improvements in the 20th century reanalysis …...improvements in the 20th century reanalysis...

Improvementsinthe20thCenturyReanalysisVersion3

LauraC.Slivinski1,2,GilbertP.Compo1,2,JeffreyS.Whitaker2,PrashantD.Sardeshmukh1,2

1CooperativeInstituteforResearchinEnvironmentalSciences2EarthSciencesResearchLab,NOAA

20th CenturyReanalysisversion3

• Availablefor1851topresent;willbekeptup-to-date• Assimilatessurfacepressureobservationsintoanensembleofmodelforecaststoprovidesubdailyglobalatmosphericconditions• NCEPGlobalForecastSystematT254(0.5deg),64verticallevels(fromT62(2deg),28verticallevelsin20CRv2c)• Assimilationmethod:ensembleKalmanfilterwith80members(from56membersin20CRv2c)

20th CenturyReanalysisversion3

• Availablefor1851topresent;willbekeptup-to-date• Assimilatessurfacepressureobservationsintoanensembleofmodelforecaststoprovidesubdailyglobalatmosphericconditions• NCEPGlobalForecastSystematT254(0.5deg),64verticallevels(fromT62(2deg),28verticallevelsin20CRv2c)• Assimilationmethod:ensembleKalmanfilterwith80members(from56membersin20CRv2c)

• Version3addressesseveralissuesin20CRv2c:fordetails,seeposter“Biasesinthe20th CenturyReanalysisVersion2c”,PosterSession#2,Wedafternoon.

20th CenturyReanalysisversion3Improvementsover20CRv2c

• Adaptivelocalization&inflation:improveuncertaintyquantification

• Improvedmethodtoavoidnumericalinstabilities

• Biascorrectionappliedtomarineobservationspriorto1870

Uncertaintyquantificationinreanalysis

Ensemblespread←→confidence• Asobservationsbecomemorefrequent,theconfidenceintheestimates

increases(uncertaintydecreases.)• Theprocessofassimilationcausestheensemblespreadtodecrease:thiscan

resultinoverconfidentestimates.• Solution:“inflate”theensemblespreadafterassimilation.

EnsemblespreadofsealevelpressureovertheUS,20CRv2c

20CRv2c:prescribedinflation

confidence = 1 − ,ens,clim

Confidence(normalizedensemblespread)ofsealevelpressure,1915

MorecertainLesscertain

ØToouncertaininArctic(largerthanclimatology->unrealistic)

ØTooconfidentoverUSandEurope

20CRv3:adaptiveinflationalgorithm20CRv3:usesrelaxation-to-prior-spread(WhitakerandHamill,2012)

• Amountofinflationisproportionaltotheamountofavailableobservations(atthattimeandlocation)

• Denseobservationnetwork->moreinflation

• Sparseobservationnetwork->lessinflation

Inflationparameter,1Feb1915

20CRv2c,prescribedinflation

20CRv3,adaptiveinflation

Confidence(normalizedensemblespread)ofsealevelpressure,1915

ØToouncertaininArctic(largerthanclimatology->unrealistic)


ØMorecertaintyinArcticØMorespreadoverUSandEurope

BasicensembleKalman filter

ToavoiddiscontinuitiesandinstabilitiesfromanalysisstepsintheensembleKalmanfilter,20CRv2cusedadigitalfiltertosmoothoutjumps.

x x

time

forecast update

observation

Toavoiddiscontinuitiesandinstabilitiesfromanalysissteps,20CRv2cusedadigitalfiltertosmoothoutjumps.

Thedigitalfilterturningon/offcausedartificial3-hourlyjumpsinprecipitationratesfromconsecutive3-hourforecastwindows.

Difference: precipitation between forecast hours 3to 6 minus hours 0 to 3, averaged over 2000-2005

Globalavg=0.73mm/day

Digitalfilterartifactsin20CRv2c

Solution:IAU(IncrementalAnalysisUpdate)toavoidunbalancedgravitywavesinanalysisstep(Bloometal,1996;LeiandWhitaker,2016)

20CRv2cdifferenceinprecipitationrate(hours3-6minushours0-3),avg.over2000-2005

20CRv3difference inprecipitationrate(hours3-6minushours0-3),avg.over2000-2005

Globalavg=0.73mm/day Globalavg=0.02mm/day

Globalsealevelpressure(SLP)

