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Chapter22

Downbursts

Downburst

• Strongdowndraft that originates within thelower partofacumulus cloud orthunderstorm anddescends totheground– Donotrequirestrongthunderstormsfordevelopment

– Responsibleforstrongstraight-linewinds–Windscanexceed100mph• DamagecanreachEF-1tornadoratingequivlancy

Microbursts

• Smallintense downbursts lessthan4kminhorizontal dimension

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DownburstHazards

• Extremely hazardous toaircraft• Evolution canbeveryquick (within amatter ofminutes ofthecumulus cloud developing)

Downburstvs Downdraft

• Downbursts aremoreintense anconcentratedoversmallareas

• Developor intensify inthelower portion ofacloud– Candevelopfromarelatively shallowcloud– Evaporativecoolingiskey

DownburstFormation

• Formbytwomechanisms– Evaporation– Dragforceoffallingprecipitation

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Evaporation

• Low-humidity airbelow the cloud isneededforthedownburst toform

• Howdoes evaporation startthedownburst?

• Whatdetermines itsspeed?

DragForceofFallingPrecipitation

• Howdoes thisaffectdownward airmotion?

EnvironmentalConditionsAssociatedwithMicroburstDevelopment

• General Characteristics– Largeenvironmentallapseratebelowthecloud– Dryairbelowthecloudbase– Increaseinmoisturecontentnearthesurface– Sub-freezingtemperaturesinmuchofthecloud

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LargeLapseRates

• Whyaretheseimportant?

DryAir

• Whyisit important tohaveitbelow cloudbase?

MoistureIncrease

• Whyisit important fornear-surface moisturecontent toincrease?

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Sub-Freezing Cloud Temps

• Whyaretheseimportant?

DownburstStructure

MicroburstTypes

• Wet– Accompaniedbymeasurablerain– Usuallyvisiblebecauseofrainshafts–Mostcommonwhere?

• Dry– Havenovisualindications,thosevirga cansometimesbeseen

–Moredangerous–Mostcommonwhere?

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WetDownburst

DryDownburst

Dryvs Wet

• Figure22.11

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Heatbursts

• Occur inasimilar manner todownbursts, butwhatdevelopment stageisdifferent? Howwarmcantheyget?

AircraftHazards

• Aircraft flyinginto amicroburst initially haveincreased airspeeds duetothe additionalheadwinds

• Inthe coreofthemicroburst, theheadwinddisappears andairspeed drops

• Onthe opposite side, atailwind isproduced,further dropping airspeed

MicroburstDetection

• Doppler radar• TDWR• WhyuseTDWRoverNWSradars?

• LLWAS

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RadarDetection

RadarDetection

Downburstversus Rotation

• Howcanyoutellthedifference between radarrotation versusadownburst?

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ForecastingMicrobursts

• Howcanaforecaster predict Microbursts?