TwistersTwisters

Review of last lectureReview of last lecture

1.1. The general size and lifetime of mesoscale The general size and lifetime of mesoscale convective systems, thunderstorms and tornadoesconvective systems, thunderstorms and tornadoes

2.2. 3 types of thunderstorms. 3 types of thunderstorms.

3.3. 3 stages of the ordinary thunderstorms. 3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft.

4.4. Formation of multi-cell thunderstorms. Downdrafts Formation of multi-cell thunderstorms. Downdrafts initiate new thunderstorms in nearby regions.initiate new thunderstorms in nearby regions.

1.1. 3 stages of the supercell thunderstorms. 3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened.

Video: A tornadoVideo: A tornado

http://www.youtube.com/watch?v=xCI1u05KD_s

TornadoesTornadoes A rapidly rotating column of air blowing around A rapidly rotating column of air blowing around

intense low pressure with circulation reaching intense low pressure with circulation reaching groundground

Wind speeds between 105 km/hr(65mph) and Wind speeds between 105 km/hr(65mph) and 450 km/hr(280mph)450 km/hr(280mph)

Rotation is almost exclusively cyclonic; a few spin Rotation is almost exclusively cyclonic; a few spin in the opposite directionin the opposite direction

Various sizes: most are 100-600 m in diameter; Various sizes: most are 100-600 m in diameter; some just a few meters; some >1 milesome just a few meters; some >1 mile

Various shapes: twisting rope-like funnels to Various shapes: twisting rope-like funnels to cylindrical funnels, to massive black funnelscylindrical funnels, to massive black funnels

Usually last only a few minutes, but some lasted Usually last only a few minutes, but some lasted several hoursseveral hours

Most move ahead of cold fronts, from SW to NE; Most move ahead of cold fronts, from SW to NE; some move in other directions some move in other directions

Moving speed is about 30 mph (some >70 mph)Moving speed is about 30 mph (some >70 mph)

Tornado FormationTornado 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 Formation3 Stages of Supercell Tornado Formation

• Before thunderstorms develop, a change Before thunderstorms develop, a change in wind direction and an increase in wind in wind direction and an increase in wind speed with increasing height creates an speed with increasing height creates an invisible, horizontal spinning effect in the invisible, horizontal spinning effect in the lower atmosphere. lower atmosphere.

• Spinning horizontal vortex tubes created Spinning horizontal vortex tubes created by surface by surface wind shearwind shear may be may be tiltedtilted and and forced in a vertical path by updrafts. This forced in a vertical path by updrafts. This rising, spinning, and often stretching rising, spinning, and often stretching rotating air may then turn into a rotating air may then turn into a mesocyclonemesocyclone..

• Most strong and violent tornadoes form Most strong and violent tornadoes form within this area of strong rotation.within this area of strong rotation.

Cloud structure of tornado-breeding Cloud structure of tornado-breeding supercell stormsupercell storm

Wall cloud Wall cloud forms underneath the forms underneath the mesocyclonemesocyclone when cold/moist when cold/moist downdraft air feeds into the updraft and condenses at a lower level downdraft air feeds into the updraft and condenses at a lower level than warm air.than warm air.

Precipitation (heating) structure of Precipitation (heating) structure of supercell stormsupercell storm

Contour is radar reflectivity (a measure of precipitation)

The most common atmospheric The most common atmospheric circulation structurecirculation structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Radiation Convection

Latent/Sensible

Conduction

Wind structure of supercell and tornadoWind structure of supercell and tornado

L

H

Life cycle of tornadoesLife cycle of tornadoes

Often evolve through a series of stages, from dust-whirl, to organizing Often evolve through a series of stages, from dust-whirl, to organizing and mature stages, and ending with the shrinking and decay stages.and mature stages, and ending with the shrinking and decay stages.

Most violent tornadoesMost 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 DamageTornado Damage

• Tornadoes are classified by the magnitude of damage they cause using the (enhanced) Fujita scale.

Tornado OutbreakTornado Outbreak The forms causing the largest damages are families of The forms causing the largest damages are families of

tornadoes; when many occur (tornadoes; when many occur (>6>6), it is a tornado outbreak), it is a tornado outbreak

The largest tornado outbreak on record, depending on the The largest tornado outbreak on record, depending on the definition applied and time elapsed between breaks in definition applied and time elapsed between breaks in tornadic activity, was the April 25–28, 2011 tornado tornadic activity, was the April 25–28, 2011 tornado outbreak, with as many as 358 tornadoes. outbreak, with as many as 358 tornadoes.

It surpasses the 1974 Super Outbreak, in which 148 It surpasses the 1974 Super Outbreak, in which 148 tornadoes were counted. tornadoes were counted.

Another big event is the November 2013 tornado outbreak, Another big event is the November 2013 tornado outbreak, with 136 tornadoes reported.with 136 tornadoes reported.

Video: Deadly 2011 tornado Video: Deadly 2011 tornado outbreakoutbreak

http://www.youtube.com/watch?v=mpGcKzI8oBI

Tornado Occurrence (global)Tornado Occurrence (global)

Tornado Occurrence (U.S.)Tornado Occurrence (U.S.)

Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually, but the highest frequency is observed in annually, but the highest frequency is observed in tornado alley of the Central tornado alley of the Central PlainsPlains. Great setting for potent mixing of air masses.. Great setting for potent mixing of air masses.

Tornadoes from T-storms in hurricanes

The timing of tornadoesThe timing of tornadoes

• Nearly 75% of tornadoes Nearly 75% of tornadoes form from March to July, form from March to July, when warm humid air is when warm humid air is overlain by cooler drier air to overlain by cooler drier air to cause strong vertical lift.cause strong vertical lift.

Trends in U.S. Tornado OccurrenceTrends 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.

FatalitiesFatalities• Fortunately, most tornadoes kill no one. Some,

unfortunately, are deadly.

SummarySummary

1.1. 3 stages of supercell tornado formation. 3 stages of supercell tornado formation.

1.1. Tornado outbreak (number>6) Tornado outbreak (number>6)

2.2. Tornado damage: Enhanced Fujita Scale (EF-0 65-85 Tornado damage: Enhanced Fujita Scale (EF-0 65-85 mph, EF-5 >200 mph)mph, EF-5 >200 mph)

3.3. Tornado occurrence: Global and U.S.. Which country Tornado occurrence: Global and U.S.. Which country has the largest number of tornadoes in the world? has the largest number of tornadoes in the world? Which state has the largest number of tornadoes per Which state has the largest number of tornadoes per unit area in U.S.? Tornado season in U.S. (March-unit area in U.S.? Tornado season in U.S. (March-July)July)

Works citedWorks cited

http://www.lakeeriewx.com/Meteo361/ResearchTopicFour/Synoptic.html

http://www.britannica.com/EBchecked/media/19397/http://www.britannica.com/EBchecked/media/19397/Tornadic-thunderstorm-The-rotating-updraft-that-Tornadic-thunderstorm-The-rotating-updraft-that-produces-the-tornado-extends produces-the-tornado-extends

http://www.geog.ucsb.edu/~joel/g266_s10/http://www.geog.ucsb.edu/~joel/g266_s10/lecture_notes/chapt08/oh10_8_3/oh10_08_3.html lecture_notes/chapt08/oh10_8_3/oh10_08_3.html

http://www.crh.noaa.gov/lmk/soo/docu/supercell.php http://www.crh.noaa.gov/lmk/soo/docu/supercell.php