• 20CRv2chasanobviouslow-pressurebiaspriorto1870

• Dryairmasswasupdatedduringassimilation

• Shipobservationswerebiasedlow

20CRv2csealevelpressureanomaly,1854(from1981-2010climatology)

globalSLPaverage:1009.0hPa

Dryairmassconservation&shipbiascorrection

20CRv3conservingdryairmass

ØNolongerincrementingdryairmass;instead,fixat98.3050kPa(Trenberth&Smith,2005)

globalSLPaverage:1009.0hPa 1011.4hPa

Sealevelpressureanomaly,1854(from1981-2010climatology)20CRv2c

incrementingdryairmass

Dryairmassconservation&shipbiascorrection

20CRv2cuncorrectedshipobs

20CRv3uncorrectedshipobs

20CRv3correctedshipobs

ØPre-processshipobservationstoremoveaveragebias,calculatedfromclimatology,fromeachshipseparately

globalSLPaverage:1009.0hPa 1011.4hPa 1011.4hPa

Sealevelpressureanomaly,1854(from1981-2010climatology)

Summary• TheTwentiethCenturyReanalysisprovidesa150+yearhistoryoftheglobalatmosphericstate• Severalbiases(“opportunitiesforimprovement”)in20CRv2c:seeposter“Biasesinthe20th CenturyReanalysisVersion2c”,PosterSession#2,Wedafternoon.

Summary• TheTwentiethCenturyReanalysisprovidesa150+yearhistoryoftheglobalatmosphericstate• Ensemblemethodprovidesameasureofconfidence• Newinflationalgorithmpreventsover-tighteningensembleindenselyobservedregions,withoutincreasingthespreadinsparselyobservedregions(Importantforassessingchanges inclimatevariability)

Summary• TheTwentiethCenturyReanalysisprovidesa150+yearhistoryoftheglobalatmosphericstate• Ensemblemethodprovidesameasureofconfidence• Newinflationalgorithmpreventsover-tighteningensembleindenselyobservedregions,withoutincreasingthespreadinsparselyobservedregions(Importantforassessingchanges inclimatevariability)

• Otherimprovementsinversion3:• Higherresolution• Improveddataassimilationalgorithm• Conservingdryairmass• Biascorrectionappliedtoearly19th C.marineobservations

References• Anderson, J.L.andS.L.Anderson, 1999:AMonteCarloimplementation ofthenonlinear filteringproblemtoproduceensembleassimilationsandforecasts.Mon.Wea.Rev.127, 2741–2758.

• Bloom, S.C., L.L. Takacs, A.M. DaSilva,and D. Ledvina, 1996: Dataassimilationusing incrementalanalysisupdates.Mon.Wea.Rev.,124, 1256–1271.

• Compo,G.P.etal,2011:TheTwentiethCenturyReanalysisProject.Q.J.R.Meteorol.Soc.,137:1–28.• Lei,L.&J.S.Whitaker,2016:Afour-dimensional incrementalanalysisupdatefortheensembleKalmanfilter.Mon.Wea.Rev.,144, 2605-2621.

• Slivinski, L.C.;G.PCompo; J.S.Whitaker;andP.D.Sardeshmukh (inprep):Improvements totheTwentiethCenturyReanalysisversion2c.

• Trenberth,K.E.andL.Smith, 2005: TheMassoftheAtmosphere:AConstraintonGlobalAnalyses. J.Climate, 18, 864–875.

• Whitaker, J.S.,G.P.Compo,X.Wei,andT.M.Hamill, 2004: ReanalysiswithoutRadiosondesUsingEnsembleDataAssimilation. Mon.Wea.Rev.,132, 1190–1200.

• Whitaker, J.S.andT.M.Hamill,2012:Evaluatingmethods toaccountforsystemerrorsinensembledataassimilation.Mon.Wea.Rev.,140, 3078-3089.

Thankyou!

Extraslides

Ensemblespread(representationofuncertainty)20CRv2c:multipliedensemblecovariancebyapredeterminedinflationparameter.

• Jumpsintime• Seamsinspace

AdaptedfromCompoetal(2011)

Ensemblespreadvstemporalspreadforairtemperature(at250hPa)overS.Atlantic

DegK

1.0

2.0

3.0

1851 20111891 1931 1971

1851 – 1890 1891 – 1920 1921 – 19501951 – 2012

Ensemblespread(representationofuncertainty)20CRv2c:multipliedensemblecovariancebyapredeterminedinflationparameter.



