Processes Influencing Rain-Field Growth and Decay after Tropical Cyclone Landfall in the United States

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Processes Influencing Rain-Field Growth and Decay after Tropical Cyclone Landfall in the United States
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Matyas, Corene
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This study measured rain-field sizes for tropical cyclones (TCs) after U.S. landfall and related changes in size to the diurnal cycle and extratropical transition (ET). For 45 TC landfalls, the spatial properties of the rain fields were calculated through an analysis of radar reflectivity returns within a geographic information system. Variables representing the conditions of the atmosphere and storm attributes were examined at three times and as changes over two time periods to account for lags between condition onset and change in raining-area sizes. Mann–Whitney U tests illustrated which of these variables had significantly different median values when the total raining area or high-reflectivity regions increased or decreased in areal extent over two 12-h periods after landfall. Results indicate that the diurnal cycle influenced changes in rain-field size. Rain-field growth occurred during the late morning and early afternoon, which is between the times for peak areal extent of oceanic- and land-based precipitation in the tropics. The rain fields of TCs completing an ET within 74 h of landfall increased in areal extent during the first 12 h after landfall and decayed during the second 12-h period as they neared the completion of ET. The availability of moisture, which was not related to either the diurnal cycle or processes associated with ET, was also important to rain-field growth or decay. In addition, it was discovered that, for the United States, landfall times have shifted from a peak before midnight during 1950–96 to after midnight during 1995–2008.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Corene Matyas.
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ProcessesInuencingRain-FieldGrowthandDecayafterTropicalCyclone LandfallintheUnitedStatesCORENEJ.MATYASDepartmentofGeography,UniversityofFlorida,Gainesville,Florida (Manuscriptreceived21June2012,innalform13December2012) ABSTRACT Thisstudymeasuredrain-eldsizesfortropicalcyclones(TCs)afterU.S.landfallandrelatedchangesin sizetothediurnalcycleandextratropicaltransition(ET).For45TClandfalls,thespatialpropertiesoftherain eldswerecalculatedthroughananalysisofradarreectivityreturnswithinageographicinformationsystem. Variablesrepresentingtheconditionsoftheatmosphereandstormattributeswereexaminedatthreetimes andaschangesovertwotimeperiodstoaccountforlagsbetweenconditiononsetandchangeinraining-area sizes.MannWhitney U testsillustratedwhichofthesevariableshadsignicantlydifferentmedianvalues whenthetotalrainingareaorhigh-reectivityregionsincreasedordecreasedinarealextentovertwo12-h periodsafterlandfall.Resultsindicatethatthediurnalcycleinuencedchangesinrain-eldsize.Rain-eld growthoccurredduringthelatemorningandearlyafternoon,whichisbetweenthetimesforpeakarealextent ofoceanic-andland-basedprecipitationinthetropics.TheraineldsofTCscompletinganETwithin74hof landfallincreasedinarealextentduringtherst12hafterlandfallanddecayedduringthesecond12-hperiod astheynearedthecompletionofET.Theavailabilityofmoisture,whichwasnotrelatedtoeitherthediurnal cycleorprocessesassociatedwithET,wasalsoimportanttorain-eldgrowthordecay.Inaddition,itwas discoveredthat,fortheUnitedStates,landfalltimeshaveshiftedfromapeakbeforemidnightduring195096 toaftermidnightduring19952008.1.IntroductionTherainfallthattropicalcyclones(TCs)produceas theymoveoverlandcanbebenecialtoalleviatedrought conditions(Maxwelletal.2012).Extensivefreshwater oodingcanalsooccur,however,especiallywhenthe stormisslowmoving(Konradetal.2002),theunderlying terrainissloped(Haggardetal.1973),and/orprevious rainshavesaturatedtheground(Sturdevant-Reesetal. 2001).Morethanone-halfofthedeathsintheUnited StatesthatarerelatedtoTCsarearesultoffreshwater ooding(Rappaport2000;Czajkowskietal.2011).When TCraineldsincreaseinsizeafterlandfall,morelocationsreceiverainfallandtheoveralldurationofrainfallis longerforagivenlocation.Bothconvectiveprecipitation andstratiformprecipitationoccurwithinTCs(Jorgensen 1984;YokoyamaandTakayabu2008),and,whetherrainfallismoderateorheavy,alongerdurationofrainfall increasesthechancesthat oodingandassociateddamage anddeathscanoccur.Abetterunderstandingofthe physicalprocessesthatareassociatedwithTCrain-eld growthanddecayasthesestormsmoveoverlandis neededtoimproverainfallforecasts. Inthetropicsandmidlatitudesduringthewarmseason, thegrowthanddecayofconvectivecloudsisstronglyassociatedwiththediurnalcycl e.Oceanicconvectionpeaks inthemorningbetween0600and1000LST,andresearchershavepostulatedthatdaynightdifferencesin radiativecoolingthatalterenvironmentallapserates withinandoutsidethestormenvironmentmayexplain thetimingofthispeak(GrayandJacobson1977;Yang andSmith2006;KikuchiandWang2008).AsTCsform overtheocean,theirdiurnalcyclesofconvectionexhibit amorningpeakcoincidentwithotheroceanicconvection asobservedbyresearchersexaminingcloud-toptemperaturesfromsatelliteobservations(Browneretal. 1977;Muramatsu1983;LajoieandButterworth1984; Sterankaetal.1984;Kossin2002).Itfollowsthatthearea coveredbyhighrainfallrateswouldbelargeraftermidnightthanafternoon.Rainfallobservationsfromislands inthetropicaloceansaswellassatellite-basedrainfall Correspondingauthoraddress: CoreneJ.Matyas,3141Turlington Hall,Dept.ofGeography,UniversityofFlorida,Gainesville, FL32611. E-mail:matyas@u.edu MAY2013MATYAS1085DOI:10.1175/JAMC-D-12-0153.1 2013AmericanMeteorologicalSociety

