Chapter 10: Chapter 10: Thunderstorms and Thunderstorms and

TornadoesTornadoes ThunderstormsThunderstorms TornadoesTornadoes Tornado formationTornado formation Observing tornadoes and Observing tornadoes and

severe weathersevere weather WaterspoutsWaterspouts

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ThunderstormsThunderstorms

•Thunderstorm: storm with lightning and thunder; they are convective storms that form with rising air in a conditionally unstable environment• The trigger needed to start air moving upward may be surface heating; topographic lift; convergence zone (e.g., sea breeze leading edge); frontal lift; divergence aloft

Q: Conditionally unstable environment means that the environmental lapse rate isa) > 10 K/km, b) < 6 K/km, c) between 6 – 10 K/km

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Ordinary Cell Ordinary Cell ThunderstormsThunderstorms

Three stages: cumulus, mature, dissipatingThree stages: cumulus, mature, dissipating

• Ordinary cell thunderstorms are sometimes called ‘air mass Ordinary cell thunderstorms are sometimes called ‘air mass thunderstorms’ or `ordinary thunderstorm’, because they thunderstorms’ or `ordinary thunderstorm’, because they form in conditionally unstable air masses and are not form in conditionally unstable air masses and are not necessarily associated with fronts or severe weathernecessarily associated with fronts or severe weather

• Shortlived (<1 hr), less than 1 km wide, low wind shear, Shortlived (<1 hr), less than 1 km wide, low wind shear, rarely produce strong wind or large hailrarely produce strong wind or large hail

Q: Is it possible to drop air temperature by 40F in an hour? Q: Is it possible to drop air temperature by 40F in an hour? A: Yes. On 7/13/1999 in California, a strong downdraft A: Yes. On 7/13/1999 in California, a strong downdraft from a mature thunderstorm dropped the air temperature from a mature thunderstorm dropped the air temperature from 97F to a chilly 57F in one hour!from 97F to a chilly 57F in one hour!

Q: Why does low wind shear produce an ordinary rather than Q: Why does low wind shear produce an ordinary rather than severe thunderstorm?severe thunderstorm?

A: because downdraft would cutoff the supply of moisture A: because downdraft would cutoff the supply of moisture 3

Moisten environment latent heat warms downdraft cuts off the cloud layer humid updraft;

Q: why is the thunderstorm downdraft usually cold?A: Entrained dry air causes raindrop evaporation, cooling the air which descends as a downdraft 4

Severe Thunderstorms and Severe Thunderstorms and SupercellSupercell

multicell stormsmulticell storms

moderate wind shear;moderate wind shear;

lasting several hours; lasting several hours;

multiple ordinary ormultiple ordinary or

supercell thunderstormssupercell thunderstorms

Severe thunderstorm is defined as a thunderstorm with at least one of the following: large hail with a diameter > ¾ inch, surface wind gusts >50 knots (58 mi/hr), and/or produces a tornado

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Q: which cloud in the figure is at the mature stage? a) A, b) B, c) C

A B C

Gust FrontGust Front gust front: leading edge of cold air originating inside a gust front: leading edge of cold air originating inside a

thunderstormthunderstorm shelf cloud and roll cloudshelf cloud and roll cloud outflow boundary: merging several gust fronts outflow boundary: merging several gust fronts

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Shelf cloud roll cloud

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MicroburstsMicrobursts downbursts (intense downdraft) and microbursts (< 4km; downbursts (intense downdraft) and microbursts (< 4km;

caused aircraft crash)caused aircraft crash)

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Dust clouds due to mictoburst tail wind headwind

Microbursts present a severe hazard to aircraft, especially duringtakeoff and landing. Several airports have installed microburst detection instruments.

