Download Chapter 13 Lake-Effect Snows Snow Belts

2/7/17
Chapter13
Lake–EffectSnows
Lake-EffectSnows
• Primary areaoffocus arethe GreatLakes
Regions, particularly thesouth andeastside
• Alsocanbefoundaround theGreatSaltLake,
theAralSeaandtheFingerLakesinNY
• Canproduce 1-5feetofsnowper event
• Ratescanexceedaninch perhour
Snow Belts
• Foundalong theshorelines ofthe GreatLakes
– Syracuse
– Rochester
– Buffalo
– Cleveland
– Erie
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Lake-EffectSnow Requirements
• Requirements:
– Verycoldairmovingacrossthelake
– Warmlake temperatures
– Non-frozenwater
• Thecolder theairandthe warmerthe water,
themostextreme the lake-effectsnow willbe
• Typicallyoccur fromNovember - January
LargeScaleWeatherPatterns
• Occursin thewakeofanextratropical cyclone
• NOTassociated with frontal boundaries
• Enhanced ifanarctic highispresent tothe
west
LakeEffectWeatherPattern
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Lake-EffectSnow Development
• Airapproaching thelakehastemperatures
from-5Cto -25C
• WaterTemperatures are0Cto4C
• Airmovingoverthelakesurface accelerates.
Why?
• Fasterairoverlakeleadsto divergenceat
surfacealong theupwind shoreline
Lake-EffectSnow Development
• Whatdoesdivergence atthe surfacemeanfor
vertical airmotion?
• Heatistransferred from thelaketotheair
– Whatdoesthisdotothesaturationvapor
pressure?
• Howmuch heating ofthe aircanoccur asit
movesacross thelake?
Lake-EffectSnow Development
• Whathappens toairstability asitmoves
acrossthe lake?Whattypesofclouds beginto
form?
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Lake-EffectSnow
• Cloud heights cantypicallygrow to(3,30,
300)kmindepth
• Canleadtodevelopment ofasnowsquall
• SnowSquall – Heavysnowaccompanied by
strong wind.Usually short-lived or
intermittent inlength
Lake-EffectSnow Development
• Whathappens whenthe windshit the
shoreline ontheother sideofthelake?
• Under what circumstances would youexpect
theheaviestsnowto develop?
Lake-EffectSnow Development
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Lake-EffectSnow Development
• Iftemperature difference between thelake
andtheair<10C,evaporation and
destabilization aregenerally insufficient for
lakeeffectsnows
• Lake-effectsnowsgenerallyonlyhappen when
winds arewesterly ornortherly. Whydon’t
yougetthem withsoutherly oreasterlyflow?
Lake-EffectSnow Climatologies
• Snowfall amounts aredependent onsix
variables
Lake-EffectSnow Climatologies
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Topography
• Playstwo rolesinenhancing precipitation
amounts. Whatarethey?
Lake-EffectSnow
Lake-EffectSnow
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ResidenceTime
• Timethatairresidess overthelake
– Affectstotalevaporation
– Affectsheattransfer
EffectsofResidenceTime
Wind Effects
• Morecomplicated thanterrain effects
• Slower winds->longer residence time
• Whateffectsdofasterwinds have?
• Speedanddirection alsoinfluence cloud
organization
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Snowfall Organization
• Clouds andPrecipitation organizeinthree
ways
– Wind-ParallelRolls
– Shore-ParallelBands
– Vortices
• Allthree aredependent onwind speedand
direction with respect tothe lake
Wind-Parallel Rolls
• Require winds tobestrong andblowacross
theshort axisofthelake
• HeatTransfer causestheairtorise, cooler air
sinksbehind
• Rising andsinkingmotions causerolls toform
thatalign parallel tothewind
Wind-Parallel Rolls
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Wind-Parallel Rolls
• Upward branches
– 1-2kmwide
– 4-6kmapart(10kmmax)
Wind-Parallel Rolls
Shore-ParallelBands
• Associated with weakwinds acrosstheshort
axisofthelake,or strongwinds acrossthe
longaxisofthelake
– Heatfromlakecausesairtoriseoverlake
– Airisdrawnfrombothshores
– Canbeinthemiddleofthelakeoralongtheshore
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Shore-ParallelBands
Vortices
• Developoverthelake
• Related to:
– HorizontalWindShear
– WindSpeedMagnitude
– VariationsinShorelineTopography
– AtmosphericStability
– Lake-AirTemperature Differences
Vortices
• Canmaintain aclosed circulation forhours
• Whatisaclosed circulation?
• Typicallyfallapartwhen theymoveinland
• Structure maybesimilar tohurricanes
– Eye
– Eyewall
– SpiralSnowBands
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Vortices
• Winds typically5– 15kts
• Diameters – 10– 100km
Vortices
Lake-Effectvs Lake-Enhanced
• Canlakesaffectsnowfall during afrontal
passage?How?
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Lake-EffectandClimateChange
• Whatimpact isclimatechange likelytohave
onLake-EffectSnows?
Lake-EffectHazards
• Whathazardsareassociated with lake-effect
snows?
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