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estimationssupportthisexpectation(Frank1977;Lajoie andButterworth1984;Jiangetal.2011). Thetimingofpeakrainfallactivityoverlandmasses differsfromthatovertheocean.Diurnalchangesinair temperaturearegreateroverthelandsurfacethanover theoceanassensibleheatuxplaysalargerrolerelative tolatentheatuxintheenergybudget.Land-based convectionpeaksintheafternoonandearlyevening between1600and1900LSTwhenupwardsensibleheat uxismaximizedsothatthelowertroposphereis destabilized(Wallace1975;Dai2001;YangandSmith 2006;Daietal.2007).Oncefullyoverthelandsurface, TCraineldsmayalsobelargerintheafternoonrelativetothemorning.Yet,duringlandfallTCsmayspend severalhoursnearthecoastwherethetimingofpeak rainfallactivitycandifferfromthatoverlandorocean (YangandSmith2006).Muramatsu(1983)showedthat, fortwotyphoonsmovingwithin250kmofOkinawa,an afternoonpeakinconvectivecloudsdevelopedaround 1500LST,whichisslightlyearlierthanthepeakidentiedforland-basedconvectionbutmuchlaterthanthat foroceanicconvection.Alargersampleoflandfalling TCsmustbeanalyzedtobetterdeterminehowthediurnalcycleinuencesrain-eldgrowthanddecayfor TCsoverland. Processesrelatedtoextratropicaltransition(ET)also affectrain-eldgrowthordecay.Duringrecurvature, interactionwithmidlatitudetroughscanrestructurea TCintoanextratropicalcyclone,causingboththewind andraineldstoexpand(Rodgersetal.1991;Jones etal.2003).HartandEvans(2001)foundthatnearly one-halfofAtlanticOceanbasinTCscompleteanET andthatmostmajorcitiesnorthof35 8 Nintheeastern UnitedStateshavereceivedrainfallfromoneormoreof thesesystems.Thechangesinenvironmentalconditions duringETincludeincreasedbaroclinicity,enhanced horizontalmoisturegradients,highverticalwindshear, increasedpositiveverticalvorticityandrelativeeddy momentumuxconvergence,andfastersteeringows (AtallahandBosart2003;Jonesetal.2003).Isentropic ascentofwarmandmoisttropicalairaheadofthestorm oftencausesraineldstoexpandinadelta-shaped areaaheadofthestormcenter(HarrandElsberry2000; Kleinetal.2000;RitchieandElsberry2001).Meanwhile, theadvectionofrelativelydrierandcoolerairaround thesouthsideofthestormreducesrainfallinthatarea. AccordingtoKleinetal.(2000),thetimelineforET spans74h:transformationaverages46h,withanother 28hrequiredforreintensication. Alimitationofmanyprevioussatellite-basedstudies ofdiurnalpeaksinrainfallistheselectionofatemperaturethresholdforanalysisofcloud-toptemperaturesas thetimingofpeakarealcoverageisdependentuponthis threshold(Muramatsu1983;LajoieandButterworth 1984).Ground-basedradarreectivitydataprovide aspatiallyaccuraterepresentationofthearealcoverage ofTCraineldsoverlandareaswithoutthislimitation. Thisstudyemployedageographicinformationsystem (GIS)tomeasurethearealextentofradarreectivity returnsassociatedwith45TClandfallsintheUnited States.Themainhypothesiswasthatthediurnalcycle willcausegrowthtooccurlaterinthemorningrelative tooceanicconvectionbutearlierintheafternoonthanis seenforland-basedconvection.Thus,makinglandfall aftermidnightshouldresultinrainfallregionsofTCs thatincreaseinarealcoverageoverthenext12h,followedbyadecreaseinarealcoveragefor1224hafter landfall,whereastheoppositeshouldoccurforlandfall afternoon.Thesecondhypothesiswasthatprocesses associatedwithETalsoinuencechangesinrain-eld size.TCsclassiedasETwithin74hoflandfallshould exhibitthispatternasthisisthetimelineforETcompletionidentiedbyKleinetal.(2000).Totestthese hypotheses,casesweregroupedaccordingtowhether thetotalrainingareaorhigh-reectivityregionsincreasedordecreasedinarealextentover12-hperiods. MannWhitney U tests(MannandWhitney1947)were employedtodiscoverwhethervariablesassociatedwith thediurnalcyclesuchastimeofdayandrelativehumidity,and/orvariableslinkedtoconditionsassociated withETsuchasfastmotiontothenorthandeasthave statisticallysignicantdifferencesinmedianvaluesfor casesinwhichraineldsgrowversusthoseinwhichthey decay.2.Dataandmethods a.RadaranalysisThenumberofstormsexaminedwasdeterminedby theavailabilityoftheradarreectivitydatautilized todelineatethespatialextentoftherainelds.Data fromtheWeatherSurveillanceRadar1988Doppler (WSR-88D)networkareavailableineachdegreeofthe 360 8 sweepevery1kmoutwardfromtheradarsitewith anouterdistancelimitof230km(OFCM2006).During 19952008,therewere45casesforwhich1)radarreectivitydatawereavailableand2)theraineldsof theTCsremainedwithinrangeoftheWSR-88Dnetworkforaminimumof24hafterlandfall.Level-IIIbase reectivityproductdatawereutilizedtoretainalarge samplesize,andthesedatawereobtainedonlinefrom theNationalClimaticDataCenter'sarchive(http:// www.ncdc.noaa.gov/nexradinv/).Thisproductisgeneratedaftertheremovalofgroundclutterandconsistsof reectivityvaluesfromthelowestscan(0.5 8 tilt)thatare1086JOURNALOFAPPLIEDMETEOROLOGYANDCLIMATOLOGYVOLUME52