Ensemblespreadvstemporalspreadforairtemperature(at250hPa)overS.Atlantic

DegK

1.0

2.0

3.0

1851 20111891 1931 19711951

Ø Ensemblespreadislargerthantemporalspread[climatology]

Ø Jumpin1951,wheninflationparameterisincreased

1851 – 1890 1891 – 1920 1921 – 19501951 – 2012

Solution:updateinflationalgorithm20CRv3:usesrelaxation-to-prior-spread(RTPS)[WhitakerandHamill(2012)]

• Amountofinflationisproportionaltotheamountofavailableobservations(atthattimeandlocation)

20CRv2c:multipliedensemblecovariancebyapredeterminedinflationparameter.


• Denseobservationnetwork->moreinflation

• Sparseobservationnetwork->lessinflation


1851 – 1890 1891 – 1920 1921 – 19501951 – 2012

Uncertaintyinsealevelpressure,JFM(1915-1918)Normalizedbyclimatologicaluncertainty


ØToomuchuncertaintyinArctic(largerthanclimatology->unrealistic)


20CRv2c

Uncertaintyinsealevelpressure,JFM(1915-1918)Normalizedbyclimatologicaluncertainty


ØToomuchuncertaintyinArctic(largerthanclimatology->unrealistic)


ØMorecertaintyinArcticØMorespreadoverUSandEuropeØUnresolvedissue:largespreadinsouthernhighlatitudes

20CRv2c 20CRv3

Actualvs.expectedRMSE(SLP)

ØActualmeansquareerror=meansquareddifferencebetweenobservationandfirst-guess

ØExpectedmeansquareerror=(varianceoffirstguessensemble)+(observationerrorvariance)

ØMeantakeninspaceandtime,thentakesquareroot

1915 1916 1917 19180

0.5

1

1.5

2

2.5

3

3.5

4

Erro

r (hP

a)

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100

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ana

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Northern Hemisphere (20N:90N)

Actualvs.expectedRMSE(SLP)

1915 1916 1917 19180

0.5

1

1.5

2

2.5

3

3.5

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Erro

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Southern Hemisphere (20S:90S)

1915 1916 1917 19180

0.5

1

1.5

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2.5

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Erro

r (hP

a)

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Northern Hemisphere (20N:90N)

v2c, actualv2c, expv3, actualv3, exp

ØConsistentimprovementfromv2ctov3ØNH:v3hassmallererrorsthanv2c;actualandexpectedRMSE’sarecloserØSH:v3actualRMSElowerthanexpectedRMSE,andlowerthanv2cactualRMSE

Nobs,v3

Nobs,v2c

Nobs,v3Nobs,v2c

Ensemblespread(representationofuncertainty)Ensemblespread(SLP)forJFM,1915-1918:20CRv2c Ensemblespread(SLP)forJFM1915-1918:20CRv3

20CRv2c:ØToomuchspreadinArctic(largerthanclimatology->unrealistic)

ØToolittlespreadoverUSandEurope(overlyconfident.)

20CRv3:ØLessspreadinArcticØMorespreadoverUSandEuropeØUnresolvedissue:largespreadinsouthernhighlatitudes

Lesscertain

Normalization:climatologicaluncertaintyofSLP

Normalization:Divideensemblespreadbyclimatologicalspread(temporalspreadofanalysisover1981-2010)andsubtractfrom1.

SLPspreadnormalizedbyclimatologicalspread

Red->morecertainty(thanclimatology)Blue->lesscertainty(thanclimatology)White=climatologicaluncertainty

20CRv3hasincreasedcertaintyintheArctic,andincreasedspreadindata-richregions(suchastheUS&Europe)


Improveddataassimilationalgorithm

ToavoiddiscontinuitiesandinstabilitiesfromanalysisstepsintheensembleKalmanfilter,20CRv2cusedadigitalfiltertosmoothoutjumps.

x x

time

forecast update

observation

00Z 09Z06Z03Z 15Z12Z

unforced Unforcedforecast

Initialanalysis(assimilatingobsin6-hourperiod,usingunforcedforecast)

Ultimateestimatefrompreviousstep

Observation

Solution:IAU(IncrementalAnalysisUpdate)toavoidunbalancedgravitywavesinKalmanupdatestep(LeiandWhitaker,2016)

00Z 09Z06Z03Z 15Z12Z

unforced

△x03△x06

△x09

Unforcedforecast


Ultimateestimatefrompreviousstep

Observation


00Z 09Z06Z03Z 15Z12Z

unforced

△x03△x06

△x09

Unforcedforecast


Ultimateestimate(forcedby△xfromstateatinitialtime)

Observation


Solution:IAU(IncrementalAnalysisUpdate)toavoidunbalancedgravitywavesinanalysisstep(LeiandWhitaker,2016)

1.0

0.0

mm/day

2000 2002 2004

20CRv2c20CRv3

Globalaverageprecip.diff(hours3:6minushours0:3)



TheIAUschemein20CRv3hasconsistentlydiminishedtheartificialjumpsinprecipitationratebetweenhours0to3andhours3to6,visiblein20CRv2c.

