PolarPolar LowsLows andand otherother HighHigh LatitudeLatitude WeatherWeather SystemsSystems JohnJohn TurnerTurner andand TomTom BracegirdleBracegirdle BritishBritish AntarcticAntarctic SurveySurvey Cambridge,Cambridge, UKUK A mosaic of infra-red satellite imagery of the Antarctic and Southern Ocean on 21 August 2006. University of Wisconsin- Madison Overlaid satellite imagery and GFS model (NCEP) analysis. University of Wisconsin- Madison Overlaid satellite imagery and GFS model (NCEP) analysis. University of Wisconsin- Madison Synoptic and mesoscale weather systems near the tip of the Antarctic Peninsula OutlineOutline •• BackgroundBackground –– history,history, definitiondefinition •• TheThe typestypes ofof polarpolar andand thethe formation/developmentformation/development mechanismsmechanisms •• ClimatologicalClimatological occurrenceoccurrence •• Analysis,Analysis, forecastingforecasting andand modellingmodelling •• FutureFuture researchresearch needsneeds andand thethe IPYIPY TheThe NatureNature ofof PolarPolar LowsLows •• OccurOccur polewardpoleward ofof thethe PolarPolar FrontFront •• MesoscaleMesoscale <1,000<1,000 kmkm diameter,diameter, <24<24 hh lifetimelifetime •• AA maritimemaritime phenomenon,phenomenon, decliningdeclining rapidlyrapidly overover landland •• HaveHave windswinds inin excessexcess ofof galegale forceforce •• UsuallyUsually occuroccur duringduring winterwinter seasonseason •• TheThe ArcticArctic systemssystems areare moremore vigorousvigorous thanthan theirtheir AntarcticAntarctic counterpartscounterparts becausebecause ofof thethe largerlarger fluxesfluxes ofof heatheat andand moisturemoisture fromfrom thethe oceanocean PossiblePossible ClimatologicalClimatological ImportanceImportance •• IndividualIndividual polarpolar lowslows cancan havehave fluxesfluxes ofof heatheat ofof upup toto 1,0001,000 Wm**2Wm**2 •• TheThe lossloss ofof heatheat fromfrom thethe oceanocean couldcould triggertrigger downwarddownward convectionconvection inin thethe oceanocean •• ButBut thethe frequentfrequent occurrenceoccurrence ofof polarpolar lowslows wouldwould bebe neededneeded inin anan areaarea forfor thisthis toto bebe ofof climatologicalclimatological importanceimportance •• ModellingModelling studiesstudies areare neededneeded toto assessassess theirtheir importanceimportance 0 400 km NomenclatureNomenclature •• PolarPolar lowslows havehave alsoalso beenbeen called:called: –– ArcticArctic hurricanehurricane –– ArcticArctic bombbomb –– ArcticArctic instabilityinstability lowslows –– ColdCold airair depressionsdepressions –– CommaComma cloudsclouds –– MesocyclonesMesocyclones •• MyMy preferredpreferred choice:choice: –– PolarPolar lowslows forfor systemssystems withwith windswinds >> galegale forceforce –– PolarPolar mesocyclonesmesocyclones forfor weakerweaker systemssystems EarlyEarly referencesreferences •• DuringDuring thethe SecondSecond WorldWorld WarWar –– thethe conveysconveys toto RussiaRussia viavia thethe BarentsBarents SeaSea reportedreported veryvery vigorousvigorous smallsmall weatherweather systemssystems •• SumnerSumner (1950)(1950) reportedreported ‘‘hurricanehurricane--likelike centrescentres withinwithin highhigh latitude,latitude, synopticsynoptic-- scalescale lowslows’’.. PeterPeter DannevigDannevig •• InIn 19541954 wrotewrote aboutabout ‘‘instabilityinstability lowslows’’ overover thethe seasea areasareas aroundaround NorwayNorway inin aa bookbook forfor pilotspilots •• ProducedProduced aa schematicschematic weatherweather chartchart showingshowing thethe relationshiprelationship betweenbetween thesethese vorticesvortices andand thethe airflowairflow aroundaround NorwayNorway TheThe HarroldHarrold andand BrowningBrowning CaseCase (1969(1969 QJ)QJ) •• ‘‘TheThe polarpolar lowlow asas aa baroclinicbaroclinic disturbancedisturbance’’ •• DevelopedDeveloped aa conceptualconceptual modelmodel forfor thethe lowlow thatthat crossedcrossed thethe UKUK onon 99 DecemberDecember 19671967 TheThe RasmussenRasmussen PaperPaper (1979(1979 QJ)QJ) •• ‘‘TheThe polarpolar lowlow asas anan extratropicalextratropical CISKCISK disturbancedisturbance’’ •• SupportedSupported thethe oldold ideaidea thatthat polarpolar lowslows werewere drivendriven byby deepdeep convectionconvection andand baroclinicitybaroclinicity playedplayed onlyonly aa smallsmall rolerole AA BaroclinicBaroclinic PolarPolar LowLow OverOver DenmarkDenmark The broken line shows the main baroclinic zone Snow cover over Jutland is indicated by the white arrow TheThe SpiraliformSpiraliform CloudCloud SignatureSignature •• AA polarpolar lowlow withwith spiraliformspiraliform cloudcloud signaturesignature offoff thethe northernnorthern coastcoast ofof NorwayNorway •• AlsoAlso indicatedindicated isis aa boundaryboundary layerlayer frontfront