lead to rapid dehydration causing
  living things to appear burnt - one of the most
  common causes of death in Tornados.

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Localised damage
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F-0 Gale tornado (40-72 mph) Some damage to
chimneys breaks branches off trees pushes over
shallow-rooted trees damages sign boards.
F-1 Moderate tornado (73-112 mph) The lower
limit is the beginning of hurricane wind speed
peels surface off roofs mobile homes pushed off
foundations or overturned moving autos pushed
off the roads attached garages may be destroyed.

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F-2 Significant tornado (113-157 mph)
Considerable damage. Roofs torn off frame houses
mobile homes demolished boxcars pushed over
large trees snapped or uprooted light object
missiles generated.
F-3 Severe tornado (158-206 mph) Roof and some
walls torn off well-constructed houses trains
overturned most trees in forest uprooted.
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F-4 Devastating tornado (207-260 mph)
Well-constructed houses leveled structures with
weak foundations blown off some distance cars
thrown and large missiles generated.
F-5 Incredible tornado (261-318 mph) Strong
frame houses lifted off foundations and carried
considerable distances to disintegrate
automobile sized missiles fly through the air in
excess of 100 meters trees debarked
steel-reinforced concrete structures badly
damaged.
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F-6 Inconceivable tornado (319-379 mph) These
winds are very unlikely. The small area of damage
they might produce would probably not be
recognizable along with the mess produced by F-4
and F-5 wind that would surround the F-6 winds.
Missiles, such as cars and refrigerators would do
serious secondary damage that could not be
directly identified as F-6 damage. If this level
is ever achieved, evidence for it might only be
found in some manner of ground swirl pattern, for
it may never be identifiable through engineering
studies.
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• Rapidly rotating (clockwise or anti-clockwise)
  funnel of winds protruding downward from a
  cumulonimbus cloud. Mostly they do not contact
  the ground but when they do they cause severe
  local damage.
• Often form in convective cells - thunderstorms,
  or right forward quadrant of hurricanes about
  200km from centre.
• Secondary phenomenon to the formation of
  supercells (much more frequent).
• 1000 per year (mostly in USA) - 70 struck
  Oklahoma in less than 24hrs in May 2000 - killed
  46, injured 800 and caused 700 million worth of
  damage..
• Width of destructive path 1km wide, metres to
  tens of km long, rarely last more than 30mins
• Translate at 50-200kmh-1 , internal wind speeds
  400-500kmh-1 . Accompanied by heavy
  precipitation.

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Other causes of convective instability

• Topography
• Convergence of air
• Most likely occurrence of instability takes place
  along cold fronts where the front undercuts warm
  moist air. (Polar front and warm tropical air)

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Other vortex winds

• Mountainado. Horizontal vortices can form over
  flat surfaces as winds at higher altitude
  overturn slower winds near the Earth's surface.
  If these hit an obstacle a mountainado can form.
• Dust devils and willy-willy's. Small vortex winds
  caused by strong convection over flat surfaces
  where solar heating is strong.
• Vortex winds can also be caused by winds shearing
  off large obstacles. This happens often in
  Boulder Colorado where strong winter winds blow
  across the Rocky mountains.
• Fire Tornadoes can also be produced by large and
  fearce fires. Typically forest fires (but also
  formed in Hiroshima bomb).

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Geological significance

• In a 1000-year period about 600 000 could have
  occurred in USA transporting large amounts of
  clay and silt particles.

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Ecological significance

• Pollen, seeds, micro-organisms, fish and
  invertebrates can be transported 100-200km and
  deposited alive. Species can be dispersed over
  large areas. Species migration between
  unconnected lakes.

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Links

• Scientific American article (contains many
  interesting links).
• Storm prediction centre database (a paper
  describing a database of information concerning
  severe weather in the USA, with some
  interpretations).
• Weather World 2010 (University of Illinois) (also
  has educational material on other weather
  phenomena - hurricanes etc.)