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roundedtothenearest5dB Z (OFCM2006).Level-II datathatincludevaluesfromallscanelevationsarenot availableforsixradarsafter2001;therefore,useofthese datawouldhavesignicantlydecreasedthenumberof casesanalyzed. Basereectivitydatacollectedduringthescannearest thetimeoflandfall t0,12hafterlandfall t1 12,and24h afterlandfall t1 24ateachradarwereimportedintoa GIS.Afterthedataweretransformedintoanequal-area projection,amosaicwascreatedforeachtimestep.The highestvaluewasretainedincasesinwhichdatafrom adjoiningradarsoverlapped.Thereectivityvalues werethencontouredin5-dB Z increments,anda10-km smoothinglterwasapplied.Aftertheconversionof contoursintopolygons(Fig.1),theirarealextentwas calculated.Intheeventthatresidualgroundclutterremained,polygonswitharealextentsoflessthan25km2wereremovedfromtheanalysis.Althoughtheauthor thenvisuallyinspectedeachcaseasanadditionalmeasureofqualitycontrol,itispossiblethatgroundclutter fromanomalouspropagationmaystillbeembedded withinprecipitation,whichcouldextendtheechoareal coverage.FromtheworkofpreviousTCresearchers, thetotalrainingareawasdenedastheareaoccupied byreectivityvaluesofgreaterthanorequalto20dB Z (Jorgensen1984;Barnesetal.1991;Matyas2007).This areaiscomposedoflow,moderate,andhighrainrates formedthroughstratiformandconvectiveprocesses (Jorgensen1984;Houze1993;Steineretal.1995).To examinethehigh-rain-rateregionsmostlikelytohave formedthroughconvectiveprocesses,theareasoccupied byreectivityvaluesgreaterthanorequalto40dB Z werealsoextractedforanalysis.Thisthresholdwasselectedbasedonpreviousresearchthatclassiedconvectiveregionsofclouds(Parrishetal.1982;Jorgensen1984; TokayandShort1996;BiggerstaffandListemaa2000). Theareaoccupiedbyreectivityvaluesatthetimeof landfallwassubtractedfromthatat t1 12tocalculatethe changeinareaovertherst12-hperiodfollowinglandfall(Per1).Areasat t1 12weresubtractedfromthoseat t1 24tocalculatethechangeinareaoverthesecond12-h periodfollowinglandfall(Per2)(Fig.2).b.Characterizationofstormattributesand environmentalconditionsAlldatapertainingtostormlocationandattributes aswellasenvironmentalconditionswereinterpolated linearlytothetimeoflandfall.Thetimeoflandfall, coordinatesofthecirculationcenter,andintensitywere obtainedfromtheHurricaneSeasonTropicalCyclone ReportsavailableonlinefromtheNationalHurricane Center(NHC;http://www.nhc.noaa.gov;Rappaportetal. 2009).Thesereportswerealsoutilizedtodetermine whetheraTCbecameextratropicalandthetimethatit didsoifapplicable.TheHurricaneDatabase(HURDAT; Jarvinenetal.1984)providedthecoordinatesofthe circulationcenter,stormheading,forwardvelocity,and intensityat0000,0600,1200,and1800UTC.Storm motionwassubdividedint onorthwardandeastward components. Atmosphericconditionswerecharacterizedthrough dataobtainedfromtheStatisticalHurricaneIntensity Scheme(SHIPS)database(DeMariaandKaplan1994; DeMariaetal.2005).TheSHIPSvariablesarederived fromtheNationalCentersforEnvironmentalPrediction (NCEP)GlobalForecastSystemmodelanalyses,and mostvariablesarecalculatedforanannularregionthatis FIG.1.Areascomposedofradarreectivityvaluesof20dB Z andhigherand40dB Z andhigherforHurricaneGaston(2004) atthethreeanalysistimesconsideredinthecurrentstudy( t0, t1 12, and t1 24). FIG.2.Timelineforanalysisofatmosphericconditionsand measurementsofrain-eldarea. MAY2013MATYAS1087

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200800kmfromthecirculationcenter.Dataareavailableatthestandardsynoptictimes.WithintheSHIPS dataset,deep-layerverticalwindsheariscalculatedover 850200hPa.Forthecurrentstudy,thisvectorwassubdividedintosouth-to-northandwest-to-eastcomponents. Vorticityat850hPaand200-hPadivergencearederived foraradiusof01000kmaboutthecirculationcenter, and200-hParelativeeddymomentumuxconvergence (REFC)isaveragedover100600km.Previousresearch (e.g.,Jonesetal.2003)suggeststhatmanyofthe10variablesderivedfromtheSHIPSdataset(Table1)should exhibitdifferencesforTCsthatbecomeextratropical within74hoflandfallascomparedwiththosethatdonot. Solarheatingduringthedayandlossoflongwave radiationduringthenightdrivethediurnalcycleofair temperaturenearEarth'ssurface,andchangesinrelativehumidity(RH)areaffectedbyairtemperatureas wellassoilmoistureandprecipitation(Dai2001).Thus, thestrongestevidencethatthediurnalcyclemayinuencechangesinthesizesofraineldswhileTCsmove overlandshouldbefoundthroughananalysisofnearsurfacetemperatureandRHvalues.Toexaminethese variablesevery3h,valueswereobtainedfromNCEP's NorthAmericanRegionalReanalysis(NARR)database(Mesingeretal.2006).AirtemperatureandRH dataat2mabovegroundlevelwereconvertedinto shapele''formatandenteredintoaGIS.Buffersof 200and800kmfromthecirculationcenterateachanalysistimewereusedtoselectonlythedatapointsinside thisdistancerange.Theaver agevalueofdatapointsover thisregionwasthenutilizedintheanalysisforcompatibilitywiththeotherSHIPSvariables. Thevaluesforallvariablesmentionedabovewere