TheThe CommaComma CloudCloud SignatureSignature •• AA rarerare highhigh latitudelatitude commacomma cloudcloud overover thethe BarentsBarents SeaSea AA ‘‘MerryMerry--GoGo--RoundRound’’ PolarPolar LowLow Greenland •• AA polarpolar lowlow withwith multiplemultiple centrescentres overover thethe GreenlandGreenland // Norway NorwegianNorwegian Seas.Seas. Iceland TheThe InstantInstant OcclusionOcclusion ProcessProcess The polar low south of Iceland moves towards the wave on the polar front to form the instant occlusion OneOne PossiblePossible PolarPolar LowLow ClassificationClassification SystemSystem 1.1. ReverseReverse ShearShear systemssystems 2.2. TroughTrough systemssystems 3.3. BoundaryBoundary LayerLayer FrontsFronts 4.4. ColdCold LowsLows 5.5. CommaComma CloudsClouds 6.6. BaroclinicBaroclinic--wavewave ForwardForward ShearShear 7.7. OrographicOrographic PolarPolar lowslows 1.1. ReverseReverse ShearShear PolarPolar LowsLows Probably triggered by short- wave, upper trough. Baroclinic instability and deep convection play a part in the developments. 2.2. TroughTrough SystemsSystems These systems often develop in trailing troughs associated with major synoptic scale lows. Again often triggered by short, UA troughs, or jet streaks. The cold air advection enhances convection. 3.3. BoundaryBoundary LayerLayer FrontFront SystemsSystems Boundary Layer fronts are frequently found over the Norwegian and Barents Seas, any many polar lows form on these features. The lows developing in these baroclinic regions are often short-lived. 4.4. ColdCold LowLow TypesTypes Forming deep in the cold air, these lows often have spiral cloud patterns. They often develop within old cold core lows. They often form when the low moved over water. A Cold Core Vortex Polar Low Over the Labrador Sea 5.5. CommaComma cloudsclouds These systems often form at much more southerly latitudes close to the polar front. They can often develop in more baroclinic regions in the cold air to the west of synoptic-scale lows. 6.6. BaroclinicBaroclinic WavesWaves A baroclinic polar low that developed on a strong thermal gradient over the eastern Weddell Sea. 7.7. OrographicOrographic PolarPolar LowsLows A baroclinic polar low that developed on a strong thermal gradient over the eastern Weddell Sea. CISKCISK WISHEWISHE TheThe SeasonalSeasonal DistributionDistribution ofof PolarPolar LowsLows inin thethe NorwegianNorwegian SeaSea PolarPolar lowlow trackstracks overover thethe NorwegianNorwegian andand BarentsBarents SeasSeas 19781978 --8282 TheThe HaroldHarold StudyStudy BasedBased onon AVHRRAVHRR ImageryImagery Two years of imagery were analysed and 4054 vortices identified! This figure shows the number of systems identified by diameter. The tracks of systems in four diameter ranges Larger vortices in the Harold study TheThe CondronCondron andand BiggBigg StudyStudy 1.1. InvestigatedInvestigated mesoscalemesoscale lowslows inin thethe ERAERA--4040 rere--analysisanalysis 2.2. UsedUsed thethe HaroldHarold datadata basebase asas ‘‘truthtruth’’ 3.3. AutomaticallyAutomatically detecteddetected vorticesvortices usingusing PMSLPMSL andand 500500 hPahPa heightheight 4.4. OctOct 19931993 –– SepSep 19951995 Automatic detection of polar lows in ERA-40 Diamonds – reanalysis (9) Circles/cross – imagery (3) TheThe CondronCondron andand BiggBigg StudyStudy -- ResultsResults 1.1. UpUp toto 80%80% ofof cyclonescyclones largerlarger thanthan 500500 kmkm areare detecteddetected inin thethe MSLPMSLP fieldfield 2.2. ThisThis fallsfalls toto 40%40% forfor 250250 kmkm vorticesvortices 3.3. OnlyOnly 20%20% werewere detecteddetected forfor 100100 kmkm scalescale systemssystems The Monthly Number of Vortices Found in ERA-40 Cyclone Density From Imagery From ERA-40 PolarPolar LowsLows inin OtherOther AreasAreas Japan January 1987 Labrador Sea 1977-93 MesoscaleMesoscale LowsLows AroundAround thethe AntarcticAntarctic All vortices in March 1992. The contours indicate MSLP anomalies UseUse ofof thethe UKMOUKMO CycloneCyclone DatabaseDatabase • A database of cyclones objectively identified in the Met Office global model. It covers the period from January 2000 to April 2004. Hewson (2001). • For each cyclone a range of diagnostics and variables are stored. • Why choose the Cyclone Database to study polar lows? • High resolution compared to reanalysis datasets. • Designed to detect many features, including very weak disturbances. Typically 50 at any one time in the North Atlantic / Europe region. Vortices identified in the Vortices identified in the UKMO cyclone data base AVHRR imagery SpatialSpatial distributiondistribution ofof polarpolar lowlow activityactivity •• PolarPolar lowlow countcount densitydensity fromfrom JanJan 20002000 toto AprilApril 20042004 calculatedcalculated forfor areaarea