Squall LinesSquall Lines squall line: multicell storms as a line squall line: multicell storms as a line

of thunderstorms extending for many of thunderstorms extending for many kilometers (up to 1000 km)kilometers (up to 1000 km)

Pre-frontal squall line may be Pre-frontal squall line may be initiated by gravity wavesinitiated by gravity waves

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Strong downdrafts of squall lines cause bow-shaped signal in Strong downdrafts of squall lines cause bow-shaped signal in a Doppler radar image, called bow echo. When the damage a Doppler radar image, called bow echo. When the damage associated with the straight-line winds extends for a associated with the straight-line winds extends for a considerable distance along the squall line’s path, the wind considerable distance along the squall line’s path, the wind storm is called a derecho (day-ray-sho).storm is called a derecho (day-ray-sho).

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mesoscale convective complex (MCC): mesoscale convective complex (MCC): multicell storms as a large circular cluster of storms;multicell storms as a large circular cluster of storms; tend to form in summer in regions where the upper-level tend to form in summer in regions where the upper-level

winds are weak;winds are weak; large size (100,000 square km);large size (100,000 square km); last for several hourslast for several hours

Q: Who first defined MCC?Q: Who first defined MCC?a) a UA scientista) a UA scientist; b) a non-UA scientist; b) a non-UA scientist

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Supercell:Strong wind shear in speed and direction;Shallow inversion above warm and humidlayer acts as a lid;

Long-lasting (hours);Larger than 1 km in diameter;Single violently rotating updraft;Produces tornado, large hail, strong gusts

Q: A thunderstorm with weak wind shear is most probablya) ordinary cell, b) multicell, c) supercell

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A model of classical supercell

Q: why is the weather pattern left is favorable for supercell?A: a) with wind shear, downdraft would not cut off the updraft; b) upper level divergence and lower level convergence strengthen the updraft

Q: Why do wind shear (e.g., increasing wind speed with height) and upward motion would cause the updraft to rotate?

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Dryline ThunderstormsDryline Thunderstorms drylinedryline

• These storms occur frequently in the southernThese storms occur frequently in the southernGreat Plains of the US. Great Plains of the US.

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Q: A circular cluster of storms is calleda) MCC, b) squall line, c) dryline

Q: Intense downdraft is calleda) Derecho, b) gust front, c) downburst

Q: What is most damaging for aviation?a) Derecho, b) gust front, c) microburst

Q: Which has the largest cloud area as seen from satellites?a) dryline, b) MCC, c) squall line, d) supercell

Q: If the vertical wind shear is weak, the thunderstorm isa) ordinary cell, b) multicell, c) supercell

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Thunderstorms and Thunderstorms and FloodingFlooding

flash floods: flash floods: floods that rise rapidly with little floods that rise rapidly with little or no advance warning or no advance warning

The Great Flood of 1993over the upper midwest

1976 Big Thompson flash flood(12 inch rain in 4 hours)

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Distribution of Distribution of ThunderstormsThunderstorms combination of warmth and moisturecombination of warmth and moisture

geographical placementgeographical placement

Q: why are the Great Plains more favorable for hails than, say, Florida (right panel above)? A: The Great Plains are more favorable for severe thunderstorms (including larger hails); the warm subcloud layer in Florida melts hail before reaching the ground.

Thunderstorm days

Hail days

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Lightning and ThunderLightning and Thunder Lightning: a discharge of electricity, a giant spark, which Lightning: a discharge of electricity, a giant spark, which

usually occurs in mature thunderstorms (and may also occur usually occurs in mature thunderstorms (and may also occur in snowstorms and dust storms); in snowstorms and dust storms);

majority of lightning strokes within clouds with only 20% majority of lightning strokes within clouds with only 20% between clouds and surface;between clouds and surface;

a lightning stroke can heat the air it travels to 30,000C, 5 a lightning stroke can heat the air it travels to 30,000C, 5 times as hot as the Sun’s surfacetimes as hot as the Sun’s surface