analyzedatthreedifferenttimesrelativetolandfall. Previousresearchhasshownthatatimelagof1224h existsbetweentheonsetofenvironmentalconditions andresultingchangesinTCstructure(e.g.,Frankand Ritchie1999;Kimball2008;Matyas2010).Toaccountfor thislageffect,environmentconditionswereanalyzedat t2 12, t0,and t1 12,whichcoincidewith24,12,and0hprior totheendofPer1and36,24,and12hpriortotheendof Per2(Fig.2).Inaddition,thechangesintheenvironmentalconditionsovereach12-hperiodwerecalculated asthesevaluesmaybemorerepresentativeofthediurnal cycleorprogressionofconditionsassociatedwithET thantheactualvalueofthevariableatanyonetime. Forexample,a12-hincreaseinRHanddecreaseinair temperatureindicatethatthemidpointoftheperiod occursneardawn,whereasincreasingstormforwardvelocitycoincideswiththeprogressionofanET.Thus,each environmentalconditionderivedfromHURDAT,SHIPS, andNARRwasexploredatthreeinstantaneoustimes ( t2 12, t0,and t1 12)andovertwoperiods( t2 12: t0and t0: t1 12),yielding90totalvariables(Table1). AsaTCmovesinland,itscenterofcirculationbecomesfartherremovedfromthewarmoceanwatersthat supplytheprimarysourceoflatentheattosustainthe storm(e.g.,Tuleya1994).Itfollowsthat,ingeneral,the TABLE1.Name,abbreviation,unitofmeasure,andsourceforvariablesthatwereusedinthestudy.VariablesfromHURDAT,SHIPS, andNARRhavevevariations(threeinstantaneoustimesandtwochangesovertime)thatweresubjectedtoMannWhitney U tests. VariableAbbreviationUnitsSource LatitudeLat 8 NHURDAT LongitudeLon 8 WHURDAT MotionnorthMotNms2 1HURDAT MotioneastMotEms2 1HURDAT Velocityofmaxsustainedwinds Vmaxms2 1HURDAT MincentralpressureMCPhPaHURDAT SoutherlyverticalwindshearShrSms2 1SHIPS WesterlyverticalwindshearShrWms2 1SHIPS Airtemperatureat200hPaT200 8 CSHIPS Zonalwindsat200hPaU200ms2 1SHIPS Divergence200hPaD200s2 13 107SHIPS Relativehumidityhigh(500300hPa)RhHi%SHIPS Relativehumiditymiddle(700500hPa)RhMd%SHIPS Relativehumiditylow(850700hPa)RhLo%SHIPS Vorticityat850hPaZ850s2 13 107SHIPS RelativeeddymomentumuxconvergenceREFCms2 1day2 1SHIPS Airtemperatureat2mT2m 8 CNARR Relativehumidityat2mRh2m%NARR Distanceinland12hafterlandfallDin12kmGISanalysis Distanceinland24hafterlandfallDin24kmGISanalysis Sineoflandfalltimeminusvalue*e.g.,SinLT-90DimensionlessNHCreport *Eightvalueswereutilized:0,45,90,135,180,225,270,and315.1088JOURNALOFAPPLIEDMETEOROLOGYANDCLIMATOLOGYVOLUME52

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greaterthedistancebetweenthenearestpointalongthe coastlineandthecirculationcenterofaTCis,thelessis thepotentialforrainfallenhancementastheavailability oflow-levelmoistureisreduced.Remainingnearthe coastlinemayallowraineldstogrowregardlessofthe timeofdayorwhetherthestormisundergoingET. WithinaGIS,theU.S.coastlinefromBrownsville, Texas,toEastport,Maine,wasconvertedintoaline featureandtheNear''functioncalculatedthedistance betweenthepositionofthecirculationcenterat12and 24hafterlandfallandthenearestpointalongtheline representingthecoast. TheinclusionofthelocaltimeofeachTC'slandfallin thestatisticalanalysisisimportantbecauseitreveals whichlandfalltimesaremostassociatedwithpostlandfallgrowthordecay.Forexample,iftheraineldsof landfallingTCsexhibitadiurnalcyclesimilartothatof ocean-basedconvectioninwhichpeakactivityoccursat 06001000LST,thenarain-eldsizethatislargerat t1 12thanatlandfallshouldoccurforlandfalltimesranging from1800to2200LST.Aftertheconversionoflandfall timefromUTCtoLST,eachhourwasmultipliedby 15toextendtherangeofvaluesto360.Sothatlandfall timesoccurringevery3hwererepresentedinthe analysis,avalueof45wassubtractedfromeachconvertedtime.Afterconvertingthesevaluesintoradians, thesineofalleightvalueswascalculated.Asaresult,the sineofthelandfalltimeminus180(SinLT-180)producesavalueof1.0forthelandfalltimeof1800LST and 2 1.0forthetimeof0600LST.Ifthediurnalcycle forlandfallingTCswascoincidentwiththatofoceanicbasedconvection,thenrain-eldgrowthwouldoccur duringthe12-hperiodsof18000600and21000900 LST,andtheseperiodswouldbeindicatedbyvalues near1.0forvariablesSinLT-180andSinLT-225.c.StatisticalanalysesThisstudyemploysnonparametricMannWhitney U tests(MWU;MannandWhitney1947)torelatedifferencesinenvironmentalconditionstorain-eldgrowth ordecay.Thistestcomparesthemediansofvariables whencasesaredividedintotwoindependentgroups,with anullhypothesisthatdatafromthetwogroupsoriginate fromthesamepopulation.Thistestwasutilizedbecause itismorerobustforskeweddatathanisanindependent samples t testandislesslikelytoyieldfalseresultswhen groupsizesaresmalland/ornotequal(Wilks1995).For eachvariable,valuesarerankedregardlessofgroup membership.Theteststatisticthencomparesthesumof theranksfromtherstgroupwiththatofthesecond.If thesamplesaredrawnfromthesamepopulation,the summedtotalofranksforeachgroupshouldbesimilar. Inthecurrentstudy,thenullhypothesiswasrejected when p valueswerelessthanasignicancelevelof 0.05,whichisacommonlyusedlevelaccordingtoWilks (1995).Althoughthevariablestestedarenotindependent,theperformanceof100individualMWUtests withineachsetasdescribedbelowdoesincreasethe possibilityofincorrectlyrejectingthenullhypothesis