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KennethE.KunkelNOAACooperativeInstituteforClimateandSatellites

NorthCarolinaStateUniversity

OverviewofDownscaling

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PURPOSEOFDOWNSCALING

Toproduceclimateinformationandprojectionsthatcanbeusedtoassesstheimpactsofclimatevariabilityandchangeonhumanandnaturalsystemswhoseprocessesoperateatfinerspatialand/ortemporalscalesthanatypicalglobalmodel

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OURPURPOSE

Toproduceclimateinformationandprojectionsthatcanbeusedtoassesstheimpactsofclimatevariabilityandchangeonhumanandnaturalsystemswhoseprocessesoperateatfinerspatialand/ortemporalscalesthanatypicalglobalmodel

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OURPURPOSE

Toproduceclimateinformationandprojectionsthatcanbeusedtoassesstheimpactsofclimatevariabilityandchangeonhumanandnaturalsystemswhoseprocessesoperateatfinerspatialand/ortemporalscalesthanatypicalglobalmodel

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OURPURPOSE

Toproduceclimateinformationandprojectionsthatcanbeusedtoassesstheimpactsofclimatevariabilityandchangeonhumanandnaturalsystemswhoseprocessesoperateatfinerspatialand/ortemporalscalesthanatypicalglobalmodel

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• WhyistheretheneedtodownscaleGlobalClimateModeldata?

• First,whathappensinaGCM?

GlobalClimateModels(GCMs)

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• Theearth’satmosphereisbrokenintogridboxes

• Typicalhorizontaldimensionsofgridboxesis100-200km

• Equationsthatdescribeatmosphericprocessesaresolvedforeachgridbox

• Eachgridboxischaracterizedbyasinglevalueoftemperature,precipitation,humidity,andotherstatevariablesateachtimestep

GlobalClimateModels(GCMs)

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• Surfacetopographyisresolvedatthesamedimensions(100-200km):thesurfaceisessentiallyasetof100-200kmplateaus

• Asaresult,detailsoftopographically-inducedclimatefeaturesarenotwellsimulatedinmountainousregions– Upwindenhancementofprecipitation;rainshadows,etc.

GlobalClimateModels(GCMs)

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• Inaddition,someimportantphysicalprocessesandmeteorologicalphenomenaoccuratsmallerscalesthanthetypicalGCMresolution

• Oneoftheseisconvectiveprecipitation,whichisthedominantforminwarmseasonsandclimates

GlobalClimateModels(GCMs)

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ExamplesofGCMsimulationdata

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FROMGLOBALTOLOCAL

GROWINGSEASONLENGTH

2mTEMPERATURE

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Example– Pune

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DOWNSCALINGintroducesnewinformation intoglobalclimatemodeloutputtogeneratehigh-resolutionclimateprojections

Whatisdownscaling?

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DOWNSCALINGintroducesnewinformation intoglobalclimatemodeloutputtogeneratehigh-resolutionclimateprojections

Wheredoesthisnewinformationcomefrom?

Whatisdownscaling?

fromhigher-resolutionmodelingofphysicalprocessesfromobservations

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DOWNSCALINGintroducesnewinformation intoglobalclimatemodeloutputtogeneratehigh-resolutionclimateprojections

Wheredoesthisnewinformationcomefrom?

Whatisdownscaling?

fromhigher-resolutionmodelingofphysicalprocessesfromobservations

REGIONALCLIMATE

MODELING

EMPIRICAL-STATISTICALMODELING

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Empirical-StatisticalDownscalingTechniques

• Usehistoricalobservationsandmodelsimulationofhistoricalperiodto“train”astatisticalmodel

• Applystatisticalrelationshipstomodelsimulationoffuture(thisassumesthattheserelationshipsremainconstantinthefuture)

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StatisticalDownscalingTechniques

• Simplebiascorrection(”delta”model):• Variancecorrection– Inflateordeflatemagnitudeofdailyvariations

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CalculatesaveragedifferencebetweenpresentandfutureGCMsimulations,thenaddsthatdifferencetotheobservedtimeseriesforthepointofinterest

DeltaChange

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StatisticalDownscalingTechniques

• Quantilemapping– useentireprobabilitydensityfunction,themajoraimbeingtoensurethattheextremevaluesareproperlyrepresented

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EmpiricalQuantileMappingProjectsPDFsformonthlyordailysimulatedGCMvariablesontohistoricalobservations

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StatisticalDownscalingAdvantages

• Computationallyinexpensive• Moderntechniquesproducegoodrepresentationofextremetails

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Dailywet-dayprecipitation

90percentile

Median

10percentile

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StatisticalDownscalingDisadvantages

• Needlengthyandaccurateobservationalrecord(minimum20-30years)

• Cannotproducenewphysics(e.g.can’tgetamesoscaleconvectivesystemifitisn’tintheGCM)

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DynamicalDownscalingTechniques

• Ahighresolutionversionofaclimatemodelisappliedforalimitedgeographicalarea

• Becausethegeographicalareaislimitedinsize,higherresolutionispossiblebecausecomputerresourcesarenotbeingusedtosimulatetheentireglobe

• HOWEVER,aglobalmodelisrequiredtoestablishtheconditionsontheboundaryofthedomain

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• CORDEX:CoordinatedRegionalClimateDownscalingExperiment

• CoordinatedbytheWorldClimateResearchProgramme

• Experimentsbeingconducted/plannedoveralllandareas

CORDEX

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CORDEXSouthAsiaDomain

Thedomaincoversapproximately10%oftheglobalsurface.

Canusecomputerresourcesforhigherspatialresolution

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CORDEXSouthAsiaDomain

Aglobalclimatemodelisneededtoprovidetheconditionsontheboundaryoftheregionalclimatemodeldomain

Specifylateralboundaryconditionsevery6hours

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DynamicalDownscalingTechniques

• OneoftheCORDEXexperimentsisa25kmresolutionsimulationfor1950-2100

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CWRF Improves NCAR Climate Change Projection

No of dry days (precipitation < 0.25 mm)

Present: 1980-2005

Future: 2035-2050

DIF

DIF

Observed Present-day NCAR Present-day NCAR Future Change

CWRF Present-day CWRF Future Change

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PCT95(mm/day)

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DynamicalDownscalingAdvantages

• Themodelcancreateitsownweather,forexample,forphenomenasuchasconvectivesystemsandtropicalcyclones

• Topographically-forcedfeaturescanbesimulatedwithmuchbetterfidelity

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DynamicalDownscalingDisadvantages

• Verycomputationallyintensive• Itisonlypracticallypossibletorunafewexperiments

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• Sometypeofdownscalingisusuallyrequiredtotransformglobalclimatemodelsimulationdataintosomethingthatcanbeappliedtoimpactsassessments

• Empirical-statisticaldownscaling:veryinexpensiveandcanthusemployallavailableGCMs

• Dynamicaldownscaling:canproducenewphysics

Conclusions