The Science of Tornadoes

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The Science of Tornadoes
Dr. John A. Knox University of Georgia Masters of
Disaster Workshop June 15, 2009
Above Doppler radar reflectivity and aftermath
of Greensburg, KS EF-5 tornado on May 4, 2007
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First the role of vertical wind shear in
thunderstorms

• Vertical wind shear change of horizontal wind
  (in both speed and direction) as you go up
• Causes of vertical wind shear
• Large-scale environmental winds (jet streams)
• Small-scale flow around thunderstorm (downdraft
  and outflow from storm, called the cold pool)
• Interaction of vertical wind shear with
  thunderstorm can radically change the character
  and life cycle of a thunderstorm

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Thunderstorm summary
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Supercell thunderstorm

• Strong shear at surface is tilted upward by
  updraft
• Result oppositely spinning pair of
  mesocyclones inside the thunderstorm (demo)
• Rotating updraft is signature of supercells

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Supercell thunderstorm schematic
http//www.nssl.noaa.gov/primer/tornado/tor_basics
.html

• Low pressure caused by tilted-upward rotation
• Low sucks in more air, accelerating rotation
  and causing storm to move to right of upper-level
  winds
• In only 30 of supercells, tornado forms
  (beneath mesocyclone)

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A few tornadoes are the problem
Graphs from www.tornadoproject.com

• 1 of all tornadoes cause 2/3rds of all the
  deaths
• Nearly all of these tornadoes are from supercell
  thunderstorms

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And so

• Focus on tornadoes caused by supercells
• Most common type
• Safety nearly all killer tornadoes are from
  supercells
• However, supercell does not tornado
• 70 or more of supercells dont produce tornadoes
• Small-scale tilted-upward spin around supercell,
  and stretching of vortex, determines tornado
  formation
• Rotating cylinders on TV weather dont mean
  theres a tornado!
• Now, lets explore tornadoes!

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What is/isnt/sort of is a tornado

• Tornado narrow, violently rotating column of low
  pressure that extends from the lowered base (wall
  cloud) of a thunderstorm to the ground
• Tornado funnel caused by condensation of water
  vapor in lower pressure also by dust, debris
• Not tornadoes (although theres debate)
• Funnel cloud not a tornado until it reaches the
  ground
• Waterspout out of cumulus, not cumulonimbus not
  as violent, not associated with mesocyclone
• Landspout similar to waterspout, over land
• Gustnado tornado associated with thunderstorm
  outflow
• Dust devil clear skies, caused by solar heating

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Thunderstorm wall cloud
Bad place to be!
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Tornado

• Most violent winds on Earth (over 200 mph 318
  mph measured above ground by Doppler radar)
• At worst, tornadoes can be 1-2 miles wide, last
  an hour, and travel about 100 miles
• Worst place on Earth for tornadoes central U.S.
  (all the ingredients for supercells are present)

Another bad place to be!
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Tornado life cycle
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National tornado climatology
TORNADO ALLEY
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Significant tornado climatology for U.S.
BAD TORNADO ALLEY
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Georgia tornado climatology
Population centers (lots of people watching the
skies)
http//www.srh.noaa.gov/ffc/images/tornado.gif
http//climate.engr.uga.edu/tornado/population.gif

Harder to explain (bona fide tornado alley?)
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Georgia tornado climatology by month
Note the secondary maximum in November!
http//www.srh.noaa.gov/ffc/images/torntotal1.PNG
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National maximum tornado threat climatology
Risk generally migrates northward with warm moist
air and the jet stream
SW Georgia part of unique fall anomaly!
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Georgia tornado climatology by hour of the day
Springtime late afternoon/ early evening max
Fall/winter late afternoon/ early evening max,
but also midnight and dawn, too
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Tornado winds

• Extreme speed because of large, rapid change in
  pressure over distance (horizontal pressure
  gradient)
• Winds usually (not always) counter-clockwise in
  NH
• At center of funnel, pressure drops about 10 vs.
  outside of funnel
• Not a vacuum!

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The multiple-vortex tornado

• Worst tornadoes tend to exhibit moving whirls
  inside of whirls, i.e. multiple vortices
• Discovered by tornado pioneer Ted Fujita

Where forward speed of storm, funnels winds and
suction vortex winds all add together, winds can
be significantly stronger than just a few feet
away
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Tornado wind estimation

• Enhanced Fujita (EF) scale uses careful
  examination of tornado damage to provide
  after-the-fact estimate of winds
• Scale EF-0 (light damage) to EF-5 (incredible
  damage)
• Use our fun applet (updated) to learn about the
  EF scale http//profhorn.meteor.wisc.edu/wxwise/t
  ornado/t.html

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EF scale winds
http//www.spc.noaa.gov/efscale/ef-scale.html
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Tornado schematic Radar reflectivity hook echo
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Tornado visualizations Radar reflectivity hook
echo
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Tornado visualizations NWS Doppler radar
velocityAtlanta, 940 pm 3/14/08
Tornado Vortex Signature
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Tornado visualizations EF path of Atlanta
tornado
Touched down near intersection of Simpson and
Burbank in Vine City.
Traveled across GWCC, CNN Center, OMNI Hotel
Crossed I-75
Hit The Equitable Building
Touched down again at Cotton Mill Lofts in
Cabbage Town.
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Tornado forecasting, in a nutshell

