Lec 10:InterpretingWeatherMaps

CaseStudy:October2011Nor’easter

FIUMET3502Synoptic

HurricaneForecasts• Genesis:onlargescaleweathermapsorsatelliteimages,lookfortropicalwaves(Africaeasterlywaves)

• Tropicalstormorhurricanedevelopment:– Environmentalconditions(CIMSSTCwebsite):

• HighSST:>=26degreeC• Lowverticalwindshear:=50mm,SAL

– Internalconditions:• OrganizedconvectiononIRimage;deepconvection(85GHz)nearcenter;ringformedin37GHzcolorimageindicatingRI(NRLTCwebsite)

• Upperlevel(200mbor300mb)outflow:anti-cyclonic(CIMSSTCwebsite)

37GHzRingandTCRapidIntensification(RI)

• Daniellewasat40kt intensity;increased33kt inthenext24hours

• RIisdefinedas30kt intensityincreasesduring24hours(KaplanandDeMaria 2003)

AMid-latitudeCyclone:Snowfalltotals

• Forecastersknewthiswascoming!!!!

“Top-down”modelanalysis

• 300mb:Jetstream• 500mb:Vorticity• 700mb:VerticalMotion• 850mb:Temperature• Surface:Pressure,Precipitation

• Note:thisexampleonlyconsidersonetimeperiodattheheightofthestorm,theevolutionofthesefeaturesthroughtimeisimportantalso

Whattolookforat300mb• Jetstreampattern(zonal,splitflow,blocking)– Note:Use200mb forsubtropicaljet,especiallyinsummer

• Ridgesandtroughs:locationandtilting(+or-)• Locationofupperlevelhighsandlows• Jetstreaks

(a) Troughwillamplify(deepen)ifjetstreakisontheleftsideofthetroughaxis

(b) 4-quadrantmodelforrising/sinkingmotion• Upper-leveldivergence=risingmotion(especiallyifthereislow-levelWAAalso)

NegativelyTiltedTrough

• Anegativelytiltedtrough:(1)indicatesalowpressurehasreachedmaturity,(2)indicatesstrongdifferentialadvection (middleandupperlevelcoolairadvectingoverlowlevelwarmairadvection).Thisincreasesthermodynamicinstability.(3)Indicatesgoodverticalwindshear.

Adeeplowpressure,anegativelytiltedtrough,andawarmandmoistwarmsectorcombinationeastoftheRockiesoftenproducesasevereweather outbreak.

4-QuadrantJetModel

• Risingmotion:RightEntranceandLeftExit• Sinkingmotion:LeftEntranceandRightExit• Risingmotionforacurvedjetstream:northofjetaxisifjetisinahighlycurvedflow

300mb

-Overallflowpattern?Tilted?-Willtroughsamplify?-4-quadrantmodel-Anyobviousareasofdivergence?

Whattolookforat500mb

• Vorticity:locationandtype– Shear,Curvature,Earth(Coriolis)

• Longwave andshortwavetroughs– Bothassociatedwithpositivevorticity

• PositiveVorticityAdvection(PVAorCVA)– Highareasofvorticitybeingadvected bystrongwinds(i.e.windblowsacrossvorticitycontours)

– Causesverticalmotion(QGOmegaequation)

500mb

-Locationandtypeofvorticity-Longwave andshortwavetroughs-Positivevorticityadvection(PVA)

Whattolookforat700mb• VerticalMotion– Shouldmatchupfairlywellwithourdiagnosticsfromtheotherlevels

– Precipitationislikelyinareasofupwardmotion,butmoistureisnecessaryalso

• Locationoffronts(at700mb)– Lookfora“kink”inheightcontoursatthebottomofatrough,usuallyonlyworksforstrongcoldfronts

• Note:Verticalmotionisnotalwayssynopticallyforced– Mountainscancreate“couplets”ofupward/downwardmotion

– Unusuallyhighupwardmotioniscommoninconvection

700mb

-Areasofrising/sinkingmotion-Locationoffronts-Basedonothermaps,whatiscausingtheverticalmotion?

Whattolookforat850mb• Warm/coldairmasses:locationandstrength• High/lowpressuresystems:locationandstrength

– Lookformagnitudeofheightgradient• Strongthermalgradient=possiblefront• Temperatureadvection(associatedwithfrontsand

cyclones):– WAA=windblowingfromwarmtocold

• WAA=risingmotion(2nd terminQGomegaequation)– CAA=windblowingfromcoldtowarm

• CAA=sinkingmotion• Temperatureadvectionisstrongestwith:

1. Closelyspacedisotherms2. Closelyspacedheightcontours(strongerwinds)3. Isothermsnearlyperpendiculartoheightcontours

• Note:850mb isessentiallyasurfacemapathighelevations

850mb

-StrengthofHighsandLows-Warmandcoldairmasses-Locationoffronts-AreasofstrongestWAAandCAA

Whattolookforatthesurface

• SurfaceHighs,Lows,Fronts• Modelforecastprecipitation– Notnecessarilyaccurate,especiallyformesoscalefeatures!!!

• 1000-500mb thickness:540dmcontouroftenapproximatestherain/snowline

• Regionsofdivergence,convergence,confluence,diffluence– Convergence=upwardverticalmotion

Surface

-SurfaceHighsandLows-Modelforecastprecipitation-Locationof540thicknesscontour

SurfaceObs.

