lec 10: interpreting weather mapsfaculty.fiu.edu/~hajian/met3502/met3502_synoptic_lec10.pdflec 10:...
TRANSCRIPT
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Lec 10:InterpretingWeatherMaps
CaseStudy:October2011Nor’easter
FIUMET3502Synoptic
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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)
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37GHzRingandTCRapidIntensification(RI)
• Daniellewasat40kt intensity;increased33kt inthenext24hours
• RIisdefinedas30kt intensityincreasesduring24hours(KaplanandDeMaria 2003)
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AMid-latitudeCyclone:Snowfalltotals
• Forecastersknewthiswascoming!!!!
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“Top-down”modelanalysis
• 300mb:Jetstream• 500mb:Vorticity• 700mb:VerticalMotion• 850mb:Temperature• Surface:Pressure,Precipitation
• Note:thisexampleonlyconsidersonetimeperiodattheheightofthestorm,theevolutionofthesefeaturesthroughtimeisimportantalso
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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)
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NegativelyTiltedTrough
• Anegativelytiltedtrough:(1)indicatesalowpressurehasreachedmaturity,(2)indicatesstrongdifferentialadvection (middleandupperlevelcoolairadvectingoverlowlevelwarmairadvection).Thisincreasesthermodynamicinstability.(3)Indicatesgoodverticalwindshear.
Adeeplowpressure,anegativelytiltedtrough,andawarmandmoistwarmsectorcombinationeastoftheRockiesoftenproducesasevereweather outbreak.
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4-QuadrantJetModel
• Risingmotion:RightEntranceandLeftExit• Sinkingmotion:LeftEntranceandRightExit• Risingmotionforacurvedjetstream:northofjetaxisifjetisinahighlycurvedflow
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300mb
-Overallflowpattern?Tilted?-Willtroughsamplify?-4-quadrantmodel-Anyobviousareasofdivergence?
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Whattolookforat500mb
• Vorticity:locationandtype– Shear,Curvature,Earth(Coriolis)
• Longwave andshortwavetroughs– Bothassociatedwithpositivevorticity
• PositiveVorticityAdvection(PVAorCVA)– Highareasofvorticitybeingadvected bystrongwinds(i.e.windblowsacrossvorticitycontours)
– Causesverticalmotion(QGOmegaequation)
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500mb
-Locationandtypeofvorticity-Longwave andshortwavetroughs-Positivevorticityadvection(PVA)
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Whattolookforat700mb• VerticalMotion– Shouldmatchupfairlywellwithourdiagnosticsfromtheotherlevels
– Precipitationislikelyinareasofupwardmotion,butmoistureisnecessaryalso
• Locationoffronts(at700mb)– Lookfora“kink”inheightcontoursatthebottomofatrough,usuallyonlyworksforstrongcoldfronts
• Note:Verticalmotionisnotalwayssynopticallyforced– Mountainscancreate“couplets”ofupward/downwardmotion
– Unusuallyhighupwardmotioniscommoninconvection
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700mb
-Areasofrising/sinkingmotion-Locationoffronts-Basedonothermaps,whatiscausingtheverticalmotion?
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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
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850mb
-StrengthofHighsandLows-Warmandcoldairmasses-Locationoffronts-AreasofstrongestWAAandCAA
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Whattolookforatthesurface
• SurfaceHighs,Lows,Fronts• Modelforecastprecipitation– Notnecessarilyaccurate,especiallyformesoscalefeatures!!!
• 1000-500mb thickness:540dmcontouroftenapproximatestherain/snowline
• Regionsofdivergence,convergence,confluence,diffluence– Convergence=upwardverticalmotion
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Surface
-SurfaceHighsandLows-Modelforecastprecipitation-Locationof540thicknesscontour
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SurfaceObs.
-Ifyouinterpretthe300,500,700,and850mbmapsproperly,youalreadyhaveamentalpictureofthismapbeforeyoulookatit-Usetheobservationstodetermineexactlocationsandcomparetomodelinitialization
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Multi-levelanalysis• Isthelowpressuresystemverticallystacked?– Comparetroughlocationat500/300mb to850/700mb
– Developinglow:verticaltilttoNWwithheight– Occludedlow:verticallystacked
• Tiltingoffrontswithheight– Importantfordetermininglocationoffrontalprecip– Usesurface,850,and700mb
• Forecastingthunderstorms/convection– Usesynopticoverviewcombinedwithverticalsounding
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Howdoupper-levelconditionshelpsurfacecyclonedevelop?
