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TheProcessofOcclusion
JimSteenburghUniversityofUtah
SupplementalReading:SchultzandMass(1993),SchultzandVaughan(2011)
ObjectionstotheNCMOcclusion• Structuresresemblinganocclusioncanbepresent
withoutapre-existingwarmsector
• Theso-called“occludedfront” isoftendrawnfromthepeakofthewarmsectortothelowcenterwithoutevidencethattheprocessofocclusionoccurred
• “Theocclusionisaddedasanaccretion”
• Thereisoftenlittleornotemperaturegradientacrossanocclusionatthesurface
• Thelargestgradientsareoftenaloft
Source:SchultzandMass(1993)
ObjectionstotheNCMOcclusion• Therearefewifanywelldocumentedcasesofcoldfrontsovertakingwarmfrontstoformocclusions
• Occludedfrontsoftenappeartoformwhenthelowcenterseparatesfromthepeakofthewarmsectoranddeepensbackintothecoldair
• Theoccludedfrontisessentiallyanewfront
• Satelliteimagerysuggeststhatoccluded-likestructuresforminnon-classicalways
Source:SchultzandMass(1993)
TheInstantOcclusion
• Occurswhenacommacloud/PVAaloftapproachesandmergeswithafrontalwave
• Structurethatformslookslikeanocclusion,butwithoutthehistoryofcatchup
Source:Reed(1979),SchultzandMass(1993),Djuric (1994)
OutstandingQuestions• Docycloneseveroccludeinaclassicalmannerinwhichthecoldfrontcatchesuptothewarmfront?
• A:Yes!
• Canoccludestructuresformfromnon-classicalmechanisms
• A:Yes!
• Mighttherebeabetterwaytoconceptualizetheocclusionprocess
• A:Yes!
EvolutionofIdealOcclusion
L LL
LL
Pre-existingwarm&coldfronts
Warmsectornarrowsascoldfrontovertakeswarmfront
Occlusionlengthensaswarm&coldfronts“zipper”
Source:SchultzandMass(1993)
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VerticalStructure
• HR18– Distinctwarmandcoldfronts
– Interveningwarmsector– Cold&upperlevelfrontssomewhatdistinct
• HR24– Warmsectornarrows– Cold&upper-levelfrontsmerge
Source:SchultzandMass(1993)
VerticalStructure
• HR30– Cold&warmfrontsmeet,formingwarm-typeocclusion
• HR33– Upper-levelcoldfrontcontinuestomovedownstream,formingelevatedcoldfrontoftheocclusion
Source:SchultzandMass(1993)
VerticalStructure
• Keypoints– Surface-basedcoldfrontcatcheswarmfrontinclassicalmanner
– Coldfrontdoesnot“rideup” overwarmfront
– Upper-levelfrontalzoneprovideselevatedcold-frontoftheocclusion
Source:SchultzandMass(1993)
Airflow• Sfc trajectoriesshowconfluenceofairacross
occlusion
• Airwithinwarmtonguealoftoriginatesinwarmsector(40,42,47,56)
• WarmsectorairremovedfromSFC
Source:SchultzandMass(1993)
Non-ClassicalOcclusion
• Deeplyoccludedsystemwithwarm-coreseclusion• Frontformsfrom“occludogenesis” ratherthancatch
up• Windfieldsgeneratethewarmtongue&seclusion
Source:Kuo etal.(1992)
Non-ClassicalOcclusion
• Confluenceoftrajectoriesalongocclusion• “Crossing” ofpost-occlusiontrajectories
– #5startstosouthinwarmerairthan13.#5endsinseclusion.• Seclusionformsfromthe“wrapping” ofcoldairoriginating
inwarm/occludedfrontalzoneSource:Kuo etal.(1992)
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ANewPerspective
Comparing the 90-yr-old Norwegian cyclone model to recent research results demonstrates that descriptions of the occlusion process in textbooks need to be rewritten.
T he Norwegian cyclone model is the foundation of observational synoptic meteorology. In the early twentieth century, the scientists at the Geophysical
Institute in Bergen, Norway, drew upon previous re-search and a mesoscale observing network to create a conceptual model for the structure and evolution of extratropical cyclones and their attendant fronts (e.g., Bjerknes 1919; Bjerknes and Solberg 1921, 1922). Even 90 yr hence, the success of the Norwegian cyclone
model is a testament to its ingenuity, simplicity, and basic accuracy. Elements of the Norwegian cyclone model continue to be presented in meteorological textbooks, whether for meteorologists (e.g., Palmén and Newton 1969; Wallace and Hobbs 1977, 2006; Carlson 1991; Bluestein 1993; Gordon et al. 1998; Barry and Carleton 2001; Martin 2006), introductory classes (e.g., Gedzelman 1980; Moran and Morgan 1997; Lutgens and Tarbuck 2001; Aguado and Burt 2001; Ackerman and Knox 2007; Ahrens 2008; Barry and Chorley 2010), or the general public (e.g., Kimble 1951; Williams 1997, 2009; Lehr et al. 2001). Figure 1 provides a typical example of how the Norwegian cyclone model is illustrated in one of the most recent books (Williams 2009).
Despite the durability and longevity of conceptual models such as the Norwegian cyclone model, their applicability needs to be continually reassessed. The body of conventional wisdom, or common knowledge shared among the members of the scientific commu-nity, is called a paradigm (Kuhn 1970). Weaknesses, inconsistencies, contradictions of existing theory, and observations that do not fit the paradigm [called anomalies by Kuhn (1970)] are occasionally revealed. As a growing number of such anomalies accumulate, eventually a new conceptual model that explains the anomalies arises. This new conceptual model replaces the old, and a paradigm shift occurs. Science advances.