Q: why would a lightning cause a thunder?Q: why would a lightning cause a thunder?A: the extreme heating from a lightning causes the air to expand A: the extreme heating from a lightning causes the air to expand

explosively, thus initiating a shock wave that becomes a explosively, thus initiating a shock wave that becomes a booming sound wave (or a thunder) booming sound wave (or a thunder)

Q: Do you see the lightning or hear the thunder first?Q: Do you see the lightning or hear the thunder first?a) see the lightning firsta) see the lightning first, b) hear the thunder first, , b) hear the thunder first, c) at the same timec) at the same time

Q: Assuming sound speed is 330 m/s and you hear the thunder 5 Q: Assuming sound speed is 330 m/s and you hear the thunder 5 seconds after seeing the lightning, what is the distance of the seconds after seeing the lightning, what is the distance of the lightning stroke? a) 330 m, b) 1 km, lightning stroke? a) 330 m, b) 1 km, c) 1 mile c) 1 mile

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Electrification of CloudsElectrification of Clouds

relationships of updrafts and downdrafts relationships of updrafts and downdrafts to electrical charges in cloudsto electrical charges in clouds

For normal fair weather, the atmosphere is usually characterized by a negatively charged surface and a positively charged upper atmosphere

There is a net transfer of positive ions (charged molecules) from the warmer (and larger) hailstone to the colder (and smaller) ice crystal or supercooled droplets which are lifted to the upper layer of clouds

Q: Why are hailstones warmer than smaller droplets?

a) because latent heat release; b) because hailstones are bigger;c) because smaller droplets are supercool 20

The Lightning StrokeThe Lightning Stroke cloud-to-ground lightningcloud-to-ground lightning stepped leaderstepped leader return stroke: large number of electrons flow to the ground and return stroke: large number of electrons flow to the ground and

a much larger, more luminous return stroke (current) to clouda much larger, more luminous return stroke (current) to cloud dart leader – subsequent leaderdart leader – subsequent leader

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Types of LightningTypes of Lightning forked lightningforked lightning ribbon lightning: ribbon lightning: hanging from clouds hanging from clouds due to windsdue to winds dry lightning:dry lightning: not producing rain;not producing rain; cause forest firecause forest fire heat lightning (in heat lightning (in summer): seen but summer): seen but not heard because not heard because sound wave sound wave propagation is affected by airpropagation is affected by air St. Elmo’s fire: a corona discharge or sparks, St. Elmo’s fire: a corona discharge or sparks, can cause the top of a ship’s mast to glow; can cause the top of a ship’s mast to glow;

also seen over power lines and aircraft wingsalso seen over power lines and aircraft wings

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Lightning Detection and Lightning Detection and ProtectionProtection

lightning direction-finder: detecting the radio waves lightning direction-finder: detecting the radio waves produced by lightningproduced by lightning

Satellites can also monitor global lightning activitiesSatellites can also monitor global lightning activities

Q: who created the U.S. National Lightning Detection Network? a) a UA scientist; b) a non-UA scientist

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Lightning ProtectionLightning Protection

Q: who invented the lightning rod?a) Jefferson, b) Lincoln, c) Washington, d) Franklin

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Q: Where do you stay under thunderstorm? A:• Not under trees;• Avoid elevated places;• Keep your head as low as possible but not touch ground• Inside a building;• Inside a car;• Not in a golf cart

Q: why don’t you want to lie down on the ground?A: lightning channels usually emanate outward at a point of lightning strike, a surface current may travel through your body and injure or kill you

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TornadoesTornadoes tornado or twistertornado or twister: : typically 100-600 m, maybe typically 100-600 m, maybe

>1 mi,>1 mi, usually moves at 20-40 knotsusually moves at 20-40 knots funnel cloudfunnel cloud:: not reach the groundnot reach the ground dust-whirl stagedust-whirl stage mature stagemature stage decay stagedecay stage

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Tornadoes ripped through the Dallas region on 4/3/2012 and a truck became airborne.

