Transcript Thunderstorms and Tornadoes - Cal State LA

Chapter 14
Thunderstorms
A storm containing lightening and
thunder; convective storms
 Severe thunderstorms: one of large hail,
wind gusts greater than or equal to
50kts, or tornado
 Ordinary Cell Thunderstorms
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 Air-mass thunderstorms: limited wind sheer
 Stages: cumulus, mature, dissipating
 Entrainment, downdraft, gust front
Thunderstorms
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Multi-cell Thunderstorms
 Thunderstorms that contain a number of
convection cells, each in a different stage of
development, moderate to strong wind
shear; tilt, over shooting top
 Gust Front: leading edge of the cold air outflowing air; shelf cloud, roll cloud, outflow
boundary
 Micro-bursts: localized downdraft that hits
the ground and spreads horizontally in a
radial burst of wind; wind shear, virga
Thunderstorms
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Multi-cell Thunderstorms
 Squall-line thunderstorms; line of multi-cell
thunderstorms, pre-frontal squall-line,
derecho
 Meso-scale Convective Complex: a number
of individual multi-cell thunderstorms grow in
size and organize into a large circular
convective weather system; summer,
10,000km2
Thunderstorms
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Supercell thunderstorms
 Large, long-lasting thunderstorm with a single
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rotating updraft
Strong vertical wind shear
Outflow never undercuts updraft
Classic, high precipitation and low precipitation
supercells
Cap and convective instability
Rain free base, low-level jet
Surface, 850mb, 700mb, 500mb, 300mb
conditions
Thunderstorms
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Thunderstorms and the Dryline
 Sharp, horizontal change in moisture
 Thunderstorms form just east of dryline
 cP, mT, cT
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Floods and 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
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Topic: Big Thompson Canyon
 July 31, 1976, 12 inches of rain in 4 hours
created a flood associated with $35.5million
in damage and 135 deaths
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Distribution of Thunderstorms
 Most frequent Florida, Gulf Coast, Central
Plains
 Fewest Pacific coast and Interior valleys
 Most frequent hail Central Plains
Thunderstorms
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Lightening and Thunder
 Lightening: 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
Thunderstorms
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Observations: Elves
 Blue jets, red sprite, ELVES
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The Lightening Stroke
 Positive charge on ground, cloud to ground
lightening
 Stepped leader, ground stroke, forked
lightening, ribbon lightening, bead lightening,
corona discharge
Thunderstorms
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Observation: Apple tree
 DO NOT seek shelter during a thunderstorm
under an isolated tree.
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Lightening Detection and Suppression
 Lightening direction finder detects
radiowaves produced by lightening, spherics
 National Lightening 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.
 Tornado life cycle
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 Organizing, mature, shrinking, decay stage
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Tornado outbreaks
 Families, super outbreak
Tornadoes
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Tornado Occurrence
 US experiences most tornadoes
 Tornado Alley (warm, humid surface; cold dry air
aloft)
 Highest spring, lowest winter
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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
 Multi-vortex tornados
Tornadoes
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Seeking shelter
 Basement or small, interior room on ground
floor
 Indoor vs. outdoor pressure
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The Fujita Scale
 Based upon the damage created by a storm
 F0 weakest, F5 strongest
 Enhanced Fujita Scale
Tornadic Formation
Basic requirements are an intense
thunderstorm, conditional instability, and
strong vertical wind shear
 Supercell Tornadoes
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 Wind sheer causes spinning vortex tube that
is pulled into thunderstorm by the updraft
 Mesocyclone, BWER, rear flank downdraft,
vertical stretching, funnel cloud, rotating
cloud, wall cloud
Stepped Art
Fig. 14-46, p. 402
Tornadic Formation

Nonsupercell Tornadoes
 Gustnadoes
 Land spout
 Cold-air funnels
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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