Twisters

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Twisters
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Review of last lecture

• The general size and lifetime of mesoscale
  convective systems, thunderstorms and tornadoes
• 3 types of thunderstorms.
• 3 stages of the ordinary thunderstorms. Downdraft
  and falling precipitation cut off the updraft.
• Formation of multi-cell thunderstorms. Downdrafts
  initiate new thunderstorms in nearby regions.
• 3 stages of the supercell thunderstorms. Winds
  aloft push downdraft/precipitation away and the
  updraft is not weakened.

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Video A tornado

• http//www.youtube.com/watch?vxCI1u05KD_s

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Tornadoes

• A rapidly rotating column of air blowing around
  intense low pressure with circulation reaching
  ground
• Wind speeds between 105 km/hr(65mph) and 450
  km/hr(280mph)
• Rotation is almost exclusively cyclonic a few
  spin in the opposite direction
• Various sizes most are 100-600 m in diameter
  some just a few meters some gt1 mile
• Various shapes twisting rope-like funnels to
  cylindrical funnels, to massive black funnels
• Usually last only a few minutes, but some lasted
  several hours
• Most move ahead of cold fronts, from SW to NE
  some move in other directions
• Moving speed is about 30 mph (some gt70 mph)

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Tornado Formation

• Tornadoes can develop in any situation that
  produces severe weather cold fronts, mesoscale
  convective systems, supercells, and tropical
  cyclones.
• The processes leading to their formation are not
  well understood
• The most intense and destructive tornadoes come
  from supercells.

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3 Stages of Supercell Tornado Formation

• Before thunderstorms develop, a change in wind
  direction and an increase in wind speed with
  increasing height creates an invisible,
  horizontal spinning effect in the lower
  atmosphere.
• Spinning horizontal vortex tubes created by
  surface wind shear may be tilted and forced in a
  vertical path by updrafts. This rising, spinning,
  and often stretching rotating air may then turn
  into a mesocyclone.
• Most strong and violent tornadoes form within
  this area of strong rotation.

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Cloud structure of tornado-breeding supercell
storm
Wall cloud forms underneath the mesocyclone when
cold/moist downdraft air feeds into the updraft
and condenses at a lower level than warm air.
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Precipitation (heating) structure of supercell
storm
Contour is radar reflectivity (a measure of
precipitation)
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The most common atmospheric circulation structure
H
L
Radiation Convection
Cooling or No Heating
Heating
Latent/Sensible Conduction
H
L

• Imbalance of heating
• Imbalance of temperature
• Imbalance of pressure
• ? Wind

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Wind structure of supercell and tornado
H
L
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Life cycle of tornadoes

• Often evolve through a series of stages, from
  dust-whirl, to organizing and mature stages, and
  ending with the shrinking and decay stages.

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Most violent tornadoes

• Most tornadoes rotate around a single core, some
  of the most violent ones have several small zones
  of intense rotations called suction vortices.

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Tornado Damage

• Tornadoes are classified by the magnitude of
  damage they cause using the (enhanced) Fujita
  scale.

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Tornado Outbreak

• The forms causing the largest damages are
  families of tornadoes when many occur (gt6), it
  is a tornado outbreak
• The largest tornado outbreak on record, depending
  on the definition applied and time elapsed
  between breaks in tornadic activity, was the
  April 2528, 2011 tornado outbreak, with as many
  as 358 tornadoes.
• It surpasses the 1974 Super Outbreak, in which
  148 tornadoes were counted.
• Another big event is the November 2013 tornado
  outbreak, with 136 tornadoes reported.

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Video Deadly 2011 tornado outbreak

• http//www.youtube.com/watch?vmpGcKzI8oBI

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Tornado Occurrence (global)
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Tornado Occurrence (U.S.)
Tornadoes from T-storms in hurricanes
Tornadoes from all 50 states of the U.S. add up
to more than 1000 tornadoes annually, but the
highest frequency is observed in tornado alley of
the Central Plains. Great setting for potent
mixing of air masses.
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The timing of tornadoes

• Nearly 75 of tornadoes form from March to July,
  when warm humid air is overlain by cooler drier
  air to cause strong vertical lift.

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Trends in U.S. Tornado Occurrence

• As population centers have expanded into formerly
  rural areas, there is a greater probability that
  a tornado would hit a structure or be observed.

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Fatalities

• Fortunately, most tornadoes kill no one. Some,
  unfortunately, are deadly.

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Summary

• 3 stages of supercell tornado formation.
• Tornado outbreak (numbergt6)
• Tornado damage Enhanced Fujita Scale (EF-0 65-85
  mph, EF-5 gt200 mph)
• Tornado occurrence Global and U.S.. Which
  country has the largest number of tornadoes in
  the world? Which state has the largest number of
  tornadoes per unit area in U.S.? Tornado season
  in U.S. (March-July)

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Works cited

• http//www.lakeeriewx.com/Meteo361/ResearchTopicFo
  ur/Synoptic.html
• http//www.britannica.com/EBchecked/media/19397/To
  rnadic-thunderstorm-The-rotating-updraft-that-prod
  uces-the-tornado-extends
• http//www.geog.ucsb.edu/joel/g266_s10/lecture_no
  tes/chapt08/oh10_8_3/oh10_08_3.html
• http//www.crh.noaa.gov/lmk/soo/docu/supercell.php