Transcript Geography 210: - Ohio State University

Twisters
Review of last lecture
1. The general size and lifetime of mesoscale
convective systems, thunderstorms and tornadoes
2. 3 types of thunderstorms.
3. 3 stages of the ordinary thunderstorms. Downdraft
and falling precipitation cut off the updraft.
4. Formation of multi-cell thunderstorms. Downdrafts
initiate new thunderstorms in nearby regions.
1. 3 stages of the supercell thunderstorms. Winds aloft
push downdraft/precipitation away and the updraft is not
weakened.
Video: A tornado
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http://www.youtube.com/watch?v=xCI1u05KD_s
Tornadoes
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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 >1 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 >70 mph)
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.
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.
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.
Precipitation (heating) structure of
supercell storm
Contour is radar reflectivity
(a measure of precipitation)
The most common atmospheric
circulation structure
H
L
Radiation
Cooling
or No
Heating
Convection
Heating
Latent/Sensible
Conduction
H
L
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Wind structure of supercell and tornado
H
L
Life cycle of tornadoes
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Often evolve through a series of stages, from dust-whirl, to organizing
and mature stages, and ending with the shrinking and decay stages.
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.
Tornado Damage
• Tornadoes are classified
by the magnitude of
damage they cause using
the (enhanced) Fujita
scale.
Tornado Outbreak
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The forms causing the largest damages are families of
tornadoes; when many occur (>6), it is a tornado outbreak
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The largest tornado outbreak on record, depending on the
definition applied and time elapsed between breaks in
tornadic activity, was the April 25–28, 2011 tornado
outbreak, with as many as 358 tornadoes.
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It surpasses the 1974 Super Outbreak, in which 148
tornadoes were counted.
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Another big event is the November 2013 tornado outbreak,
with 136 tornadoes reported.
Video: Deadly 2011 tornado
outbreak
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http://www.youtube.com/watch?v=mpGcK
zI8oBI
Tornado Occurrence (global)
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.
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.
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.
Fatalities
• Fortunately, most tornadoes kill no one. Some, unfortunately,
are deadly.
Summary
1. 3 stages of supercell tornado formation.
1. Tornado outbreak (number>6)
2. Tornado damage: Enhanced Fujita Scale (EF-0 65-85
mph, EF-5 >200 mph)
3. 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. (MarchJuly)
Works cited
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http://www.lakeeriewx.com/Meteo361/ResearchTopicFo
ur/Synoptic.html
http://www.britannica.com/EBchecked/media/19397/Torn
adic-thunderstorm-The-rotating-updraft-that-producesthe-tornado-extends
http://www.geog.ucsb.edu/~joel/g266_s10/lecture_notes/
chapt08/oh10_8_3/oh10_08_3.html
http://www.crh.noaa.gov/lmk/soo/docu/supercell.php