Tornado OccurrenceTornado Occurrence tornado alley: Great Plainstornado alley: Great Plains time of day: most frequent at 4-6pm LTtime of day: most frequent at 4-6pm LT times of year: May and Junetimes of year: May and June

Annual number per state;Annual number per 100 mi by 100 mi 27

Tornado WindsTornado Winds multi-vortex multi-vortex

tornadoestornadoes suction vorticessuction vortices

Q: what is the wind speed at A or C?a) 100 knots, b) 112 knots, c) 150 knots

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Seeking ShelterSeeking Shelter tornado watch: tornado watch: likely to formlikely to form

tornado warning: tornado warning: spotted visually or by spotted visually or by radarradar• It’s always a good idea to know what to do if a It’s always a good idea to know what to do if a

tornado watch or warning is issued for your area. tornado watch or warning is issued for your area. • Take shelter in the basement or small room in the Take shelter in the basement or small room in the middle of the house at a lower level and cover your headmiddle of the house at a lower level and cover your head• Lie flat on the ground in a ditch (but not lie down on a Lie flat on the ground in a ditch (but not lie down on a

flat surface) flat surface)

• Don’t stay under a highway overpassDon’t stay under a highway overpass• Don’t stay near window or wall Don’t stay near window or wall • Don’t stay in a mobile home Don’t stay in a mobile home

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The Fujita ScaleThe Fujita Scale tornado classification based on damagetornado classification based on damage

• The “F-scale” was named after Prof. Ted Fujita.The “F-scale” was named after Prof. Ted Fujita.• Wind damage is proportional to the square of wind speedWind damage is proportional to the square of wind speed Q: How many times is the damage from F3 (160 knots) as that from F0 (40 knots)? a) 2, b) 4, c) 8, d) 16

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Tornado families: different tornados spawned by the same thunderstorm

Tornado outbreaks: 6 or more tornados over a particular region

Tri-state tornado outbreak on 3/18/1925: at least 7 tornadoes traveled a total of 437 miles across portions of Missouri, Illinois, and Indiana, causing 747 human casulties.

Meteorologists are doing a better job now in protecting lives.

• Tornado outbreaks

Tornado FormationTornado Formation

mesocyclonesmesocyclones bounded weak echo region: inside bounded weak echo region: inside

mesocyclone (no precip) mesocyclone (no precip) Radar hook echo: rotating precipitation around mesocycloneRadar hook echo: rotating precipitation around mesocyclone wall cloudwall cloud

• A rotating wall A rotating wall cloud is an cloud is an unforgettable unforgettable sight - justsight - justask a successful ask a successful storm chaser. storm chaser.

Supercell Tornadoes

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hook echo

Q: What is the percentage of supercellsproducing tornadoes: a) 100%, b) 70%,c) 40%, d) 15%

Q: Is mesocyclone caused by the vertical tilting of the horizontal vortex tube?a) yes, b) no

Nonsupercell TornadoesNonsupercell Tornadoes Gustnadoes: tornado along a gust frontGustnadoes: tornado along a gust front Landspouts: weak and short-lived, from Landspouts: weak and short-lived, from

congestuscongestus cold air funnels: cold air aloft; short-cold air funnels: cold air aloft; short-

lived; weaklived; weak Waterspouts: similar to landspoutsWaterspouts: similar to landspouts

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Observing Tornadoes and Severe Observing Tornadoes and Severe WeatherWeather

Doppler shift: similar to change of sound frequency as a train Doppler shift: similar to change of sound frequency as a train approaches the observerapproaches the observer

tornado vortex signature: rapidly changing wind directiontornado vortex signature: rapidly changing wind direction Doppler lidar: use light beam (instead of microwave in Doppler lidar: use light beam (instead of microwave in

radar), higher spatial resolutionradar), higher spatial resolution NEXRAD: >150 Doppler radars overNEXRAD: >150 Doppler radars over

continental U.S. (www.weather.gov)continental U.S. (www.weather.gov)

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Yellow and red:moving away

Green and blue:moving toward the radar