whentheexperiment-wiseerrorrateisconsidered.However,thetechniqueisvalidgiventhattheacceptanceof thetwomainhypothesesofthestudydoesnotrelysolely ontheresultsofanyonetest;anumberofvariablesare representativeofthediurnalcycleandET.All p values meetingthe0.05thresholdarereportedsothatresultscan alsobescrutinizedmorestringentlyifdesired. ForeachMWUtest,casesweregroupedaccordingto whetherrainfallareasincreasedordecreasedduring a12-hperiod.Twodifferent12-hperiodsandtworadar reectivitythresholdstocalculatetherain-eldarea wereanalyzed,yieldingfoursetsofMWUtestsinwhich all100variables(discussedinsection2b)weregrouped accordingtorain-eldgrowthordecay.Set1examined rain-eldgrowthordecayfortotalrainingareaduring Per1,andset2comparedgrowthanddecayforhighreectivityregionsduringPer1.Insets3and4,cases weregroupedaccordingtorain-eldgrowthordecay duringPer2,withset3(4)focusingontotalrainingarea (high-reectivityregions).Variablesforwhichthenull hypothesisisrejectedidentifytheconditionsassociated withrain-eldgrowthordecayduringeachperiod.The inuenceofthediurnalcyclemaybeindicatedbyvariablessuchas12-hchangesinnear-surfaceairtemperatureandRHalongwiththevariationsinsineofthe landfalltime.Inotherwords,ifthemeanrankofthe 12-hchangeinairtemperatureorRHforrain-eld growthcasesdiffersfromthemeanrankofthe12-h changeforrain-elddecaycases,thenadiurnaleffect maybeoccurring.In20cases,TCsmadethetransition toextratropicalcycloneswithin74hoflandfall.Ifthe nullhypothesisisrejectedforvariablescharacterizing stormmotion,vorticity,REFC,andverticalwindshear, itindicatesthatprocessesassociatedwithETinuence rain-eldgrowthanddecay.Notethatlandfalloccurred atvarioustimesofdayforTCsthatbecameextratropicalwithin74hoflandfall.3.Results a.Rain-eldsizesandstatisticsAtthetimeoflandfall,themedianraineldhadan areaof109263km2andthemedianvalueforhighreectivityregionswas10473km2(Fig.3).Hurricane Floyd(1999)wasanoutlier(notvisibleinFig.3),having thelargestextentofbothmeasuresofrainfallarea.TCsMAY2013MATYAS1089

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suchasFloydthatbecameextratropicalwithin74hhad thelargestrainelds(Fig.3).Nineof13TCswithtotal rainingareasthatwerelargerthan150000km2and11 of16TCswithhigh-reectivityregionsthatspanned morethan15000km2completedanETwithin60hof landfall.Spearman'srankcorrelationcoefcients(not shown)demonstratethatthetimeoflandfallwasnot signicantlycorrelatedwithrain-eldsize.Yet,thediurnalcyclemaybeinuencingthetimingofrain-eld growthanddecaygiventhatoverPer1largerrainelds decreasedwhilesmallerraineldsincreasedinareal coverage.Asaresult,themeanandmedianareaswere similartothevaluesatlandfall.DuringPer1,23(21)of 45casesexperiencedincreasesintotalrainingarea (high-reectivityregions).Rain-eld(highreectivity) growthoccurredfor14(16)of45casesduringPer2. Althoughby t1 24medianrainfallareashaddecreased fromthatatlandfall,19(12)TCsincreasedinareal coveragefortotalrainingarea(high-reectivityregions) duringthisperiod,withameanincreaseof27734km2(4228km2).b.ResultsoftheMannWhitney U testsTheoutcomesofthefoursetsofMWUtestssupport thehypothesesthatthediurnalcycleandETaffectraineldgrowthanddecay.Therewere51variableswith p valuesoflessthanorequalto0.05(Tables25).Of these,20wereuniquetoonesetandtwovariationsof thesineoflandfalltimeweresignicantinallfoursetsof tests,indicatingthatrain-eldgrowthoccurredwhen eitherPer1orPer2begannear0000and0300LST.The statisticallysignicantvariablesarebroadlycategorized asrelatingto1)thediurnalcycle,2)ET,and3)the availabilityofmoisture.Atleastonevariablefromeach categoryappearedineachofthefoursets,suggesting thatallthreeprocessesinuencerain-eldgrowthand decayafterlandfall.Theresultsalsodemonstratethe importanceofincludingbothinstantaneousandchangeinvariablesaswellas12-and24-htimelagsbetween conditiononsetandrain-eldresponse.Amorestringent p levelof0.025(0.01)yields21(9)signicantvariablesacrossthefoursetsoftests.1)RAIN-FIELDGROWTHANDDECAYANDTHE DIURNALCYCLETCsmakinglandfallnearmidnightand0300LST tendedtogrowduringPer1anddecayduringPer2, whereastheoppositewastrueforlandfallsoccurring nearnoonand1500LST(Fig.4).Statisticalevidence supportingthispatternwasfoundthroughthetimerelatedvariablesandnear-surfacetemperatureandRH valuesashypothesized.Casesinwhichlandfalloccurred closertomidnight(valueofSinLT-270closeto1.0)or 0300LST(valueofSinLT-315closeto1.0)wereassociatedwithgrowthduringPer1(Tables2and3)and decayduringPer2(Tables4and5).Landfallsoccurring FIG.3.ArealcoverageofraineldsforTCsthatbecameextratropical(labeledET)within74hoflandfallascomparedwiththosethat didnot(nonET'')at t0, t1 12,and t1 24andthechangeinareaforTCsthatdecayed(decreasing)andgrew(increasing)during t0: t1 12and t1 12: t1 24for(a)totalrainingareaand(b)high-reectivityregions.HurricaneFloyd(1999)wasanoutlier,withvaluesthatwouldappear offthetopofeachpanel.1090JOURNALOFAPPLIEDMETEOROLOGYANDCLIMATOLOGYVOLUME52