• First-ever forecast March 25, 1948
  http//www.nssl.noaa.gov/briefings/vol2_no2/fiftie
  th.html
• Tinker AFB, OK, hit by tornado on 3/20/48 (10
  million damage)
• Major Fawbush and Captain Miller charged with
  creating forecast method
• They predict a tornado on 3/25/48
• Incredibly enough, another tornado hits Tinker on
  that day!
• Tornado forecast approach today
• Identify likely locations for supercell
  development (trigger, buoyancy, shear)
• More likely with more buoyancy shear
• Mid-level cap (inversion) can create even
  stronger updraft if cap broken

CAP
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Tornado safety watches and warnings

• Tornado watch it could happen soon
• Issued by National Weather Service
• Issued hours ahead of bad weather
• Watch box covers large region (e.g., half a
  state see example at right)
• Tornado warning its happening now
• Issued by National Weather Service
• Time span of an hour or so
• Warning area covers local region (e.g., a county
  or two)
• Warnings for ATL tornado http//www.youtube.com/w
  atch?vAe29tz7K6vc

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Tornado safety what to do in a

• Frame house go to basement
• Only 20-30 deaths in basement out of 11,000
  tornado-related deaths since 1880, acc. to Tom
  Grazulis
• Parkersburg, IA 5/25/08 tornado at right even
  basements are not safe from an EF-5
• Small windowless room with strong walls (pipe in
  bathroom walls) best
• Put as many walls between you and the tornado
• Hide under a table
• Protect your head

http//www.crh.noaa.gov/Image/dmx/ParkersburgTorna
do/IMG_2006.JPG
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Tornado safety what to do in a

• School, workplace or church follow severe
  weather plan
• Large free-span roofs are to be avoided
• Hallways without doors or windows
• Avoid glass
• Crouch, protect your head
• Buses should not try to outrun storm
• At right F4 destroys metal fabrication plant in
  Illinois in 2004, no deaths and no injuries
  thanks to planning

http//www.crh.noaa.gov/images/ilx/pdf/Miller_Abst
ract.pdf 150 employees evacuated to shelters
(in yellow) in just 3 minutes! For video of
tornado hitting a MO school http//www.youtube.co
m/watch?vUdSjzWVevI8
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Tornado safety what to do in a

• Vehicle
• Get out.
• Lie in a ditch
• DONT hide under an overpass (it channels winds
  and makes them stronger)
• Dont sightsee, dont try to outrun tornado, etc.

Car pulverized by tree in April 1998 Birmingham
tornado http//www.tornadoproje
ct.com/past/birmdam2.htm
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Tornado safety what to do in a

• Mobile home
• Get out.
• Take shelter nearby, preferably in
  steel-reinforced surroundings and/or underground
• Tornadoes are NOT attracted to mobile homes
• Instead, low-income people in relatively poor
  tornado alleys who cant afford frame homes are
  attracted to mobile homes

http//www.texastech.edu/images/story-photos/torna
do-path-large.jpg
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Tornado safety what to do in a

• Arena or skyscraper
• Stay away from windows!
• Follow instructions, if any http//www.youtube.com
  /watch?vK4K5e9wqVB8
• Go to lower floors (wind speed increases with
  height)
• Westin Peachtree Plaza tower swayed 2 feet during
  tornado!

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Main sources of information

• Ackerman, S.A., and J.A. Knox, Meteorology
  Understanding the Atmosphere (2nd edition),
  Brooks/Cole, 2007.
• Grazulis, T.P., The Tornado, University of
  Oklahoma Press, 2001.
• http//www.meted.ucar.edu (requires registration)
• http//www.spc.noaa.gov/faq/tornado/

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Questions?
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Worst tornado outbreak ever (35 years ago)
April 3, 1974 Superoutbreak
Context Tornadoes ahead of cold front, the day
before Hank Aaron tied Babe Ruths home run
record in Cincinnati (above left) 148 tornadoes
in 13 states and Canada 49 killer tornadoes (F5
in Xenia, OH above right, F5 in Cincy, F4 north
of Atlanta, I spent night in tornado shelter)
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April 3, 1974 Superoutbreak (35 years ago)
Extratropical cyclone (comma cloud above) with
pre-cold front squall lines 2,598 miles of
tornado paths (above right, with F-scales labeled
and morning Lifted Index contoured) 315 killed,
5,484 injured, 600 million in damage
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April 3, 1974 Superoutbreak How rare? From my
textbook
Recently, research meteorologists calculated
that more violent tornadoes developed on the one
day of April 3, 1974 than during any 4-week
period during the past 130 years. The
Superoutbreak was probably a once-in-a-millenniu
m event, and is the single most extreme weather
event we discuss in this textbook.