MET4300

Lecture 18Cold Waves (CH14)

ColdWaves,orPolarOutbreaks• Influxofunusuallycoldairintomiddle

andlowerlatitudes• Generallycoverlargerareasthan

blizzardsoricestorms• Onaverageclaim30livesayear• Moregenerally~600peopledieyearly

fromhypothermia– Men,elderly,inthesouth

• Impactsonagricultureinthebillions(Citrusetc.)

• Measure:Actualtemperatureitselfisnotameaningfulmeasureofacoldwave’sintensity&impact.Instead,weusethedeparturefromnormaltemperatureasameasure.Forexample,20deg FisdangerouslycoldtosouthFlorida,butnormaltothenorthGreatPlains.

RecordLowestTemperature(F)inEachStateasof2010-11Winter

FormationofColdAirmasses• Thecoreofcoldwaveisastronghighpressure

centerthatformsduringwinterinhighlatitudes

• Surfacehighpressurecentersformbycoolingofairinthelowtroposphere

• Cooling isfavoredbythelongpolarnightswhenwindsarelightandtheskyiscloudfree(Cooledbyinfraredradiationtospaceduringlongpolarnightsoversnowcoveredsurface)

• Generallyshallow,1-2km• Polarairmasses thatformoversnow-covered

northern landareasinSiberia,northCanada,andAlaskaarecolderthanArcticairmass thatformsovertheArticoceanbecausesomeoceanheatleaksthrough cracksintheice

• InNorthAmerica,mountainspreventintrusionofwarmairfromthePacific

• 3physicalfactorsthatfavorcoldairmassformation: longnight, clearskies,lightwinds–allcontribute toradiativecooling

• 2dynamicalfactors:Upper-airconvergence;subsidence

TypicalWeatherPatternpriortoacoldoutbreakUpper level-convergence right above the surface high.

Intensify the sinking and spreading of denser air

A ridge over the western North America is associated with a storm system in the North Pacific Ocean

Give the cold airmass a southward push (steering winds)

OutbreaksofColdAir• Twofactorscontributingtothesouthwardmoveofthe

coldairmass intotheUS1)Thetendencyofadenserfluidtosinkrelativetoalessdensefluidandtospreadlaterallyatlowlevels2)Steeringwindsinthemiddle&upperlevel– aupperlevelridgeprovidesbothconvergenceandsteeringwinds

• Intensificationofupper-levelridge:1)StrongcyclonesintheAleutianregionofAlaska2)StrongcyclonesoriginatingeastoftheRockiesoralongtheEastCoast3)TheN-SalignmentoftheRockiesfavorstheenhancementoftheridgesanddownstreamtroughs.

InitialStagesofaCycloneandColdRidgetotheWest

Surface 500 mb• Surfacehighhasdevelopedandbeginstomovesouthward

DevelopingCycloneandColdRidge

Surface 500 mb• StrongsurfacecyclonemovingfromthecentraltotheeasternUScontributestotheintensificationoftheridge&trough

EndStageoftheCycloneandColdRidge

Surface 500 mb

• AsthecyclonemovesNEandoccluded,thesurfacehighmovesSEward• Thenortherlywindsbetweenthelowandthehighbringcoldairtothe

centralandeasternUS,deepeningthetroughandmovingiteastward.• Theprogressionoccursoveraperiodofonly2-3days,causingcoldin

theeastandwarminthewesternUS

TrajectoriesofairparcelsreachingthecentralUSinthecoreofpolar/Arcticairmassesduringtwomajorcoldwaves

Channeling Effect: cold air follows a track just east of the Rokies;Subsidence of the air by several hundred mbs due to moving from polar to the mid latitudes. This causes a little bit warming.

Summaryofallfactorsthatcontributetomajorcoldoutbreaks

• Thebuildupofaridgeinthejetstream overwesternNorthAmerica,oftenasaresultofwarmairtransportnorthwardinthelowertroposphereeastofacycloneintheNorthPacific

• FormationofasurfacehighovernorthernCanadaorAlaskathroughrapidcoolingoftheairnearthesurfaceandconvergencealoftdownstreamoftheridge.

• Movementofthecoldairmass SEward inresponsetosteeringbytheupper-levelwindsandchannelingofthecold-airpoolbytheRockies

• Amechanism(oftenastrongwintercycloneovertheUS)toenhancethewindsthattransportthecoldairSEward,therebyreducingthetransittimeofthecoldair.

• ExtensivesnowcoverovercentralNorthAmericatokeepthepolarairmass “refrigerated”.

ColdWavesofthe20th Century

ArcticOutbreakofFebruary1899

Outbreak covered 2/3 of USSub-freezing in MiamiSnow in Ft. Myers34 in of snow in DCIce flowed into the Gulf of Mexico from the Mississsippi

Weather Conditions: 1) extensive snow cover built up in central & eastern US earlier; 2) Polar airmass formation to the north; 3) Gulf coast cyclone to the south The greatest Arctic Outbreak in history; mother of all cold waves;

record high pressure of 1064mb; set many coldest record.

ExtremeColdinEurasian(Europe&Asia)

Cold airmass over SiberiaSiberiacoldairmassformation:• Largedistance

tothenearestunfrozenocean

• Mountainstotheeast&south,trappingthecoldair

• Coldaircanstayformonthsduringwinter

WhatCausesUnusuallyColdWeatherinEurope?

COLD

WARM

Subpolar Low

Subtropical High

• Normallycoldweatherwon’thappen (leftpanel)duetothewest-to-eastflowbringingmildmaritimeair

• Butduring thenegativephaseofNorthAtlanticOscillation,thesubtropicallow&subtropicalhighweakens(rightpanel),enablingthewestwardmovementoftheSiberiaairmass toEurope.• When Siberia cold air mass moves

SWward to Europe

WindChill

• Windchillfactor:accountfortheeffectofbothtemperatureandwind.

• Windchilltemperature:thefasterthewind,thelowerthewindchilltemperature

• Windchillindex:providesanestimateofperceivedtemperaturebasedonwindspeedandactualtemperature.Anempiricalformulasthatequatestherateofheatlossatagiventemperatureandwindspeedtotherateofheatlosswithnowind.

Summary• ColdWaves,Polar(orArctic)outbreaks

– Coldairpushessouthwardbehindacoldfront– Arcticorpolarairformsunderclearskiesinananticycloneoversnow-coveredground

– Mountainsprotecttheheartlandfromwarmmaritimeair

• Causehumansufferinganddeathsinunpreparedareas

• Cyclone-anticyclonedoublewhammy• Wind-Chillfactorcombineseffectsoftemperatureandwind:– At0oF,a20kt windcauses-22oFwindchill