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approximately12hoffsetfromthesetimes(valuesof SinLT-270andSinLT-315closeto 2 1.0)experienced decayatrstfollowedbygrowthduringPer2.Forboth analysisperiods,rain-eldgrowthwasassociatedwith decreasingairtemperaturesandincreasingRHvalues nearthesurfaceduringtheprevious12h,indicatingthat thisprecedingperiodwasduringthenight.DuringPer2, growthwasassociatedwithcaseshavinglowerRH valuesatthetimeoflandfall(whichoccurredduringthe day)andvaluesdecreasingpriortoandincreasingafter landfall.Correspondingly,near-surfacetemperatures increasedpriortolandfallanddecreasedafterlandfall forTCswhoseraineldsgrewduringPer2.Overall, growthtendedtooccurduringthelate-morningandearlyafternoonhoursasarealcoveragewasgreaterat1200and 1500LSTthanitwasat0000and0300LST. Thesetimesforrain-eldgrowthoccurbetweenthe timesreportedbypreviousresearchersforoceanicand land-basedconvection.Becausemostoceanicconvection peaksbetween0600and1000LST(GrayandJacobson 1977;YangandSmith2006;KikuchiandWang2008), landfallingTCsfollowingthispatternwouldhavehad valuesnear1.0forvariablesSinLT-180andSinLT-225, indicatinglandfallat1800or2100LSTandrain-eld growthoverthenext12h.Thisscenariodidnotoccur. Afternoonpeaksinrainfalloverlandcorrespondtotimes of16001900LST(Wallace1975;Dai2001;Daietal. 2007).TCsmakinglandfallnear0600LSTdidnottend tohavelargerrainingareas12hlaterastheanalysis ofvariableSinLT-0didnotproducestatisticallysignicantresultsintheMWUtests.Oneexplanationforthese ndingsisthatTCsaremakingthetransitionfromthe diurnalcycleofanocean-basedstormtothatofalandbasedstormduringlandfall.AsTCsadvectmoistureladentropicalairmassesoverthelandsurfaceduringthe morninghours,itispossiblethatinstabilityisenhanced sothatconvectiondevelopsearlierinthedaythanwhen thenormalcontinentalairmassisinplace.Knowledge TABLE2.ResultsofMannWhitney U testsforrain-eldgrowthvsdecayfortotalrainingareaduringPer1.FormeaningoftheabbreviatedvariablenamesseeTable1. VariableTimeMedianvaluegrowMedianvaluedecay U SignicanceAssociation Din12 t1 1266.22144.101340.007Moisture SinLT-270* t00.71 2 0.26367.50.009Diurnal RhMd t2 12: t02.00 2 1.253640.011Moisture SinLT-315** t00.71 2 0.263590.016Diurnal MotE t1 120.28 2 2.063500.026ET Z850 t2 12: t010.25 2 5.213490.027ET RhLo t2 12: t02.30 2 0.10346.50.031Moisture Lon t0: t1 120.50 2 0.70345.50.033ET RhHi t2 12: t01.00 2 1.653400.045Moisture MotE t2 12: t00.780.143380.050ET REFC t2 12: t05.00 2 0.383380.050ET *Highpositivevaluesindicatelandfalltimecloseto0000LST. **Highpositivevaluesindicatelandfalltimecloseto0300LST. TABLE3.AsinTable2,butforhigh-reectivityregionsduringPer1. VariableTimeMedianvaluegrowMedianvaluedecay U SignicanceAssociation Din12 t1 1245.64145.751090.001Moisture ShrS t1 122 1.341.921100.002ET REFC t2 125.338.581360.008ET SinLT-270* t00.71 2 0.26358.50.015Diurnal Rh2m t2 12: t07.04 2 2.313510.024Diurnal SinLT-315** t00.71 2 0.26349.50.026Diurnal RhMd t2 12: t02.00 2 0.65345.50.033Moisture T2m t2 12: t02 2.740.061600.036Diurnal RhHi t2 12: t00.80 2 2.15341.50.042Moisture Lon t1 122 83.50 2 88.743400.045ET Rh2m t082.7778.023400.045Diurnal Lon t2 122 83.30 2 88.58339.50.047ET Lon t02 83.65 2 88.733380.050ET *Highpositivevaluesindicatelandfalltimecloseto0000LST. **Highpositivevaluesindicatelandfalltimecloseto0300LST. MAY2013MATYAS1091

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thatrainfallmaycommenceearlierinthedaythanexpectedforalocationallowsweatherforecasterstoalert thepublicaspreparationsaremadeforthearrivalofTC conditions. Thecurrentstudyisbelievedtobethersttoanalyze radarreectivityreturnsforalargesampleoflandfalling TCsintheUnitedStatesandtoassociaterain-eldgrowth anddecaywiththediurnalcycle.Jiangetal.(2011)providedtherstdocumentationo fdiurnalvariationsofglobal TCrainfallusingdatafromtheTropicalRainfallMeasuring Mission(TRMM)satellite.TheTRMManalysisshowed thatvolumetricrainfallfromTCsoverlandpeakedat0100 0730LSTand16301930LST.However,Jiangetal.(2011) examinedglobalTCrainfallvariations.TheirFig.12shows littlediurnalvariationforAtlantic-basinstormsascomparedwiththeotherTCbasins.Atmosphericconditionsdo varyclimatologicallyamongthedifferentTCbasins.For example,VincentandFink(2001)discussdifferencesin theprecipitablewatercontentforthewesternandeastern PacicTCbasins.Thus,itis possiblethatthetimingof thepeakoverlandrainfallfromAtlantic-basinTCsdiffers fromthatobservedinotherbasinswhereTCrainratescan behigher.Also,studieshaveshownthattheshapeofthe coastlineaswellastopographyofthelandsurfacecanalter stormstructure.TheuniquefeaturesoftheU.S.coastline maycontributetoapeakinrainfallthatdiffersintiming fromotherlandfalllocations(e.g.,RogersandDavis1993; Cubukcuetal.2000;Liuetal.2007;Kimball2008;AuYeungandChan2010).2)CONDITIONSRELATEDTOEXTRATROPICAL TRANSITIONOfthe45casesexamined,20becameextratropical within74hoflandfall(Fig.4).ThisstudyndsthatET contributestooverallrain-eldgrowthearlyinthe processanddecayastheprocessnearscompletionwhile TABLE4.AsinTable2,butfortotalrainingareaduringPer2. VariableTimeMedianvaluegrowMedianvaluedecay U SignicanceAssociation Rh2m t077.1982.161070.007Diurnal SinLT-315* t00.71 2 0.441190.016Diurnal RhLo t0: t1 123.45 2 0.30315.50.017Moisture T2m t2 12: t00.19 2 2.713130.019Diurnal Rh2m t2 12: t02 2.867.041290.031Diurnal RhMd t0: t1 121.90 2 1.303070.033Moisture SinLT-270** t00.61 2 0.791320.037Diurnal Rh2m t0: t1 126.77 2 