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Chapter22
Downbursts
Downburst
• Strongdowndraft that originates within the
lower partofacumulus cloud or
thunderstorm anddescends totheground
– Donotrequirestrongthunderstormsfor
development
– Responsibleforstrongstraight-linewinds
– Windscanexceed100mph
• DamagecanreachEF-1tornadoratingequivlancy
Microbursts
• Smallintense downbursts lessthan4kmin
horizontal dimension
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DownburstHazards
• Extremely hazardous toaircraft
• Evolution canbeveryquick (within amatter of
minutes ofthecumulus cloud developing)
Downburstvs Downdraft
• Downbursts aremoreintense anconcentrated
oversmallareas
• Developor intensify inthelower portion ofa
cloud
– Candevelopfromarelatively shallowcloud
– Evaporativecoolingiskey
DownburstFormation
• Formbytwo mechanisms
– Evaporation
– Dragforceoffallingprecipitation
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Evaporation
• Low-humidity airbelow the cloud isneeded
forthedownburst toform
• Howdoes evaporation startthedownburst?
• Whatdetermines itsspeed?
DragForceofFallingPrecipitation
• Howdoes thisaffectdownward airmotion?
EnvironmentalConditionsAssociated
withMicroburstDevelopment
• General Characteristics
– Large environmentallapseratebelowthecloud
– Dryairbelowthe cloudbase
– Increaseinmoisturecontentnearthesurface
– Sub-freezingtemperaturesinmuchofthecloud
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LargeLapseRates
• Whyaretheseimportant?
DryAir
• Whyisitimportant tohaveitbelow cloud
base?
MoistureIncrease
• Whyisitimportant fornear-surface moisture
content toincrease?
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Sub-Freezing Cloud Temps
• Whyaretheseimportant?
DownburstStructure
MicroburstTypes
• Wet
– Accompaniedbymeasurablerain
– Usuallyvisiblebecauseofrainshafts
– Mostcommonwhere?
• Dry
– Havenovisualindications,thosevirga can
sometimesbeseen
– Moredangerous
– Mostcommonwhere?
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WetDownburst
DryDownburst
Dryvs Wet
• Figure22.11
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Heatbursts
• Occur inasimilar manner todownbursts, but
whatdevelopment stageisdifferent? How
warmcantheyget?
AircraftHazards
• Aircraft flyinginto amicroburst initially have
increased airspeeds duetothe additional
headwinds
• Inthe coreofthemicroburst, theheadwind
disappears 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 radar
rotation versusadownburst?
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ForecastingMicrobursts
• Howcanaforecaster predict Microbursts?
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