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.)
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 “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).
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!!
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
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
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)
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 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 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.
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_part_adv.mpg http://www.psc.edu/research/graphics/gallery/case4100_vol_iso.mpg http://whyfiles.org/013tornado/3.html
Red Rock, Okla.26 April 1991 Tornado