Ordinary Cell Thunderstorms - Concordia University...

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Chapter 14

1 Concordia University Geog/Sci-381 Chapter 14

Thunderstorms

 A storm containing lightning and thunder; convective storms

 Severe thunderstorms: At least one:   large hail  wind gusts greater than or equal to 50 kt   Tornado


Ordinary Cell Thunderstorms

 AKA Air-mass thunderstorms: form with limited wind sheer, vertically stacked   Stages: cumulus, mature, dissipating  Cumulus Stage (Growth Stage) ○  Warm air rises. Cloud droplets evaporate at

top and make air more humid, allowing higher growth.

○  Condensation releases heat, which causes more rising.

○  No precipitation (held aloft by updrafts), no lightning.


Ordinary Cell Thunderstorms   Mature Stage

  Droplets grow large enough to fall.   Drier air is drawn into cloud from sides (entrainment). This

causes evaporates some drops, which cools air.   Cooling air leads to downdrafts, also enhanced by falling

precipitation.   Anvil-Shaped cumulonimbus is formed, cloud top can be

40000’ high.   Updrafts collide with downdrafts- turbulence.   Cool downdrafts spread sideways at the ground along a

gust front. ○  Turbulence along gust front. ○  Warm air rises over gust front.


Ordinary Cell Thunderstorms

 Dissipating Stage   Storm generally dissipates after 15-30

minutes.  Gust front moves away from the storm and

no longer enhances updrafts.  Downdrafts dominate, no more updrafts to

fuel the storm.  Whole process can last only one hour.   Thunderstorms bring summer rain and

welcome temperature relief, though brief.

5 Concordia University Geog/Sci-381 Chapter 14 6 Concordia University Geog/Sci-381 Chapter 14

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Thunderstorms   Multi-cell Thunderstorms

  Thunderstorms that contain a number of convection cells, each in a different stage of development

  Caused by moderate to strong wind shear, which produces tilt   Sometimes there is an over shooting top   Mammatus clouds can form below the anvil

  Gust Front: leading edge of the cold air out-flowing air   Can form shelf clouds or roll clouds.   Combined edge of gust fronts is called the outflow boundary.

  Downbursts: localized downdraft that hits the ground and spreads horizontally in a radial burst of wind   Microbusts: 4km spread or less.   Cause wind shear, dangerous to planes.

  Plane crash in Dallas, 1985: 100 deaths.   Virga may form.   Warm downbursts are called heat bursts.


9 Concordia University Geog/Sci-381 Chapter 14 Fig. 14-4, p. 375 10 Concordia University Geog/Sci-381 Chapter 14

Fig. 14-4, p. 375 11 Concordia University Geog/Sci-381 Chapter 14 12 Concordia University Geog/Sci-381 Chapter 14

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Multi-cell Thunderstorms   Squall-line thunderstorms: a line of multi-cell thunderstorms

  Pre-frontal squall-line   Bow Echo: a bow-shaped squall line   Derecho: High winds along several hundred kilometers of squall

line   The Rear Inflow Jet

  Damaging straight-line winds   Meso-scale Convective Complex (MCC): a number of

individual multi-cell thunderstorms grow in size and organize into a large circular convective weather system   Happens in summer   Can last 12 hours, cover 10,000 km2


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Supercell Thunderstorms   Large, long-lasting thunderstorm with a single

rotating updraft   Strong vertical wind shear: horizontal rotation

becomes vertical.   Outflow never undercuts updraft   The Mesocyclone and the Overshooting Top   Wall clouds   Three types of supercell:

  Classic   High precipitation   Low precipitation

  Rain free base, low-level jet   Surface, 850mb, 700mb, 500mb, 300mb conditions




Inversion layer caps air, convective instability: breakthrough


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Floods   Flash Floods

  Flash floods rise rapidly with little or no advance warning; many times caused by stalled or slow thunderstorm

