Download Chapter 12

1/31/17
FreezingPrecipitationandIce
Storms
Chapter 12
FreezingPrecipitation
• Rain ordrizzle thatisliquid inform,but
freezesimmediately tosurfaceswhen it
contacts them
• FreezingRain isasignificant hazardto
electrical utilities, airlines androadsurface
transportation
IceStorms
• Iceformed duetofreezing precipitation that is
often undetectable
• Whatarethetwotypesofice?
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Supercooled Water
• Waterthatremains liquid attemperatures
below freezing
• Howdoes thishappen?
• Atwhat temperature willwater freeze
regardless?
IceNuclei
• Soil, organic particles, pollutants, bacteria, etc
• What is special about ice nuclei particles that
makes water freeze tothem?
• What is the difference between CCNand IN?
• What temperatures are ice nuclei most effective
at creating ice? Marginally effective? Not
effective atall?
FreezingPrecipitationFormation
• Formedbytwoprocesses
– Meltingprocess
– Supercooled warmrainprocess
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MeltingProcess
• Snowfallsinto alayerwhere thetemperature
exceedsO°C
• Snowmeltsin thislayerandthenfallsback
into asub-freezing layerbelow
• Ifthedepth ofthesubfreezing layeris
sufficiently shallow, the meltedsnow won’t
refreeze
Supercooled WarmRainProcess
• Cloud droplets growtoprecipitation sizeby
collision/coalescence process
• Snow/frozen precipitation isnot involvedin
thisprocess
MeltingProcessvs Supercooled Warm
RainProcess
• Precipitation produced bymeltingprocess is
typically freezingrain
• Precipitation produced bysupercooled warm
rainprocess istypicallyfreezing drizzle
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FreezingRain
• Needtohaveaninversion intheatmosphere
forfreezing rainprocesses tooccur
– Alsoreferredtoasa“warmnosealoft”
PrecipitationAcrossaFront
FreezingRain
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IcePellets
• Formedwhen melted snowrefreezesbefore
hitting the ground
• Alsoreferred toasSleet
• Contrary tothe book,theyareasignificant
hazardtoaviation.Why?
FreezingDrizzle
• Light precipitation commonly mistaken as Mist
• Drizzle drop diameters are typically (0.002to
0.005mm, 0.02– 0.05mm, 0.2 – 0.5mm) in
diameter
• Forms in cloudlayers typically 1-3km indepth
• Entire cloud temperature istypically below
freezing
• What are the common cloudtop temperatures
associated with freezing drizzle?
FreezingDrizzle
• Causesiceaccretion onroadwaysurfaces
• Biggesthazard istoaircraft both onthe
surfaceandaloft. Why?
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FreezingPrecipitationWeather
Patterns
• Typicallyfound eastoftheRockyMountains.
Why?
ArcticFront/ArcticHigh
• HighPressure overtheMidwest
• Stalledfront along theRockies, GulfandEast
Coasts
• Warmairriding overthefront createsshallow
cloud layer
• Canproduce either FZRAorFZDZ
• Accounts for1/3ofallfreezingprecipitation
events
ArcticFront/ArcticHigh
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WarmFronts(NoHighPressuretothe
North)
• FreezingPrecipitation occurs north ofwarm
fronts
• Typicallyoriented parallel tofront
• Warmfronts account for1/3ofallfreezing
precipitation events
– ½ofthosehavenohighpressuretothenorth
– ½ofthosedohaveahighpressuretothenorth
WarmFronts(NoHighPressuretothe
North)
WarmFrontw/CanadianHigh
PressuretotheNorth
• Similar setuptoasituation with nohigh
pressure tothenorth
• Results instronger temperature gradients
– Resultsinstrongersurfacewinds,whichcan
increaseaccretionrates
– Canleadtosignificantdestructionoftreesand
powerlines
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WarmFrontw/CanadianHigh
PressuretotheNorth
WesternQuadrantofArcticHigh
Pressure
• Southerly flow atsurface andalofttypically
occurs onwestern sideofarctic airmasses
• If0°Cisotherm islocated inthisregion, warm
moist airbeingcarried overthecold airbelow
canleadtofreezingprecipitation events
• Typicallyoccurs inamorecircular region
• 1/10ofallfreezingprecipitation eventsoccur
fromthis setup
WesternQuadrantofArcticHigh
Pressure
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Cold AirDamming
• 15%ofallfreezingprecipitation eventsoccur
becauseofthis
• Phenomena that occurs whenboundaries
(topography, density gradients, etc)trap airin
acertain location
• Developsintwo ways
Cold AirDamming
Cold AirDamming
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FreezingPrecipitationCoverage
• Typicallyconfined tonarrow bandsthatmove
alongwith theweather systems
• Whatcausesmajor icestorms?
DetectingFreezingPrecipitation
• Twomethods
– Sfc based
– Radarbased
Radar-basedDetectionofFZDZ
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Radar-BasedDetectionofFZDZ
• Whatisthedrawbacktousing radarin
detecting FZDZ?
Radar-basedDetectionofFZRA
Surfacebased
• FreezingPrecipitation Detection Sensor
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FreezingPrecipitationSensor
• Howdoes itwork?
• WhattypesofPrecipitation canitdetect?
• Drawbacks?
FreezingRainSensor
• Howisfrequency changerelated toice
thickness onthesensor?
FreezingRainDistribution
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FreezingDrizzleDistribution
Whatmakesfreezingprecipitation
hazardous?
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