5.893020.037Diurnal Lat t0: t1 121.391.731350.044ET T2m t0: t1 122 2.410.811380.050Diurnal *Highpositivevaluesindicatelandfalltimecloseto0300LST. **Highpositivevaluesindicatelandfalltimecloseto0000LST. TABLE5.AsinTable2,butforhigh-reectivityregionsduringPer2. VariableTimeMedianvaluegrowMedianvaluedecay U SignicanceAssociation Lon t0: t1 122 1.120.62103.50.002ET REFC t2 12: t02 2.463.501090.004ET Lon t2 12: t02 1.160.23113.50.005ET MotE t1 122 2.630.94128.50.014ET T2m t2 12: t00.36 2 2.713360.014Diurnal Rh2m t076.4882.031290.015Diurnal Lat t0: t1 121.222.13131.50.017ET T2m t027.8126.153300.020Diurnal MotN t2 12: t02 0.081.631410.031ET U200 t2 12: t02 1.254.481410.031ET MotE t02 1.930.681450.039ET T2m t0: t1 122 1.730.411450.039Diurnal SinLT-315* t00.71 2 0.44146.50.042Diurnal Rh2m t2 12: t02 1.366.971480.046Diurnal RhMd t0: t1 120.68 2 1.65318.50.047Moisture Rh2m t0: t1 126.41 2 4.983150.049Diurnal SinLT-270** t00.71 2 0.13150.50.050Diurnal *Highpositivevaluesindicatelandfalltimecloseto0300LST. **Highpositivevaluesindicatelandfalltimecloseto0000LST.1092JOURNALOFAPPLIEDMETEOROLOGYANDCLIMATOLOGYVOLUME52

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high-reectivityregionsmainlydecayed.DuringPer1, ETstormsexhibitinggrowthofthetotalrainingarea movedfastertotheeastwhilepositivevorticityand REFCincreased(Table2).Thisnetgrowthcorresponds todescriptionsofthedevelopmentofthedelta-shaped rainshieldthatoccursaheadofTCsintheearlystagesof ET(HarrandElsberry2000;Kleinetal.2000;Ritchie andElsberry2001).Thetropicalairmassthatisadvectedoverthecooleranddrierairmassaheadofthe stormcentercreatesastableenvironmentthatfavors stratiformratherthanconvectiveprocesses.Thisexplainswhyhigh-reectivityregionsdecayedforET cases.HighervaluesofverticalwindshearandREFC aswellaslongitudeslocatedfarthertotheeastascomparedwithcasesinwhichhigh-reectivityregionsgrew supportthendingthatETcontributestothisdecrease inhigh-reectivityregions(Table3).FortheveET casesfeaturinggrowthinhigh-reectivityregionsduring Per1,landfalloccurrednearmidnightLST,indicating thatthediurnalcyclemayhavecontributedtothe growthoftheseregionsinthelate-morninghours.Thus, itislikelythatthediurnalcycleaffectsconvectiverainfallgrowthanddecayevenwhenETisoccurring. Theadvectionofrelativelycoolanddryairaroundthe westernandthensouthernsideoftheTCdecreases rainfallproductionduringstep3ofthetransformation stageofET(Kleinetal.2000).Thecurrentstudysupportsthispreviouswork.Saveforonecaseinwhichthe diurnalcyclemayhavecontributedtorain-eldgrowth, allTCscompletingETwithin30hoflandfallexperienceddecayinbothrainfallareasduringPer2.TCs movingfarthertothenorthdecayedintotalrainingarea ascomparedwiththosethatremainedsouth(Table4). Forhigh-reectivityregions,decayduringPer2wasassociatedwithETasfastnorthwardandeastwardmotion andfastwesterlywindsat200hPawithincreasedREFC beforelandfall(Table5).3)IMPORTANCEOFMOISTUREFORGROWTH ANDDECAYDuringPer1,oneofthemostsignicantdifferences betweencasesexperiencingrain-eldgrowthordecay wasthedistanceinlandtraveledbystormsineachgroup. TCsremainingclosertothecoastlinegrewwhilethose locatedmorethantwiceasfarinlanddecayed(Tables2 and3).PreviousresearchhasshownthatTCsremaining closertothecoastlinecanhavelargerraineldsthat producemorerainfallbecauseremainingnearthecoastlineallowsTCstodrawinlow-levelmoisturefromthe oceantoincreaserainfallproduction(Lonfatetal.2007; Matyas2007;Medlinetal.2007).Yet,thedistancetraveled inlandby t1 24wasnotsignicantforPer2,andSpearman's rankcorrelationcoefcientscalculatedbetweenvariable Din24andtheactualarealcoveragerainfall(notshown) didnotproducestatistically signicantresults.Previous researchhasshownthatrainf allproductioncanstillincreaseforTCslocatedmorethan500kminlandifalowleveljetadvectsmoistureintothestorm,soilmoistureis highenoughtosupplymorelatentheatuxthanisexperiencedoverdryland,ororographicenhancementof rainfalloccurs(Haggardetal.1973;BluesteinandHazen 1989;Tuleya1994;Emanueletal.2008;Arndtetal.2009; Kellneretal.2012).Thus,trackingfarinlanddoesnot necessarilymeanthatraineldswilldecreaseinareal coverage. FIG.4.Tracksofthe45TCs,indicatingwhetherlandfalloccurred closertonoonormidnight,whetherEToccurredwithin74hof landfall,andwhetherraineldsgrewordecayedduringPer1and Per2for(a)totalrainingareaand(b)high-reectivityregions. MAY2013MATYAS1093

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Moistureabovetheboundarylayerisalsoimportant asrain-eldgrowthwasassociatedwithincreasingRH valuesinthelow,middle,orhightroposphereduringthe previous12h(Tables25).ArelativelymoistsurroundingenvironmentiskeytoincreasedTCrainfall production(Jiangetal.2008;HillandLackmann2009; Matyas2010).Theadvectionofdryairintothesystem enhancesevaporation,whichleadstoadecreasein rainfallproduction(Chanetal.2004;Kimball2008). AdditionalMWUtestsperformedwiththeseRHvariablesandcasesgroupedaccordingtowhetherornotET occurredwithin74hoflandfallandwhetherlandfall occurredduringthedayornightindicatedthatchanges inRHabovetheboundarylayerwerenotstronglyassociatedwitheitherthediurnalcycleorET.c.DiurnalvariationsinlandfalltimesAnexaminationofthedistributionoflandfalltimes