  Large floods can be created by training of storm systems, Great Flood of 1993


Thunderstorms

 Distribution of Thunderstorms  Most frequent Florida, Gulf Coast, Central

Plains   Fewest Pacific coast and Interior valleys  Most frequent hail Central Plains



Thunderstorms

  Lightning and Thunder   Lightning: discharge of electricity in mature

storms (within cloud, cloud to cloud, cloud to ground)

  Thunder: explosive expansion of air due to heat from lightening

  Electrification of Clouds: graupel and hailstones fall through supercooled water, ice crystals become negatively charged

 Upper cloud positive, bottom cloud negative


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Thunderstorms

 The Lightning Stroke   Positive charge on ground, cloud to ground

lightning   Stepped leader, ground stroke, forked

lightening, ribbon lightning, bead lightning, corona discharge



Thunderstorms

 Observation: Apple tree  DO NOT seek shelter during a thunderstorm

under an isolated tree.   Lightning Detection and Suppression

  Lightning direction finder detects radiowaves produced by lightning: Spherics

 National Lightning Detection Network   Suppression: seed clouds with aluminum


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Tornadoes

 Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground.

 Funnel cloud: A tornado that has not reached the ground

 Tornado life cycle  Organizing, mature, shrinking, decay stage

 Tornado outbreaks   Families, super outbreak


Tornadoes   Tornado Occurrence

  US experiences most tornadoes   Tornado Alley (warm, humid surface; cold dry air

aloft, wind shear helped by jet stream)   Highest in spring, lowest in winter

  Tornado winds   Measurement based upon damage after storm or

Doppler radar   For southwest approaching storms, winds strongest

in the northeast of the storm, 220 kts maximum   If a tornado approaches, on which side is the wind

fastest?   Multi-vortex tornados




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Tornadoes

 Seeking shelter   Basement or small, interior room on ground

floor   Indoor vs. outdoor pressure, p. 398

 The Fujita Scale   Based upon the damage created by a storm   F0 weakest, F5 strongest   Enhanced Fujita Scale

49 Concordia University Geog/Sci-381 Chapter 14 Table 14-1, p. 399 50 Concordia University Geog/Sci-381 Chapter 14

Table 14-2, p. 400 51 Concordia University Geog/Sci-381 Chapter 14 Table 14-3, p. 400 52 Concordia University Geog/Sci-381 Chapter 14

Tornadic Formation

 Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind shear

 Supercell Tornadoes  Wind sheer causes spinning vortex tube that

is pulled into thunderstorm by the updraft  Mesocyclone, BWER (bounded weak echo

region), rear flank downdraft, vertical stretching, funnel cloud, rotating cloud, wall cloud


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Stepped Art Fig. 14-46, p. 402


Tornadic Formation

 Nonsupercell Tornadoes  Gustnadoes ○  Form along gust front, short-lived and weak

  Land spout ○  Common over East-Central Colorado

 Cold-air funnels ○  Formed by cold air aloft, common along US

West Coast



Severe Weather and Doppler Radar  Doppler radar measures the speed of

precipitation toward and away radar unit  Two Doppler radars can provide a 3D

view  TVS, Doppler lidar  NEXRAD


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Fig. 14-49, p. 405 61 Concordia University Geog/Sci-381 Chapter 14 Fig. 14-50, p. 405 62 Concordia University Geog/Sci-381 Chapter 14

Waterspouts

 Rotating column of air that is connected to a cumuliform cloud over a large body of water

 Tornadic waterspout

63 Concordia University Geog/Sci-381 Chapter 14 Fig. 14-51, p. 406 64 Concordia University Geog/Sci-381 Chapter 14

Homework for Chapter 14   Chapter 14 Questions for Review, p. 407

  #2-6, 19, 25, 29

  Chapter 14 Questions for Thought, p. 408   #7

  Chapter 14 Problems and Exercises, p. 409   #4


Project for Chapter 14   None


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