chapter 14. thunderstorms a storm containing lightening and thunder; convective storms severe...
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Chapter 14
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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 ThunderstormsAir-mass thunderstorms: limited wind sheerStages: cumulus, mature, dissipatingEntrainment, downdraft, gust front
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Thunderstorms 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 out-flowing 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
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Thunderstorms
Multi-cell ThunderstormsSquall-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
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Thunderstorms Supercell thunderstorms
Large, long-lasting thunderstorm with a single rotating updraft
Strong vertical wind shearOutflow never undercuts updraftClassic, high precipitation and low precipitation
supercellsCap and convective instabilityRain free base, low-level jetSurface, 850mb, 700mb, 500mb, 300mb
conditions
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Thunderstorms Thunderstorms and the Dryline
Sharp, horizontal change in moistureThunderstorms form just east of drylinecP, mT, cT
Floods and Flash FloodsFlash 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
Topic: Big Thompson CanyonJuly 31, 1976, 12 inches of rain in 4 hours
created a flood associated with $35.5million in damage and 135 deaths
Distribution of ThunderstormsMost frequent Florida, Gulf Coast, Central
PlainsFewest Pacific coast and Interior valleysMost frequent hail Central Plains
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Thunderstorms
Lightening and ThunderLightening: 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
Observations: ElvesBlue jets, red sprite, ELVES
The Lightening StrokePositive charge on ground, cloud to ground
lighteningStepped leader, ground stroke, forked
lightening, ribbon lightening, bead lightening, corona discharge
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Thunderstorms
Observation: Apple treeDO NOT seek shelter during a thunderstorm
under an isolated tree.
Lightening Detection and SuppressionLightening direction finder detects
radiowaves produced by lightening, sphericsNational Lightening Detection NetworkSuppression: 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.
Tornado life cycleOrganizing, mature, shrinking, decay stage
Tornado outbreaksFamilies, super outbreak
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Tornadoes Tornado Occurrence
US experiences most tornadoesTornado Alley (warm, humid surface; cold dry air
aloft)Highest spring, lowest winter
Tornado windsMeasurement based upon damage after storm
or Doppler radarFor southwest approaching storms, winds
strongest in the northeast of the storm, 220 kts maximum
Multi-vortex tornados
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Tornadoes
Seeking shelterBasement or small, interior room on ground
floorIndoor vs. outdoor pressure
The Fujita ScaleBased upon the damage created by a stormF0 weakest, F5 strongestEnhanced Fujita Scale
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Tornadic Formation
Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind shear
Supercell TornadoesWind sheer causes spinning vortex tube that
is pulled into thunderstorm by the updraftMesocyclone, BWER, 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 TornadoesGustnadoesLand spoutCold-air funnels
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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