-Ifyouinterpretthe300,500,700,and850mbmapsproperly,youalreadyhaveamentalpictureofthismapbeforeyoulookatit-Usetheobservationstodetermineexactlocationsandcomparetomodelinitialization

Multi-levelanalysis• Isthelowpressuresystemverticallystacked?– Comparetroughlocationat500/300mb to850/700mb

– Developinglow:verticaltilttoNWwithheight– Occludedlow:verticallystacked

• Tiltingoffrontswithheight– Importantfordetermininglocationoffrontalprecip– Usesurface,850,and700mb

• Forecastingthunderstorms/convection– Usesynopticoverviewcombinedwithverticalsounding

Howdoupper-levelconditionshelpsurfacecyclonedevelop?

• Atperturbation stage,inthecenterof thiscirculation, thereismassconvergence.Whenallthatairhitsthecenter,wehaverisingmotionbecauseithasnowhereelsetogo.Iftheupper levelsarefavorableforcyclonedevelopment, then thereisaregionofdivergencealoftabovethedeveloping Low-pressurecenter.Thiswillhelppull theairthatisconvergingatthesurfaceupwardandcontinue todevelop thesurfacecyclone.Theupperlevelsalsosteerthesystemandmakeitprogresseast.

Summary:ContributingfactorstoverticalmotionoverNortheast

• Upperleveldivergenceduetoregionbeinglocatedinrightentrance+leftexitregionoftwojetstreaks

• 500mb PositiveVorticityAdvection(PVA)• 850mb WarmAirAdvection(WAA)• SurfacelowpressuresystemcontinuingtodeependuetoNWtiltingcreatingdivergenceoversurfacelow

Otherfactorstoconsiderforpredictingexactsnowfalltotals:

• WarmSoiltemperature– Maylimitsnowfallaccumulationduetomelting

• Warmoceantemperature– Limitingfactorifflowisonshore

• Risingmotionin“Dendritic GrowthZone”– Snowflakesgrowfastestin-12to-18°Crange

• Ifsnowfallsduringdaytime,thehighsunanglemaymeltsnowoffdarksurfaces

• Note:inaborderlinerain/snowcase,heavysnowfallrateswillcanceloutlimitingfactorsduetohighratesofadiabaticcooling(fromintenserisingmotion)andlatentheatoffusion(fromflakesmeltingastheyfallintowarmerair)

Sources/Notes• Informationoninterpretingweathermapscanbefoundat:

http://www.theweatherprediction.com--readingassignment:

http://www.theweatherprediction.com/charts/

• MathematicalderivationsandinterpretationsareinMartin(2006)

• Weathermapslikethiscanbefoundat:• http://aos.wisc.edu/weather/wx_models/Models.htm or

http://sol.aos.wisc.edu• Disclaimer:Allverticalmotiondiagnosticsarefilledwith

assumptions.Theomegatermscanalsocanceleachotheroutinsomecases.Itisusuallyeasytodiagnoseastrongsystem,itistheweakercasesthataremorechallenging.

Jetstreamanalysis

Identifyingthejetstreamonwatervaporimagery

• The6.7µmwatervaporchannelissensitivetomoistureintheupperatmosphere(above500mb),whichmakesitespeciallyusefulinidentifyingthejetstream

• Jetstreamsareoftenfound:–Wherewatervaporfeaturesaremovingrapidly(streakyfeatures)

– Nearmoisturegradients– Inthevicinityofdevelopingfrontsorcyclones

Whereisthejetstream?

Commontypesofjetstreampatterns

• ZonalFlow• Meridional Flow• SplitFlow• CutoffLow• BlockingHigh• OmegaBlock• RexBlock• PineappleExpress

ZonalFlow

• Dominantwesttoeastflowpattern

• Lowpressuresystemsaregenerallyweakandmovequickly

• Weatherisgenerallyquietwithfewextremes

300mb

850mb

SplitFlow• Similartozonalflowexceptwithdistinctlyseparatepolarandsubtropicaljets

• Quietweatherbetweenjets

• Fastmoving,activeweatherelsewhere

300mb

850mb

Meridional Flow• Dominantnorth-southflowpattern

• Lowpressuresystemsarestrongerandmoveslowerthanzonalflow

• Associatedwithtemperatureextremes

300mb

850mb

Cutofflow

• Startsasanormaltrough,persistswhentroughcontinuesto“dig”whilerestofjetstreamresumesridgingorzonalflow

• Associatedwithpersistentcold,cloudyweather

300mb

850mb

BlockingHigh• Formsfromaridgewhenwarmadvectionwrapsaroundtothepoleward andwestsidesofthehigh

• AssociatedwithsummerheatwavesineasternUS

• Example:TheBermudaHighisasemi-permanentblockinghigh

OmegaBlock

• Jetstreamtakesshapeofomega:Ω(low-high-low)

• Largestandleastcommonblockingpattern,usuallyseeninsummer

• Associatedwithmostpersistent/extremedroughts

300mb

850mb

RexBlock• Blockinghighontopofcutofflow

• Associatedwithpersistentridging(dry/warmweather)overmostofUS

5/28/2009

4½dayslater…

Dissipatingablockingpattern

• Deformationzoneupstream(west)ofblockisthekeytomaintainingtheblockingfeature– Flowtravelsaroundblock,eithertothenorthorsouth

– Deformationzoneappearsasastrongmoisturegradientinwatervaporimagery

• Numericalmodelstendtobreakdownblockingpatternstooquickly

PineappleExpress• StrongjetstreamfromHawaiitoPacificcoastcreatespersistentmoistflowtoUSPacificcoast

• Associatedwithveryheavyrainfall

• MorecommoninElNinoyears

300mb

IRSatellite