Remaining Course Schedule

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Remaining Course Schedule Thu, 16
November Supercells, part 2 Tornadoes, part
1 Wed, 22 November no class (study for the
exam) Thu, 23 November no class (make-up
date/time/place TBA Forecasting) Wed, 29
November Tornadoes, part 2 Thu, 30
November Hail other hazards Wed, 06
December Hurricanes, part 1 Thu, 07
December Hurricanes, part 2 Wed, 13
December Exam review Thu, 14 December Final
exam Thu, 10 January Final exam (location TBA
maybe Wegener Ctr.)
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Tornadoes! Theory Formation
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Tornadoes!

• A violently rotating column of air, in contact
  with the ground, either pendant from a cumuliform
  cloud or underneath a cumuliform cloud, and often
  (but not always) visible as a funnel cloud.
  (Glossary of the American Meteorological Society,
  2004)
• Develop primarily from supercell thunderstorms
• This formative mechanism will be our focus
• Squall lines and multi-cell thunderstorms can
  also sometimes produce tornadoes
• Tornadoes produced when hurricanes come ashore
  are essentially formed by supercell-like
  structures that form in the outer rainbands as
  they encounter surface friction

4
Precursors to development

• Classic case
• Strong upper-level trough moves over the western
  U.S.
• Provides two important ingredients
• (1) Upper divergence/jetstreak divergence
• (2) Vertical wind shear
• Surface low pressure develops just east of the
  Rocky Mountains
• Air circulation (counter-clockwise) around the
  low does several things
• (1) Draws warm, moist, conditionally unstable air
  northward from the Gulf of Mexico
• (2) Sets up frontal boundaries between the
  various air masses
• Dryline and warm front are most common locations
  for supercell development

5
What exactly causes tornadoes to form?

• Field studies (many from OU scientists) have
  discovered that a very low percentage (maybe as
  low as 5-10) of supercell thunderstorms go on to
  produce tornadoes!
• Remember that supercell thunderstorms themselves
  are rare, accounting for lt5 of all thunderstorms
  that develop annually
• So why do so few thunderstorms produce tornadoes?

6
What exactly causes tornadoes to form?

• We know the ingredients that cause supercell
  thunderstorms to form
• Conditionally unstable air
• Strong vertical wind shear (both speed
  directional shear)
• Lifting mechanism a dryline or warm front
• And this trigger mechanism must be enough to
  overcome the capping inversion (region where
  temperature increases w/height) that exists 5000
  feet above ground
• What seems to be missing in the 90 of
  supercells that do not produce tornadoes?
• Low-level (the near-surface) wind shear seems to
  hold the key!
• Lets examine how tilting leads to the formation
  of the rotating mesocyclone and possible tornado

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Tilting ? Mesocyclone formation
Horizontal roll
Low-level wind shear causes large areas of air to
slowly turn. The thunderstorm updraft tilts
this area of rotation, and it becomes the
mesocyclone (rotating updraft in a supercell
thunderstorm).
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Conservation of angular momentum
As the horizontal roll gets tilted into the
updraft, it stretches. Here is where Physics
takes over as the updraft stretches the roll,
conservation of angular momentum comes into
play Important result the skinny updraft
rotates much much faster!
Skinny, fast
Large, slow
Conservation principle as the radius decreases,
the speed MUST INCREASE!!
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Why does the tornado form in the hook echo
region of a supercell?

• Tornado location is tied directly to the
  interface between updraft and downdraft regions
  in the supercell
• Notice the two downdraft regions, FFD and RFD,
  meet/collide at the hook
• When the downdrafts collide, they aid in
  generating the updraft (act as a small-scale
  trigger mechanism)
• Key to maintaining the tornado updraft region
  must not become separated from the warm/moist air

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One theory of vortex formation
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Rear flank downdraft leads to tornado formation!
Direction of motion of air associated with the
rear flank downdraft
Dust generated by the rear flank downdraft
New tornado forming
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• Current theory of tornadogenesis
• (Markowski 2003)
• Not only do we think the strength (wind
  velocities) of rear-flank downdraft (RFD) is
  critical, we also think the temperature of the
  RFD is important
• Warm RFD adds buoyancy stretching to the updraft

