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

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

Tornadoes

        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 Convection Cooling or No Heating 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

 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

 The forms causing the largest damages are families of tornadoes; when many occur ( >6 ), it is a tornado outbreak  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.  It surpasses the 1974 Super Outbreak, in which 148 tornadoes were counted.  Another big event is the November 2013 tornado outbreak, with 136 tornadoes reported.

Video: Deadly 2011 tornado outbreak

 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. (March July)

Works cited

    http://www.lakeeriewx.com/Meteo361/ResearchTopicFo ur/Synoptic.html

http://www.britannica.com/EBchecked/media/19397/Torn adic-thunderstorm-The-rotating-updraft-that-produces the-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