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National Aeronautics and Space Administration
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
GMAO RESEARCH BRIEF
Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States Allison Collow, Mike Bosilovich, and Randy Koster
BACKGROUND
SUMMARY Overthepast15years,thenortheasternUnitedStateshasseenastatistically significant increase in the frequency of extremeprecipitation events that is larger and more widespread thananywhereelseinthecountry.Thisincreaseineventsismorelikelytobeassociatedwithfrontalandlow-pressuresystems,ratherthanbeingcausedbymoretropicalcyclonesimpactingtheregion.
Extreme precipitation events are characterized by a statisticallyexceptionalamountofprecipitationfallinginagiventimeperiodas compared to climatology. Varying definitions of extremeprecipitationcanbefoundintheliteraturebasedondifferenttimeand spatial scales and statistical methods. Regardless, extremeprecipitationeventsneedtobemonitoredandpredictedbecauseof their local and regional impacts, such as major floods andlandslides.ThisstudyexamineschangesintheoccurrenceofsucheventsinthenortheasternUnitedStates,demonstratingthattheyarebecomingmorefrequent.
#2017-1, February 2017
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
2
Introduction
Extremeprecipitationeventscanhavealargeimpactonsocietythroughfloodingthatcanresultinpropertydestruction,croplosses,economiclosses,thespreadofwater-bornediseases,andfatalities(Handmeretal.,2012).However,thereisconsiderableuncertaintyregardinghowsucheventsmaychangeinthefuture(IPCC,2013;Janssenetal.,2014).Muchinformationcanbegainedbystudyingpasteventsandhowtheyhavechangedovertime.WhileobservationsindicatethatextremeprecipitationeventshaveincreasedinthecontiguousUnitedStates,thelargestandmostwidespreadincreasehasbeenobservedduring the summer in the northeastern United States, encompassing an area fromsoutheastern New York to northern Maine. The trend in northeastern U.S. extremeprecipitationeventscarriesastatisticalsignificanceof99percentandhasbeenconfirmedinmultipledatasets(e.g.,Ageletal.,2015;Freietal.,2015;Collowetal.,2016).Thestudyof extreme precipitation events is complicated by the wide range of meteorologicalcauses,includingextratropicalcyclones,tropicalcyclones,frontalsystems,andmesoscaleconvectivesystems(Konrad,2001;Kunkeletal.,2012;Ageletal.,2015).UsingNASA’sModern-EraRetrospectiveAnalysisforResearchandApplications–version2(MERRA-2),observedextremeprecipitationeventsinthenortheasternU.S.wereclassifiedbytheirmeteorologicalcauseinordertoevaluatehowthedifferenttypesofeventshavechangedovertime.Extremeprecipitationeventsweredefinedasoccurringwhentheamountofrainfallisgreaterthan90percentofcorrespondingvaluesforthecalendardayoftheyearbasedonobservationsfromtheNOAAClimatePredictionCenter(CPC)griddedUnifiedGauge-BasedAnalysiswithinaclimatologyperiodof1981through2010.
TimeSeriesofExtremePrecipitationEvents
AsseenbytheblacklineinallpanelsofFig.1,whichshowsthetotalnumberofextremeprecipitationeventspersummer,extremeprecipitationeventsinthenortheasternU.S.havebecomemorecommonovertime.Thefrequencyofoccurrencehasincreasedbyanaverageof1.48eventsperdecadeoverthe35-yeartimeperiodextendingbackto1980,butitisevidentfromFig.1thatthenumberofeventshasincreasedoverthelast15yearsinparticular.
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
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Each observed extreme event was classified as tropical, frontal, low pressure, orunassociatedinthecasesthatdidnotfitintooneofthefirstthreecategories.AtropicaleventwasdefinedasoneinwhichastormidentifiedbytheNationalHurricaneCenter’sNorthAtlantichurricanedatabase(HURDAT)wasco-locatedwithin5°oftheNortheastregion,followingthemethodologyofKunkeletal.(2012).FrontaleventswereidentifiedviathedetectionofalowertroposphericwindshiftandtemperaturegradientwithintheNortheastdomain.Lowpressure-relatedeventsweredefinedusingthemethodologyofPooketal.(2006),whichrequiresaclosedregionofsea-levelpressurebelow1008hPaandanegativeanomalyin500hPaheightoraclosedareaof500hPaheight.
Figure1.ThetotalnumberofextremeprecipitationeventsintheNortheastduringJune,July,andAugustaswellasthosecausedby(a)fronts,(b)closedlowpressuresystems,(c)othertypesofsystems(unassociated),and(d)tropicalcyclones.
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
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TropicalcyclonesareknowntocausethemostintenseextremeprecipitationeventsintheNortheast (Konrad, 2001). Aside froma scarcity of tropical cyclones in the 1980s,thereislittlevariabilityinthefrequencyoftropical-relatedextremeprecipitationevents,as therewerenevermore than twoevents inagivenyear (Fig.1a).Although there isinterannualvariabilityinthenumberofunassociatedevents,mostoccurredpriortotheincreaseseenoverthepast15years(Fig.1b).Unlikethetropicalstorm-relatedsystemsand the unassociated events, the number of events related to frontal systems hasincreased by 0.69 events per decade (statistically significant at 95%), accounting forroughlyhalfoftheobservedincreaseinthetotalnumberofextremeprecipitationevents(Fig. 1c). The other half of the trend in the total number of events is related to astatisticallysignificantincreaseinthenumberofclosedlowpressuresystems(Fig.1d).
