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 1Thu, 07 December Hurricanes, part 2

Wed, 13 December Exam reviewThu, 14 December Final exam

Thu, 10 January Final exam (location TBA – maybe Wegener Ctr.)

Tornadoes! Theory & Formation

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

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

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 <5% of all thunderstorms that develop annually

• So why do so few thunderstorms produce tornadoes?

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!– Let’s examine how tilting leads to the formation of the rotating

mesocyclone and possible tornado

Tilting Mesocyclone formation

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).

“Horizontal roll”

Conservation of angular momentumAs 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!

Conservation principle: as the radius decreases, the speed MUST INCREASE!!

Large, slow

Skinny, fast

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

One theory of vortex formation

Rear flank downdraft leads to tornado formation!

Dust generated by the “rear flank

downdraft”

New tornado forming

Direction of motion of air associated with the “rear flank downdraft”

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

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

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.

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)

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)

Multiple vortex tornado

Additional photographs:

multiple vortex tornado

Fujita Scale of Tornado IntensitySCALE

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 33–49Moderate 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 50–69Considerable 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 70–92 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 93–116Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large

missiles generated.

5 117–142Incredible 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.

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

April 3-4, 1974 “SUPER-

OUTBREAK” of Tornadoes

148 Tornadoes in 24 hours!!

Tornado Statistics

Note: While F4/F5 tornadoes account for only 1% of all tornado occurrence, they account for 67% of all tornado deaths!!

Tornado Alley

Significant tornado (F2 or greater) days per century

Violent tornado (F4 or greater) days per millenium

Significant Tornado (>F2) ClimatologyTornado Occurrence in May

Worldwide occurrence of tornadoes

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

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.

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

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 later330 people were dead5,484 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.

Fujita Scale of Tornado IntensitySCALE

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 33–49Moderate 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 50–69Considerable 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 70–92 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 93–116Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large

missiles generated.

5 117–142Incredible 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.

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_part_adv.mpg

http://www.psc.edu/research/graphics/gallery/case4100_vol_iso.mpg

http://eiger.mae.wvu.edu/for_NGTV2/anim2740A.mpeg

http://whyfiles.org/013tornado/3.html

Xenia, Ohio F5 tornado

Red Rock, Okla.26 April 1991

Red Rock, Okla.26 April 1991

Tornado

Attica, KS12 May 2004

Attica, KS12 May 2004

Attica, KS12 May 2004

Attica, KS12 May 2004

Northern Iowa11 June 2004

Northern Iowa11 June 2004

Woonsocket, SD24 June 2003

Woonsocket, SD24 June 2003

Mulvane, KS

13 June 2004

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