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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?