forTCsinthecurrentstudyyieldsaninterestingresult whencomparedwithapreviousstudy.One-thirdofthe landfallsinthecurrentstudyoccurredbetween0000and 0300LST(Fig.5).Incontrast,Konrad(2001)foundthat for195096nearlyone-halfofTCslandfallingalongthe GulfofMexicoandAtlanticcoastlinesoftheUnited Statesdidsobetween1700and0000LSTwhereasonly fourlandfallsoccurredbetween0100and0300LST. Afterbinningthelandfalltimesinthecurrentstudy accordingtothetimegroupslistedinTable1ofKonrad (2001),achi-squaretestshowedstatisticallysignicant differencesbetweentheobservedandexpecteddistributionsofcasesamongthelandfall-timecategoriesfor thetwodatasets( x25 10.912; p 5 0.012).Toruleoutthe possibilitythatthecurrentstudy'ssamplewasnotrepresentativeofalllandfallsduring19952008,anadditional chi-squaretestrevealedthatthedistributionofthecurrent studyverystronglyresemblesthedistributionoftheentire population( x25 0.836; p 5 0.858).Thus,itappearsthat ashiftinlandfalltimeshastakenplacefrom195096, whenthemajorityoflandfallsoccurredintheevening hourspriortomidnight,to19952008,whenmanyoccurredatorjustaftermidnight. AshiftinthemajorityofTClandfallsfrombeforeto aftermidnighthasimportantimplicationsforthereceipt ofhurricanewarningmessagesbythepublic.Priorto 2010,theNHCissuedhurricanewarningswhenhurricaneconditionswereexpectedfortheareawithin24h (Sheets1990;Rappaportetal.2009).Withpeaklandfalls occurringbetween1700and0000LST,thesewarnings wouldlikelyhavereachedpeopleduringthelateafternoonandeveninghours,givingthemadequatetimeto makepreparations.Duringthemorerecentperiodwhen landfallspeakedatandaftermidnight,thesewarnings weremorelikelytohavebeenissuedafterpeoplehad retiredfortheevening.Uponawakeningthenextmorning, theymaythenhavehadlessthan18htocompletepreparationsforthearrivalofh urricane-forcewinds.In2010, theNHCchangeditsleadtimesothathurricanewarnings arenowissued36hinadvanceoftheanticipatedonsetof tropicalstormforcewinds(NHC2012).Thenewlonger leadtimesreducetheproblemswithTCsmakinglandfall aftermidnight,whichhasbecomemorecommoninrecent years(Fig.5).4.ConclusionsandfutureresearchInthisstudy,thearealcoverageofTCraineldswas examinedafterU.S.landfallthroughaGIS-basedspatialanalysisofradarreectivityreturns.Thegrowthand decayofthetotalrainingareaandhigh-reectivityregionswereexaminedovertwo12-hperiods.Variables characterizingthetimeofday,conditionsoftheatmosphere,anddistancefromthecoastlinewereexamined infoursetsofMannWhitney U testsinwhichcases weregroupedaccordingtowhetherrainingareasincreasedordecreasedovera12-hperiod.Changesin rain-eldsizewereassociatedwiththediurnalcycleas growthoccurredduringthelatemorningandearlyafternoonaftermaximumRHandminimumairtemperatureshadbeenreached.Raineldsdecreasedinareal extentinthelate-eveninghours.TCscompletinganET within74hoflandfallhadthelargestraineldsatthe timeoflandfall,andtotalrainingareaincreasedduring therst12hafterlandfallbutdecreasedduringPer2. RegionsofhighreectivitydecreasedinarealcoverageforETcasessavewhenthediurnalcyclelikely contributedtogrowthinthelatemorningandearlyafternoon.Moisturewasalsoimportantforrain-eld FIG.5.Frequencyoflandfalltimesforthe45casesinthestudy.1094JOURNALOFAPPLIEDMETEOROLOGYANDCLIMATOLOGYVOLUME52

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growth.Abovetheboundarylayer,increasingRHinthe preceding12hwasrelatedtorain-eldgrowthaswas remainingnearthecoastlineduringPer1.Inaddition, thisstudyfoundthatmoreTCsmadelandfalljustafter midnightascomparedwith195096whenmostlandfalls occurredjustpriortomidnight.Thisndingislikelydue toimprovementsinobservingsystems. Thisstudyhasprovidedobservationalevidencefor diurnalvariationsofrainingareasofTCsevenasthey facerapidlychangingenvironmentalconditionswhile movingovertheUnitedStates.Associationsbetween environmentalconditionsandrain-eldgrowthand decaywereexploredonabroadspatialscaleintermsof theenvironmentalvariablesandonatemporalscalein termsoftheradaranalysis.Thelogicalnextsteptoward conrmingalinkbetweenthediurnalcycleandTCrain eldsoverlandthroughobservationaldataistoexamine rain-eldevolutionatahighertemporalresolution. Furtheranalysesutilizingradarreectivitydatafromthe WSR-88DnetworkarejustiedbecausetheprecipitationradarswathusedtocreatetheTRMMdatasetis only250kmwide.Also,WSR-88Dreectivitydataare availableevery56min,whichisahighertemporal resolutionthanisavailablefromtheTRMMdata.The analysisofWSR-88Ddataoverhourlyperiodswillallow thetimingofpeakrainfallwithinthediurnalcycleto bemorepreciselydenedalongwiththerateatwhich growthanddecayoccur.Inclusionoflevel-IIreectivity datainfutureanalysiswouldallowtheverticaldevelopmentofTCraineldstobeanalyzedtoaidinthe identicationofconvectiveregionsoftherainelds wherethehighestrainratesoccur.Itwillalsobeadvantageoustomodelatmosphericconditionsatahigher spatialresolutionthanthe200800-kmaverageprovidedbytheSHIPSdataset,particularlyinlightofthe associationsbetweenmoistureandrain-eldgrowth identiedbythecurrentstudy.Itwouldbeappropriate tocharacterizeenvironmenta lconditionsutilizingNARR datagiventhattheyareavailableevery3hata32-km spatialresolution(Mesingeretal.2006). 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