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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