20CRv2cdifferenceinprecipitationrate(hours3-6minushours0-3),avg.over2000-2005

20CRv3difference inprecipitationrate(hours3-6minushours0-3),avg.over2000-2005

Averageprecipitation&comparison




Both20CRv2cand20CRv3slightlyoverestimateannualprecipitation,but20CRv3avoidsthespectralringingvisiblein20CRv2c.

Comparison tomonthlydatafromtheGlobalPrecipitationClimatologyProject(GPCP)

Timeseriesofglobally-averagedsealevelpressure(SLP)DJF MAM

JJA SON

20CRv2cAMIPERA20CERA-Interim

Shading=+/- onestandarddeviation(forensembleestimates)

Timeseriesofglobally-averagedsealevelpressure(SLP)DJF MAM

JJA SON

20CRv2chasanobviouslow-pressurebiasinthe1850s(4hPainallseasons)

20CRv2cAMIPERA20CERA-Interim


Biasedshipobservationsin1850s

Pa

20CRv2c:assimilatedall(QC’d)shipobservations Identicalexperimentto20CRv2c,butassimilated~10%fewershipobservations

Analysisincrements(analysisminusfirst-guess)ofSLP,time-averagedover1851-1853

Assimilatingtheshipobservationsinthistimeperiodacttopulltheanalysisdown,suggestingthattheshipsarebiasedlow.

Biasedshipobservationsin1850sHistogramsofassimilatedshipobservations(1851-1860)minusmodernclimatology(1951-1999)forDJF

90S:40S 40N:90N40S:40N

-50 0 50 -50 0 50 -50 0 50

600 9000 400

hPa hPa hPa

Mean=-3.8 Mean=-2.3 Mean=-2.9

Regardlessoflatitude,theshipobservationsarebiasedlowby2-4hPaonaverage.

Cause:biasedshipobservations

20CRv2c:assimilatedall(QC’d)shipobservations Identicalexperimentto20CRv2c,butassimilated~10%fewershipobservations

Analysisestimateminusmodernclimatology[1951-2000]ofSLP,averagedover1851-1853

AssimilatingmoreshipsinthistimeperiodbringstheoverallSLPdownbyabout3hPa,whencomparedtoamodernclimatology.

hPa

Obsminusfirst-guess,Jan1856 Obsminusclimatology(1951-2000),Jan1856

Ø SignandsizeofbiasvariesacrossshiptracksØ Theoverallbehaviorof(obs– f.g.)biasissimilarto(obs– clim)biasØ Potentialbiascorrectionfor20CRv3:pre-processtheobservationsbasedon

(obs– clim)biasescalculatedusingthe20CRv2cclimatology

Individualshiptracks

hPa

Preliminarytest– biascorrectionbyship

fromPhilipBrohan,UKMetOffice

hPa

-10

5

0

-5

1800 1850 1900

AnnualaverageSLPbiasinICOADS3observations(+/- 2std.dev.)

Ø “Bias”– differencebetweenobservationsandmodernclimatology(1981-2010,from20CRv2c)Ø Preliminarybiascorrection– removethemeanoffsetfromeachindividualship

Preliminarytest– biascorrectionbyship

fromPhilipBrohan,UKMetOffice

hPa

-10

5

0

-5

-10

5

0

-5

1800 1850 1900 1800 1850 1900

AnnualaverageSLPbiasinICOADS3observations(+/- 2std.dev.)

AnnualaverageSLPbiasinICOADS3observations(+/- 2std.dev.)afterprelim.biascorrection

Ø “Bias”– differencebetweenobservationsandmodernclimatology(1981-2010,from20CRv2c)Ø Preliminarybiascorrection– removethemeanoffsetfromeachindividualshipØ Producesdesiredeffect,butcouldremoverealsignalsØ Onlypossibleforidentifiableships

TimeseriesofSLP,averagedover60S:90S20CRv2cAMIPERA20CERA-Interim

DJF MAM

JJA SON


Highpressurebiasinsouthernhighlatitudesinearly1900sin20CRv2candERA20C.ParticularlyapparentinERA20CduringDJFseason.

improvements in the 20th century reanalysis …...improvements in the 20th century reanalysis...

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