OCCLUDED FRONTS AND THE OCCLUSION PROCESSA Fresh Look at Conventional Wisdom
BY DAVID M. SCHULTZ AND GERAINT VAUGHAN
AFFILIATIONS: SCHULTZ—Division of Atmospheric Sciences, Department of Physics, University of Helsinki, and Finnish Meteorological Institute, Helsinki, Finland, and Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom; VAUGHAN—National Centre for Atmospheric Science, University of Manchester, Manchester, United KingdomCORRESPONDING AUTHOR: Dr. David M. Schultz, Centre for Atmospheric Science, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Simon Building, Oxford Road, Manchester M13 9PL United KingdomE-mail: [email protected]
The abstract for this article can be found in this issue, following the table of contents.DOI:10.1175/2010BAMS3057.1
A supplement to this article is available online (10.1175/2010BAMS3057.2)
In final form 13 November 2010©2011 American Meteorological Society
443APRIL 2011AMERICAN METEOROLOGICAL SOCIETY |
ConventionalWisdom• Theoccludedfrontformsandlengthensasthecoldfront
overtakesthewarmfront
• Theocclusionmaybewarmorcoldtypedependingonthetemperaturesbehindthecoldfrontandaheadofthewarmfront
• Theformationofanocclusionsignifiesanendtothecyclonedeepeningphase
• Theoccludedfrontfeaturesthepre-frontalweatherofawarmfront(widespreadcloudsandprecipitaiton)andthepost-frontalweatherofacoldfront(clearskiesanddrying)
Source:SchultzandVaughan(2011)
Catch-UpProcess
• Question– Isthecatch-upprocessanexplanationofocclusionformationoraconsequenceoftheunderlyingphysicalprocess?
• AlternativeAnswer– Lengtheningandnarrowingofwarmtongue,andcatch-upofcoldandwarmfrontsbetterexplainedbydifferentialdeformationandrotationaroundlowcenter
Source:SchultzandVaughan(2011)
DeformationandRotation
Advantage:Explains“occludogenesis”,strengtheningofinstantocclusion,lengtheningofocclusionaslowdeepensintocoldair,catchup process,etc.
Source:Schultzetal.(1998),SchultzandVaughan(2011)
Reconcilingconceptualmodels• Keyphysicalprocessoperatingintraditionalocclusion
andShapiro-Keysermodelis“wrap-up”ofthethermalwavebydifferentialrotationanddeformation
• IntheNCM,thenarrowingofthewarmsectorand“catch-up”ofthecoldfronttothewarmfrontisaconsequenceofthisprocess
• IntheS-Kmodel,theseparationofthelowcenterfromthewarmsector,developmentoftheinterveningwarmfront,andformationofbent-backfrontareaconsequenceofthisprocess
Source:SchultzandVaughan(2011)
AlternativeS-KModel
Source:SchultzandVaughan(2011)
Eliminatespeskyuseof“warmfront”and”bent-backwarmfront”
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WarmandColdTypeOcclusion• Temperature“rule”forocclusionformationdoesn’tworkbecausefrontsarenotzeroorderdiscontinuitiesintemperature
• Staticstabilityisabetterdiscriminatorforocclusiontype(lowstaticstabilitygoesoverhigh)– Explainswhywarm-typeismorecommon
• Coldfrontisusuallylessstable– Suggeststhatcold-typeocclusionsmaybemorecommoninareaswherewarmfrontstendtobeweak(e.g.,California)
Source:Stoelinga etal.(2002),SchultzandVaughan(2011)
OcclusionEndsCycloneDeepening
• NorwegianModel– Occlusionindicatesendofdeepeningphasebecausecyclonenolongerhasaccesstopotentialenergystoredinwarmsector• “Aftertheocclusion,thecyclonesoonbeginstofillup”(Bjerknes andSolberg1922)
• Reality– Cyclonesoftendeepenafterocclusionformation– Cyclogenesis betterviewedfromQGorPVperspective
Source:SchultzandVaughan(2011)
OcclusionWeatherandPrecip
• NorwegianModel/CW– Occlusionsareassociatedwithwidespreadclouds/precip followedbyclearingaftersurfacefrontalpassage
• Reality– Occlusionsareassociatedwithavarietyofcloud/precip patterns,includingdryslotsandbandedprecipitation
Source:SchultzandVaughan(2011)
OcclusionWeatherandPrecip
Source:SchultzandVaughan(2011)
TheDryAirstreamGradientsin“weather”notnecessarilycollocatedwithsurfacefront
Don’tusebackedgeofcirrostratustolocatesurfacefront!
OcclusionWeatherandPrecip
Source:MassandSchultz(1993),SchultzandVaughan(2011)
Summary• OcclusionsformasdescribedbyNorwegianCycloneModel(i.e.,Catchup)
• Thealsoformthroughnon-classicalprocesses
• Formationandlengtheningofwarmtongueandcatch-upprocessbetterexplainedbythewrap-upofthebaroclinic zonebydifferentialdeformationandrotation
• Paradigmcanalsoexplain“occludogenesis”andS-Kmodel
• Moveintothe21st century– Warm/coldtypedeterminedbystaticstability– ThinkQG/PVtounderstandcyclonedevelopment– Recognizecloud/precip patternswithocclusionsarevaried
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ClassActivity• UsetheReal-Time-Wx ->Analyses->Global-10daybundleandfindaNCMandaS-Kmodelcyclone
• Evaluatetheapplicabilityofdifferentialrotationanddeformationasaparadigmforexplainingthethermalwavewrapup ineachcyclone
• AnalyzetheS-Kcycloneusingthealternativemodelpresentedherein
