GPMandAirborneRadarInvestigations:PIAEstimationPI:StephenL.Durden

SimoneTanelliJetPropulsionLaboratory,CaliforniaInstituteofTechnology

Email:sdurden@jpl.nasa.gov

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• TheAPR-2airborneprecipitationradarhasbeeninoperationsince2001,collectingdatawithsamefrequenciesandgeometryasDPR

• In2015APR-2wasaugmentedwithW-band(referredtoasAPR-3)OriginallydueforcompletionJuneof2016AcceleratedscheduletoaccommodateOLYMPEX

• APR3acquireddataonallDC8flights,Nov12– Dec19• Amongaccomplishmentsincludetwounderflights ofGPMfor

observationsofprecipitation• TheAPR3Ku/Ka-bandchannelsallowdirectcomparisonwithDPR• TheW-bandchannelprovideshighresolutionobservationsoflight

precipitationandclouds,allowingdual-frequency(Ka/W)retrievals

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Introduction

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The authors gratefully acknowledge support by the Precipitation Measurement Missions Program. The research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Copyright 2016 California Institute of Technology. Government sponsorship acknowledged.

• NUBFeasiertodetectwithDPRbecauseNUBFchangesexpectedratiooftheKu/Ka PIAs

• Calculationbelow(left)ofexpectedvalueofPIAasafunctionofrainrateforlog-normalrain

• Atright,GPMdatashowsamebehavior

NUBFisimportanttounderstandandcorrectsinceitcausesbiasesinradarestimates:

• WhenthereisNUBF,wewouldliketogettheaverageoftheprecipitation• Instead,wemeasuretheaverageofradarreflectivityandsurfacebackscatter• Nonlinearrelationsbetweenprecipitationquantitiesandradarquantitiescausebiases

whenusingaverageradarquantitiestoestimateaverageprecipitation

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ExamplefromNovember18:Ku,Ka, W,DFRKu/Ka,DFRKa/W

Shallowpost-frontalconvectionwithanoverrunningaltostratusice-cloudlayer

• APR-3Ka/WDFRiniceshouldbeindicativeoficeparticlesize;plotsbelowcompareMASCbrightnesstemperatureswithDFR(horizontalaxisistime)– Dec3duringGPMunderflight

• Generally,largerDFRisassociatedwithlowerTb;datasetshouldallowmulti-frequencyand/ormulti-instrumentretrievalsforcomparisonwithGPM

(Co-alignmentofdatacourtesyJ.Turk)

Left:dataforGPMunderflight Dec3Below:Dec19underflight

Underflights allowdirectcomparisonofAPR3andDPRforassessmentofnon-uniformbeam-filling(NUBF)effects

StudiesusingairborneradardatashowthatPIAestimatedwithSurfaceReferenceTechnique(SRT)areespeciallyimpactedbyNUBF

Right:plotofPIAErrorversusstandarddeviationofPIAinsidesimulatedGPMfootprint(Durden&Tanelli 2008)

ExamplecasesuggestsPIAKA~5xPIAKUwhenNUBFissmall(blue)

Datainprecip notshown

• Hence,theory,simulations,andDPRdataconfirmimpactofNUBFonSRT-estimatedPIA

• AnalternativemethodofPIAestimationusesonlytheradarprofile,notthesurfacemeasurement(suggestedbyZ.Haddad)

• Advantages:• Shapeofverticalprofileshouldbeless

impactedbyNUBF• Couldprovidealternativeoverland,

whereSRTislessreliable

Why?Consideracaseinwhichhorizontalvariationisthesameatallaltitudes;simplestexampleishalf-filled,half-empty

Near-surfacerangebinswillbeattenuatedbythePIAduetotherain;surfacebinattenuatedbyonly3dB

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Todevelopprofile-onlyPIAestimate,weneededadatabasewithknownprecipitation,PIA,etc;syntheticdatabase:–Usedmorethan1yearofTRMMPRreflectivityprofiles–Foreachprofileoverocean,allowdropsizedistributiontovary–SimultaneouslyestimateR,Zka,attenuations,PIAs–Result:manyrainprofilesthatareconsistentwiththemeasuredTRMMKu-bandZprofile

• Examinedvariousprofileshapemetrics,includingprincipalcomponents,slope,maxZ,maxdual-frequencyratio(DFR)• BestpredictorofPIAisDFRnearsurface• SmallererrorforGPMPIAestimatewhenusingpredictortrainedfromAPRKu/Ka databasethansynthetic• ScatterplotisKa-bandSRTPIA(vert axis)versuspredictedKa PIAforGPM cases;NUBFcasesareblue• RMSerrorusingGPMKa-bandis2.6dBforcaseswithsmallNUBF• Increasesto4.2dBforlargerNUBF(>5dBstd dev)• SRTbiascanbe15dBinsimilarcases(Durden&Tanelli2008andplotinpanel5,above)

• Summary• APR3collecteddatasetinOLYMPEXthatwillbeusefulformulti-frequency/instrumentretrievals,includingdirectcomparisonwithGPM• Initialanalysisofnon-SRTPIAestimateindicatesmaybepossibletopredictPIAtowithinafewdBusingonlytheobservedreflectivityprofile• OurhypothesisisthatPIAestimationfromprofilemayhavelesserrorthanSRTincaseswithlargeNUBF• TestsofPIAestimationusingDFRnearsurfaceprovides2.6dBRMSerroratKa-band(smalleratKu-band)• ErrorforNUBFcasesislargerbutlikelybetterthanSRTinsomecases

MASCradiometer

APR3W-band

Secondapproachtodatabase:usedAPRKu/Ka-bandmeasurementsoveroceanconvection

NUBF

Windshearwillviolatethisassumption;however,impactmaybelessthanwithSRT

SyntheticdatabaseKa PIA(verticalaxis)versusmeasuredDFRnearsurface(horizontalaxis)

Profile-onlyPIA • Plans:directlyassessNUBFviaOLYMPEXcases,furtherinvestigateutilityofPIAestimationusingprofile-onlyapproach