Chapter 10

Thunderstorms

A storm containing lightning 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

Fig. 10-1, p. 275

Fig. 10-2, p. 276

Fig. 10-2a, p. 276

Fig. 10-2b, p. 276

Fig. 10-2c, p. 276

Stepped Art

Fig. 10-2, p. 276

Fig. 10-3, p. 277

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

Fig. 10-4, p. 278

Fig. 10-5, p. 278

Fig. 10-6, p. 279

Fig. 10-7, p. 279

Fig. 10-8, p. 280

Fig. 10-9, p. 280

Fig. 10-10, p. 281

Fig. 10-11, p. 281

Stepped Art

Fig. 10-11, p. 281

Thunderstorms

Mutli-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

Fig. 10-12, p. 282

Fig. 10-13, p. 282

Fig. 10-14, p. 282

Fig. 10-15, p. 283

Fig. 10-16, p. 283

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

Fig. 10-17, p. 284

Fig. 10-18, p. 284

Fig. 10-19, p. 285

Fig. 10-20, p. 285

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

Fig. 10-21, p. 286

Fig. 10-22, p. 287

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

Fig. 10-23, p. 289

Fig. 10-24, p. 289

Thunderstorms

Lightning and ThunderLightning: discharge of electricity in mature

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

Thunder: explosive expansion of air due to heat from lightning

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

Upper cloud positive, bottom cloud negative

Fig. 10-25, p. 290

Fig. 10-26, p. 291

Fig. 10-27, p. 291

Thunderstorms

Types of lightningBlue jets, red sprite, ELVES

The Lightning StrokePositive charge on ground, cloud to ground

lightningStepped leader, ground stroke, forked

lightning, ribbon lightning, bead lightning, corona discharge

Fig. 10-28, p. 292

Fig. 10-28a, p. 292

Fig. 10-28b, p. 292

Fig. 10-28c, p. 292

Fig. 10-29, p. 293

Fig. 10-30, p. 294

Fig. 10-31, p. 294

Fig. 10-32, p. 295

Thunderstorms

Lightning Detection and SuppressionLightning direction finder detects radiowaves

produced by lightning, sphericsNational Lightning Detection NetworkSuppression: seed clouds with aluminum

Observation: Apple treeDO NOT seek shelter during a thunderstorm

under an isolated tree.

Fig. 10-33, p. 295

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

Fig. 10-34, p. 297

Tornadoes Tornado Occurrence

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

aloft)Highest in spring, lowest in winter

Tornado windsMeasurement based upon damage after storm or

Doppler radarFor southwest approaching storms, winds strongest

in the northeast of the storm, 220 kts maximumMulti-vortex tornados

Tornado outbreaksFamilies, super outbreak

Fig. 10-35, p. 298

Fig. 10-36, p. 298

Fig. 10-37, p. 299

Fig. 10-38, p. 299

Fig. 10-39, p. 301

Tornados

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

Table 10-1, p. 300

Table 10-2, p. 301

Table 10-3, p. 301

Tornadic Formation

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

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

Fig. 10-41, p. 303

Fig. 10-42, p. 303

Fig. 10-42a, p. 303

Fig. 10-42b, p. 303

Stepped Art

Fig. 10-42, p. 303

Fig. 10-43, p. 304

Fig. 10-44, p. 304

Tornadic Formation

Nonsupercell TornadoesGustnadoesLand spoutCold-air funnels

Fig. 10-45, p. 305

Fig. 10-46, p. 306

Fig. 10-47, p. 306

Fig. 10-47a, p. 306

Fig. 10-47b, p. 306

Observing Tornadoes and Severe Weather Doppler radar measures the speed of

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

view TVS, doppler lidar NEXRAD

Fig. 10-48, p. 307

Fig. 10-49, p. 308

Waterspouts

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

Tornadic waterspout

Fig. 10-50, p. 308