• Atperturbation stage,inthecenterof thiscirculation, thereismassconvergence.Whenallthatairhitsthecenter,wehaverisingmotionbecauseithasnowhereelsetogo.Iftheupper levelsarefavorableforcyclonedevelopment, then thereisaregionofdivergencealoftabovethedeveloping Low-pressurecenter.Thiswillhelppull theairthatisconvergingatthesurfaceupwardandcontinue todevelop thesurfacecyclone.Theupperlevelsalsosteerthesystemandmakeitprogresseast.
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Summary:ContributingfactorstoverticalmotionoverNortheast
• Upperleveldivergenceduetoregionbeinglocatedinrightentrance+leftexitregionoftwojetstreaks
• 500mb PositiveVorticityAdvection(PVA)• 850mb WarmAirAdvection(WAA)• SurfacelowpressuresystemcontinuingtodeependuetoNWtiltingcreatingdivergenceoversurfacelow
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Otherfactorstoconsiderforpredictingexactsnowfalltotals:
• WarmSoiltemperature– Maylimitsnowfallaccumulationduetomelting
• Warmoceantemperature– Limitingfactorifflowisonshore
• Risingmotionin“Dendritic GrowthZone”– Snowflakesgrowfastestin-12to-18°Crange
• Ifsnowfallsduringdaytime,thehighsunanglemaymeltsnowoffdarksurfaces
• Note:inaborderlinerain/snowcase,heavysnowfallrateswillcanceloutlimitingfactorsduetohighratesofadiabaticcooling(fromintenserisingmotion)andlatentheatoffusion(fromflakesmeltingastheyfallintowarmerair)
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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.
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Jetstreamanalysis
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Identifyingthejetstreamonwatervaporimagery
• The6.7µmwatervaporchannelissensitivetomoistureintheupperatmosphere(above500mb),whichmakesitespeciallyusefulinidentifyingthejetstream
• Jetstreamsareoftenfound:–Wherewatervaporfeaturesaremovingrapidly(streakyfeatures)
– Nearmoisturegradients– Inthevicinityofdevelopingfrontsorcyclones
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Whereisthejetstream?
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Commontypesofjetstreampatterns
• ZonalFlow• Meridional Flow• SplitFlow• CutoffLow• BlockingHigh• OmegaBlock• RexBlock• PineappleExpress
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ZonalFlow
• Dominantwesttoeastflowpattern
• Lowpressuresystemsaregenerallyweakandmovequickly
• Weatherisgenerallyquietwithfewextremes
300mb
850mb
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SplitFlow• Similartozonalflowexceptwithdistinctlyseparatepolarandsubtropicaljets
• Quietweatherbetweenjets
• Fastmoving,activeweatherelsewhere
300mb
850mb
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Meridional Flow• Dominantnorth-southflowpattern
• Lowpressuresystemsarestrongerandmoveslowerthanzonalflow
• Associatedwithtemperatureextremes
300mb
850mb
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Cutofflow
• Startsasanormaltrough,persistswhentroughcontinuesto“dig”whilerestofjetstreamresumesridgingorzonalflow
• Associatedwithpersistentcold,cloudyweather
300mb
850mb
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BlockingHigh• Formsfromaridgewhenwarmadvectionwrapsaroundtothepoleward andwestsidesofthehigh
• AssociatedwithsummerheatwavesineasternUS
• Example:TheBermudaHighisasemi-permanentblockinghigh
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OmegaBlock
• Jetstreamtakesshapeofomega:Ω(low-high-low)
• Largestandleastcommonblockingpattern,usuallyseeninsummer
• Associatedwithmostpersistent/extremedroughts
300mb
850mb
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RexBlock• Blockinghighontopofcutofflow
• Associatedwithpersistentridging(dry/warmweather)overmostofUS
5/28/2009
4½dayslater…
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Dissipatingablockingpattern
• Deformationzoneupstream(west)ofblockisthekeytomaintainingtheblockingfeature– Flowtravelsaroundblock,eithertothenorthorsouth
– Deformationzoneappearsasastrongmoisturegradientinwatervaporimagery
• Numericalmodelstendtobreakdownblockingpatternstooquickly
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PineappleExpress• StrongjetstreamfromHawaiitoPacificcoastcreatespersistentmoistflowtoUSPacificcoast
• Associatedwithveryheavyrainfall
• MorecommoninElNinoyears
300mb
IRSatellite