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Multiple tornadoes from one supercell
Often updraft region DOES become separated from
the warm, moist air However, a new updraft forms
on the flanking line and rotates into the main
body of the supercell A new tornado can form from
this updraft
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Supercell cycle two wall clouds
These two wall clouds illustrate the life cycle
of a supercell. The one on the right is the old,
dying updraft. The wall cloud on the left is
just forming and eventually will become the main
updraft of the supercell.
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Supercell cycle 8 tornadoes from one supercell
thunderstorm!
Notice the apparent jumps in tornado
tracks This is explained nicely by our theory of
downdraft/updraft interaction in the hook
region of a supercell (see slide 6)
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Vortex breakdown phenomenon
These smaller suction vorticies can also lead
to incredible gradients of damage (i.e., one
house is demolished while another is left
seemingly unscathed)
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Multiple vortex tornado
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Additional photographs multiple vortex tornado
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Fujita Scale of Tornado Intensity
SCALE WIND SPEED (m s-1) DAMAGE
0 18-32 Light damage. Some damage to chimneys breaks branches off trees pushes over shallow-rooted trees damages sign boards.
1 3349 Moderate damage. The lower limit is the beginning of hurricane-force wind speed peels surface off roofs mobile homes pushed off foundations or overturned moving autos pushed off roads.
2 5069 Considerable damage. Roofs torn off frame houses mobile homes demolished boxcars pushed over large trees snapped or uprooted light-object missiles generated Cars lifted off ground
3 7092 Severe damage. Roofs and some walls torn off well-constructed houses trains overturned most trees in forest uprooted heavy cars lifted off the ground and thrown.
4 93116 Devastating damage. Well-constructed houses leveled structures with weak foundations blown off some distance cars thrown and large missiles generated.
5 117142 Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate automobile-sized missiles fly through the air in excess of 100 meters trees debarked incredible phenomena will occur.
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Geographic location of American F5 tornadoes from
1950 to 2000
Notice ALL F5 tornadoes in the U.S. are between
the Rocky Mountains and the Appalachian
Mountains Also notice that while most F5
tornadoes occur April-June, they have occurred in
all seasons of the year
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April 3-4, 1974 SUPER-OUTBREAK of Tornadoes 148
Tornadoes in 24 hours!!
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Tornado Statistics
Note While F4/F5 tornadoes account for only 1
of all tornado occurrence, they account for 67
of all tornado deaths!!
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Tornado Alley
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Significant tornado (F2 or greater) days per
century
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Violent tornado (F4 or greater) days per millenium
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Significant Tornado (gtF2) ClimatologyTornado
Occurrence in May
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Worldwide occurrence of tornadoes
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Tornado statistics
Two points (1) both Austria and Oklahoma have
peak tornado occurrences in late afternoon
(16.00-20.00 local time). (2) Notice the large
difference in tornado occurrence Oklahoma had
1300 tornadoes between 16.00 and 20.00 in the 46
years from 1950-1996 Austria had 60 tornadoes
in the same period. (Austria had 92 total
tornadoes from 1910-1998). Source
http//www.tordach.org/at
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Tornado statistics
Average number of tornadoes per month in
Oklahoma Peak occurrence is from April-June.
Total number of tornadoes, by month, in
Austria Peak occurrence is from June-August.
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Biggest outbreak in the fall 143 tornadoes broke
out in 41 hours of continuous tornado activity
from November 21 to 23, 1992 (Galway (1977) has
defined ten or more tornadoes as constituting an
outbreak) Most tornadoes spawned from a
hurricane 117 tornadoes spun out of Hurricane
Frances upon landfall in Florida in September
2004. The old record was 115 from Hurricane
Beulah in 1967 Most significant coincidence The
small town of Codell, Kansas was hit by a
tornado on the exact same date three years
straight. A tornado hit on May 20, 1916, 1917,
and 1918. The U.S. gets 100,000 storms a year
only 1 produces a tornado. The odds of this
coincidence occurring again is practically
infinitesimal
Photo courtesy of NASA
33

• Biggest outbreak of tornadoes The Super Outbreak
• 148 tornadoes affected 13 states (Alabama,
  Georgia, Illinois, Indiana, Kentucky, Michigan,
  Mississippi, North Carolina, Ohio, South
  Carolina, Tennessee, Virginia, and West Virginia)
  and one Canadian province on April 3 and 4, 1974.
  
• the outbreak was an unprecedented producer of
  large, long-track, and intense tornadoes
• Before it was over 16 hours later
• 330 people were dead
• 5,484 were injured
• left adamage path covering more than 2,500 miles.

At that time, National Weather Service
forecasters could see only green blobs on their
radar scopes and had to wait for visual
confirmation of the tornado before issuing a
tornado warning.
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Fujita Scale of Tornado Intensity
SCALE WIND SPEED (m s-1) DAMAGE
0 18-32 Light damage. Some damage to chimneys breaks branches off trees pushes over shallow-rooted trees damages sign boards.
1 3349 Moderate damage. The lower limit is the beginning of hurricane-force wind speed peels surface off roofs mobile homes pushed off foundations or overturned moving autos pushed off roads.
2 5069 Considerable damage. Roofs torn off frame houses mobile homes demolished boxcars pushed over large trees snapped or uprooted light-object missiles generated Cars lifted off ground
3 7092 Severe damage. Roofs and some walls torn off well-constructed houses trains overturned most trees in forest uprooted heavy cars lifted off the ground and thrown.
4 93116 Devastating damage. Well-constructed houses leveled structures with weak foundations blown off some distance cars thrown and large missiles generated.
5 117142 Incredible damage. Strong frame houses lifted off foundations and carried considerable distance to disintegrate automobile-sized missiles fly through the air in excess of 100 meters trees debarked incredible phenomena will occur.
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http//eiger.mae.wvu.edu/for_NGTV2/anim2740A.mpeg
http//esminfo.prenhall.com/science/geoanimations/
animations/Tornadoes.html
http//www.pbs.org/wgbh/nova/tornado/damage.html
http//www.psc.edu/research/graphics/gallery/gel_p
art_adv.mpg
http//www.psc.edu/research/graphics/gallery/case4
100_vol_iso.mpg
http//whyfiles.org/013tornado/3.html
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Xenia, Ohio F5 tornado
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Red Rock, Okla.26 April 1991
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Red Rock, Okla.26 April 1991
Tornado
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Attica, KS12 May 2004
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Attica, KS12 May 2004
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Attica, KS12 May 2004
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Attica, KS12 May 2004
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Northern Iowa11 June 2004
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Northern Iowa11 June 2004
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Woonsocket, SD24 June 2003
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Woonsocket, SD24 June 2003
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• Mulvane, KS
• 13 June 2004

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http//www.stormeyes.org/tornado/faq/notahose.htm