Download pdf - chapter14estrada.cune.edu/.../brent.royuk/sci381/docs/Chapter14_3.pdf · 2019. 4. 10. · 4/10/19 2 Ordinary Cell Thunderstorms Mature Stage Droplets grow large enough to fall. Drier

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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.


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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.



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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.



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Fig. 14-4, p. 375 10 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


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Inversion layer caps air, convective instability: breakthrough


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


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Thunderstorms

� Distribution of Thunderstorms � Most frequent Florida, Gulf Coast, Central

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



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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


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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


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


Table 14-1, p. 399 54 Concordia University Geog/Sci-381 Chapter 14

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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


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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




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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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Waterspouts

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

� Tornadic waterspout




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Homework for Chapter 14 �  Chapter 14 Questions for Review

�  #2-6, 19, 25, 29

�  Chapter 14 Questions for Thought �  #7

�  Chapter 14 Problems and Exercises �  #4


Project for Chapter 14 �  None
