Part I Numerical Simulation and General Evolution

AHigh-ResolutionModelingStudyofthe24May2002DrylineCaseduringIHOP.PartI:NumericalSimulationandGeneralEvolutionoftheDrylineandConvectionMINGXUESchoolofMeteorology,andCenterforAnalysisandPredictionofStorms,UniversityofOklahoma,Norman,OklahomaWILLIAMJ.MARTINCenterforAnalysisandPredictionofStorms,UniversityofOklahoma,Norman,Oklahoma(Manuscriptreceived8November2004,infinalform21June2005)ABSTRACTResultsfromahigh-resolutionnumericalsimulationofthe24May2002drylineconvectiveinitiation(CI)casearepresented.Thesimulationusesa400km
700kmdomainwitha1-kmhorizontalresolutiongridnestedinsidea3-kmdomainandstartsfromanassimilatedinitialconditionat1800UTC.Routineaswellasspecialupper-airandsurfaceobservationscollectedduringtheInternationalH2OProject(IHOP_2002)areassimilatedintotheinitialcondition.Theinitiationofconvectivestormsataround2015UTCalongasectionofthedrylinesouthoftheTexaspanhandleiscorrectlypredicted,asisthenoninitiationofcon-vectionatacold-front–drylineintersection(triplepoint)locatedfarthernorth.ThetimingandlocationofpredictedCIareaccuratetowithin20minand25km,respectively.Thegeneralevolutionofthepredictedconvectivelineupto6hofmodeltimealsoverifieswell.Mesoscaleconvergenceassociatedwiththeconfluentflowaroundthedrylineisshowntoproduceanupwardmoisturebulge,whilesurfaceheatingandboundarylayermixingareresponsibleforthegeneraldeepeningoftheboundarylayer.Theseprocessesproducefavorableconditionsforconvectionbuttheactualtriggeringofdeepmoistconvectionatspecificlocationsalongthedrylinedependsonlocalizedforcing.Interactionoftheprimarydrylineconvergenceboundarywithhorizontalconvectiverollsonitswestsideprovidessuchlocalizedforcing,whileconvectiveeddiesontheimmediateeastsidearesuppressedbyadownwardmesoscaledrylinecirculation.Acompanionpaperanalyzesindetailtheexactprocessesofconvectiveinitiationalongthisdryline.1.IntroductionThedryline,definedasthenarrowzoneofstronghorizontalmoisturegradientatandnearthesurface,isfrequentlyobservedinthewesternGreatPlainsoftheUnitedStates.Inthisregion,thelineisusuallyabound-arybetweenwarm,moistairfromtheGulfofMexicoandhot,drycontinentalairfromthesemiaridsouth-westernstatesortheMexicanplateau.Thedrylineisoftenthefocusofconvectioninitiation(CI).Rhea(1966)foundthatconvectiondevelopedwithin200nmiofthedryline70%ofthetime.Surfacewindconver-gencecommonlyassociatedwithdrylinesisbelievedtobeanimportantreasonforfrequentCIalongtheline.Anotherreasonisthatthedrylinerepresentsthewest-mostboundaryofthemoistairfromtheGulf,anditiswhereconditionsfirstbecomefavorableforupper-leveldisturbancespropagatingoutoftheRockiestotriggerconvection.Inspiteofanumberofexistingstudies(e.g.,BluesteinandParker1993;ZieglerandHane1993;Ziegleretal.1995;Shawetal.1997;Atkinsetal.1998;ZieglerandRasmussen1998;Haneetal.2002;Peckhametal.2004),theexactprocessesbywhichcon-vectionisinitiatedarestillnotwellunderstood(Haneetal.1993).Theexacttimingandlocationthatconvec-tionisinitiatedalongdrylinesareevenhardertopre-dict.Duringthe2002InternationalH2OProject(IHOP_2002;Weckwerthetal.2004),adrylineformedon24Mayandintenseconvectionwasinitiatedalongthelineintheafternoon.TheeventwasintensivelyCorrespondingauthoraddress:Dr.MingXue,SchoolofMe-teorology,UniversityofOklahoma,100E.Boyd,Norman,OK73019.E-mail:[e-mail]2006XUEANDMARTIN149©2006AmericanMeteorologicalSocietyMWR3071observedduringthefieldexperimentforthepurposeofstudyingCIandthepossibleinteractionofthedrylinewithanintersectingcoldfront(seeahighlightofthecaseinWeckwerthetal.2004).Arichsetofspecialobservationswascollectedduringthefieldexperimentandadditionaldataweregatheredfromvariousnet-worksofsurfacestations[seeWeckwerthetal.(2004)forlistsandmapsofIHOPspecificinstrumentsandtheirdeploymentduringIHOP_2002].Thespecialob-servationalplatformsincludeseveralresearchaircraftswithairborneDopplerradars,lidars,dropsondesandotherinstruments,theS-banddual-polarizationDopp-lerradar(S-Pol),twoDoppler-on-Wheels(DOW),SharedMobileAtmosphericResearchandTeachingRadar(SMART-R),andmobileX-bandpolarimetricradar(X-Pol)radars,mobilesurfaceandupper-airsoundingsystems,andMobileIntegratedProfilingSys-tem(MIPS).Inaddition,theprimaryregionsofCIfallwithintherangeofseveralWeatherSurveillanceRa-dar-1988Dopplerunits(WSR-88Ds),inparticular,thoseofAmarillo(KAMA)andLubbock(KLBB),Texas,andFrederick(KFDR),Oklahoma(seeFig.1foramapofthevicinity).Atthesurface,theOklahomaMesonetandWestTexasMesonetprovideadditionalobservationsnearandaroundtheCIregionsalongthedryline.ThedetailedobservationaldataprovideuswithanunprecedentedopportunitytostudytheCIpro-cessesfrombothobservationalandmodelingperspec-tives.Infact,therearecurrentlyseveralstudiesonthiscase,eachwithasomewhatdifferentfocus(e.g.,Geertsetal.2006;Holtetal.2006;Wakimotoetal.2006).Inthisstudy,anonhydrostaticmesoscalemodelisusedtosimulatetheevolutionofthedrylineandtheintersectingcoldfrontaswellastheinitiationandde-velopmentofconvectivestormsalongandnearthedrylineandcoldfront.Anested700km
400kmmodeldomainat1-kmhorizontalresolutionisused,whichislargeenoughtocovertheentiredrylineandtheportio