ChangeinCompositedMeteorologyDuringExtremeEvents
As frontal systems and closed low pressure systems are primarily responsible for theobserved increase in extreme precipitation events, the atmospheric conditionssurrounding these typesof eventswereevaluated, aswell ashow themeteorologicalconditionshavechangedovertime.Thiswasdonebycompositingmeteorologicalfieldsduringobservedextremeprecipitationeventsthatoccurredwithinthesecondhalfoftheanalysisperiod,1997through2014,andcomparingwithcorrespondingfieldsfromtheperiodof1980through1996.Suchcompositingwasdoneforalleventsaswellasthoserelated toa frontalora lowpressure system.As seen inFig.2a,anarcof statisticallysignificantlowersealevelpressurespanningfromMainetotheFlorida/Georgiaborderisseen inthemorerecenttimeperiodwhenalleventsareconsidered.Thischangeovertimeprimarilystemsfromthelowpressuresystem-relatedevents(Fig.2b).WhileeventsrelatedtoafrontalpassagearenotassociatedwithlowerpressureovertheNortheastinmorerecentyears,thereisamorepronouncedareaoflowersealevelpressureintheHudsonBay,which likelycorrespondstothe locationof theparent lowfor the frontalsystems(Fig.2c).Thereisanincreasedgradientintheheightof500hPainthemorerecenttimeperiod,withsignificantlylowerheightsjustsouthoftheregionandsignificantlyhigherheightstothenortheast(Fig.3a).AswiththesealevelpressurechangesshowninFig.2,the500hPa height changes are more related to low pressure events than frontal events.However,botheventtypesareassociatedwithhigherheightsnortheastoftheregioninmore recentyears (Figs.3band3c).Also, in themore recent timeperiod, theareaof
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
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lower500hPaheightslagsslightlybehindfromthechangeovertimeatthesurface.Forextratropicalcyclones,thespatialalignmentofpressurepatternsnearthesurface(sealevelpressure)and inthemiddletroposphere(500hPaheights)arerelatedtothe lifecycleofthesystemandenhancedbaroclinicity.ThechangesovertimeshowninFigs.2and3areindicativeofagreatertiltinthepressurepatternsinmorerecentyears,withsystemsinthestrengtheningphaseoftheirlifecycle.The Eady mean growth rate (Hoskins and Valdes, 1990) is a measure of baroclinicinstabilitywithin theatmosphere that canbeusedasananalogue for stormtracks todeterminehowthepathofastormhaschangedovertime.Theparameter,which isafunctionof theatmosphere’swindand temperatureprofile,hasbeencomputed fromMERRA-2 fieldscorresponding toeachextremeprecipitationevent.Ascomparedwiththeearlierperiodexamined,morerecentextremeprecipitationeventsarecharacterizedbyasmallerEadymeangrowthratetothenorthandeastoftheregion,andlargervaluestothesouthbetween35°and40°N.Thisisindicativeofsystemsassociatedwithextremeeventshavingshiftedsouthinmorerecentyears(Fig.4a).ThesmallervaluesintheEadymeangrowthratetothenorthandeastareseeninbothfrontalandlowpressureevents.ButonlythelowpressureeventsareassociatedwithanincreaseintheEadygrowthratetothesouthinmorerecentyears.Alowpressuresystemdevelopingtothesouthinthemorerecentperiod,wouldadvectwarm,moistoceanicairmoredirectlyintotheNortheastregion,inacounter-clockwisemotionaboutthecenterofthecirculation.Theinteractionoftheadvectedatmosphericmoisture with topography would enable extreme precipitation events to occur. Thissouthward shift in the storm track for extreme events caused by closed lowpressuresystemsisakeyfactorfortheobservedincreaseinextremeprecipitationeventsinthenortheasternU.S.Thecompositeanomaliesforfrontalsystemsare lessconclusiveandfurtherworkwillaimtodiscoverwhethertherehasbeenanincreaseinthenumberoffrontalsystemsmovingthroughtheregion,oriffrontalsystemshavebecomestrongerovertime.Insummary,observationsindicatethattherehasbeenanincreaseinthefrequencyofextreme precipitation events in the northeastern United States. Although tropicalcyclones can cause the most intense events in the region, the recent increase thefrequency of extreme events is related to frontal systems and closed low pressure
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
6
systemsthatareextratropicalinnature.Asouthwardshiftinthetracksoflowpressuresystemshascontributedtotheincreaseinextremeprecipitationeventshoweverthereisstillmoretolearnregardingfrontalsystems.
Figure2.ThedifferenceinsealevelpressureduringobservedextremeprecipitationeventsintheNortheasternUnitedStatesduring1997through2014and1980through1996for(a)allevents,(b)eventscausedbyalowpressuresystem,and(c)eventscausedbyfrontalsystems.Hatchingdenotesareaswithastatisticalsignificanceof95percent.
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
7
Figure3.Thedifferencein500hPaheightonthedayofanextremeprecipitationeventintheNortheasternUnitedStatesduring1997through2014and1980through1996for(a)allevents,(b)eventscausedbyalowpressuresystem,and(c)eventscausedbyfrontalsystems.Hatchingdenotesareaswithastatisticalsignificanceof95percent.
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
8
Figure4.ThedifferenceinEadymeangrowthrateonthedayofanextremeprecipitationeventintheNortheasternUnitedStatesduring1997through2014and1980through1996for(a)allevents,(b)eventscausedbyalowpressuresystem,and(c)eventscausedbyfrontalsystems.Hatchingdenotesareaswithastatisticalsignificanceof95percent.
GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
9
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GMAO RESEARCH BRIEF Synoptic Scale Influences on Increasing Summertime Extreme Precipitation Events in the Northeastern United States
Global Modeling and Assimilation Office NASA Goddard Space Flight Center
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