freezing precipitation and ice storms · freezing precipitation • rain or drizzle that is liquid...
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FreezingPrecipitationandIceStormsChapter 12
FreezingPrecipitation
• Rain ordrizzle thatisliquid inform,butfreezesimmediately tosurfaceswhen itcontacts them
• FreezingRain isasignificant hazardtoelectrical utilities, airlines androadsurfacetransportation
IceStorms
• Iceformed duetofreezing precipitation that isoften undetectable
• Whatarethetwotypesofice?
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Supercooled Water
• Waterthatremains liquid attemperaturesbelow freezing
• Howdoes thishappen?
• Atwhat temperature willwater freezeregardless?
IceNuclei
• Soil,organic particles, pollutants,bacteria, etc• What is specialabout icenuclei particles thatmakes water freeze tothem?
• What is thedifference between CCNand IN?
• What temperatures are icenuclei mosteffectiveatcreating ice? Marginally effective? Noteffective atall?
FreezingPrecipitationFormation
• Formedbytwoprocesses–Meltingprocess– Supercooled warmrainprocess
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MeltingProcess
• Snowfallsinto alayerwhere thetemperatureexceedsO°C
• Snowmeltsin thislayerandthenfallsbackinto asub-freezing layerbelow
• Ifthedepth ofthesubfreezing layerissufficiently shallow, the meltedsnow won’trefreeze
Supercooled WarmRainProcess
• Cloud droplets growtoprecipitation sizebycollision/coalescence process
• Snow/frozen precipitation isnot involvedinthisprocess
MeltingProcessvs SupercooledWarmRainProcess
• Precipitation produced bymeltingprocess istypically freezingrain
• Precipitation produced bysupercooled warmrainprocess istypicallyfreezing drizzle
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FreezingRain
• Needtohaveaninversion intheatmosphereforfreezing rainprocesses tooccur– Alsoreferredtoasa“warmnosealoft”
PrecipitationAcrossaFront
FreezingRain
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IcePellets
• Formedwhenmelted snowrefreezesbeforehitting the ground
• Alsoreferred toasSleet• Contrary tothe book,theyareasignificanthazardtoaviation.Why?
FreezingDrizzle
• Lightprecipitation commonly mistaken asMist• Drizzledropdiameters are typically (0.002to0.005mm, 0.02– 0.05mm, 0.2– 0.5mm) indiameter
• Forms incloudlayers typically 1-3km indepth• Entire cloud temperature istypically belowfreezing
• Whatare thecommon cloudtop temperaturesassociated with freezing drizzle?
FreezingDrizzle
• Causesiceaccretion onroadwaysurfaces• Biggesthazard istoaircraft both onthesurfaceandaloft. Why?
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FreezingPrecipitationWeatherPatterns
• Typicallyfound eastoftheRockyMountains.Why?
ArcticFront/ArcticHigh
• HighPressure overtheMidwest• Stalledfront along theRockies, GulfandEastCoasts
• Warmairriding overthefront createsshallowcloud layer
• Canproduce either FZRAorFZDZ• Accounts for1/3ofallfreezingprecipitationevents
ArcticFront/ArcticHigh
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WarmFronts(NoHighPressuretotheNorth)
• FreezingPrecipitation occurs north ofwarmfronts
• Typicallyoriented parallel tofront• Warmfronts account for1/3ofallfreezingprecipitation events– ½ofthosehavenohighpressuretothenorth– ½ofthosedohaveahighpressuretothenorth
WarmFronts(NoHighPressuretotheNorth)
WarmFrontw/CanadianHighPressuretotheNorth
• Similar setuptoasituation with nohighpressure tothenorth
• Results instronger temperature gradients– Resultsinstrongersurfacewinds,whichcanincreaseaccretionrates
– Canleadtosignificantdestructionoftreesandpowerlines
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WarmFrontw/CanadianHighPressuretotheNorth
WesternQuadrantofArcticHighPressure
• Southerly flow atsurface andalofttypicallyoccurs onwestern sideofarctic airmasses
• If0°Cisotherm islocated inthisregion, warmmoist airbeingcarried overthecold airbelowcanleadtofreezingprecipitation events
• Typicallyoccurs inamorecircular region• 1/10ofallfreezingprecipitation eventsoccurfromthis setup
WesternQuadrantofArcticHighPressure
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ColdAirDamming
• 15%ofallfreezingprecipitation eventsoccurbecauseofthis
• Phenomena that occurs whenboundaries(topography, density gradients, etc)trap airinacertain location
• Developsintwo ways
ColdAirDamming
ColdAirDamming
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FreezingPrecipitationCoverage
• Typicallyconfined tonarrow bandsthatmovealongwith theweather systems
• Whatcausesmajor icestorms?
DetectingFreezingPrecipitation
• Twomethods– Sfc based– Radarbased
Radar-basedDetectionofFZDZ
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Radar-BasedDetectionofFZDZ
• Whatisthedrawbacktousing radarindetecting FZDZ?
Radar-basedDetectionofFZRA
Surfacebased
• FreezingPrecipitation Detection Sensor
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FreezingPrecipitationSensor
• Howdoes itwork?
• WhattypesofPrecipitation canitdetect?
• Drawbacks?
FreezingRainSensor
• Howisfrequency changerelated toicethickness onthesensor?
FreezingRainDistribution
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FreezingDrizzleDistribution
Whatmakesfreezingprecipitationhazardous?