1. Remaining Course Schedule Thu, 16 NovemberSupercells, part 2; Tornadoes, part 1 Wed, 22 Novemberno class (study for the exam) Thu, 23 Novemberno class (make-up; date/time/place TBA; Forecasting) Wed, 29 NovemberTornadoes, part 2 Thu, 30 NovemberHail; other hazards Wed, 06 DecemberHurricanes, part 1 Thu, 07 DecemberHurricanes, part 2 Wed, 13 DecemberExam review Thu, 14 DecemberFinal exam Thu, 10 JanuaryFinal exam (location TBA – maybe Wegener Ctr.)

2. Tornadoes! Theory & Formation

3. 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 <5% 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! –Let’s examine how tilting leads to the formation of the rotating mesocyclone and possible tornado

7. 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”

8. 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! Conservation principle: as the radius decreases, the speed MUST INCREASE!! Large, slow Skinny, fast

9. 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

10. One theory of vortex formation

11. 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”

13. 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

15. 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

16. 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.

17. 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)

18. 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)

19. Multiple vortex tornado

20. Additional photographs: multiple vortex tornado

21. Fujita Scale of Tornado Intensity SCALE WIND SPEED (m s -1 ) DAMAGE 018-32 Light damage. Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards. 133–49 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. 250–69 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 370–92 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. 493–116 Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated. 5117–142 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.

22. 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

23. April 3-4, 1974 “SUPER- OUTBREAK” of Tornadoes 148 Tornadoes in 24 hours!!

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

25. Tornado Alley

26. Significant tornado (F2 or greater) days per century

27. Violent tornado (F4 or greater) days per millenium

28. Significant Tornado (>F2) Climatology Tornado Occurrence in May

29. Worldwide occurrence of tornadoes

30. 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/athttp://www.tordach.org/at

31. 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.

32. 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.

34. Fujita Scale of Tornado Intensity SCALE WIND SPEED (m s -1 ) DAMAGE 018-32 Light damage. Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards. 133–49 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. 250–69 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 370–92 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. 493–116 Devastating damage. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated. 5117–142 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.

44. 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

45. Xenia, Ohio F5 tornado

46. Red Rock, Okla. 26 April 1991

47. Tornado

48. Attica, KS 12 May 2004

52. Northern Iowa 11 June 2004

54. Woonsocket, SD 24 June 2003

56. Mulvane, KS 13 June 2004

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