floating frozen particle orientation inferred from global...
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FloatingFrozenParticleOrientationInferredfromGlobalPrecipitationMeasurementMicrowaveImager(GMI)andCompactScanningSubmillimeter-wave
ImagingRadiometer(CoSSIR)Polarimetric MeasurementsJieGong1,2,DongL.Wu2
1.UniversitiesSpaceResearchAssociation,Columbia,MD,USA2.NASAGoddardSpaceFlightCenter,Greenbelt,MD,USA
IPWG/IWSSMWorkshop
1. Motivationv Cloudprocessesplayaninstrumentalroleindetermining
thesurfaceprecipitationcharacteristics,butcloudiceprocessesareamongtheleastunderstood.
v Retrievingfrozenparticlemicrophysicalpropertieshasbeenagreatchallengeforspaceborne remotesensing,especiallyforpassivesensors.Itdependsnotonlyonmacrophysical variables(e.g.,columnintegratedmassamount,particlesizedistribution,etc.),butalsoonfrozenparticleshapeandorientation.
v Toinferfrozenparticleshapeandorientationproperties,polarimetric measurementsfromhighfrequencymicrowave(MW)channels(>89GHz)areneeded.GlobalPrecipitationMeasurementMicrowaveImager(GMI)’s89and166GHzpolarimetric measurementsprovideusanunprecedentedopportunitytoinvestigatethesepropertiesonaglobalbasis.
2.DataandMethodologyv GMIisaconical-scanningmicrowaveradiometer.The
Polarimetric Differences(PDs),definedasBrightnessTemperature(TB)differencebetweenverticallyandhorizontallypolarizedchannelpairs(i.e.,TBv - TBH),arecalculatedfor89and166GHz.
v CompactScanningSubmillimeter-waveImagingRadiometer(CoSSIR)flownintheTC4campaignhada640GHz(V,H)channelpair,whichisonlysensitivetocloudicecloud.CoSSIR sharedthesameviewinggeometrywithGMI.
3.UniversalBell-curvePD-TBRelationship 5.RTMSimulationsSupporttheAssumption
4.AConceptualTwo-layerModel 6.Conclusions
v Publication:Gong,J.andWu,D.L.:MicrophysicalPropertiesofFrozenParticlesInferredfromGlobalPrecipitationMeasurement(GPM)MicrowaveImager(GMI)Polarimetric Measurements,Atmos.Chem.Phys.Discuss.,doi:10.5194/acp-2016-787,2016.v Acknowledgement:thisworkandtheauthor’stravelaresupportedbyNASA’s
NNH12ZDA001N-INVESTfund.
Fig.2:JointProbabilityDensityFunctions(PDFs)ofPD- TBV statisticsfromthe[0°,10°N]latitudebinfortheJuly,2014- 2015climatology.
Fig.1:Asqualllinecase(April29,2014)overSouthernU.S.showsthatPDissmallinsideandawayfromthedeepconvectivecores,whileitisthelargestattheanvil/stratiformprecipitationregion.
Fig.3:Bell-curvePD- TBrelationshipholdsacrossdifferentlatitudes,surfaceconditionsandfrequencies.
v Undercloudy-skyconditions,thePD– TBrelationshipfollowsabottom-upbellcurvethatisrobustacrossdifferentlatitudes,surfaceconditionsandfrequencies.ThelargestPDamplitudeisindependentoffrequency(~10K).
v Assumingallicecrystalsarehorizontallyaligned,andV- andH- polextinctionsaredominatedbyscattering,naturallyV- andH-polradiancespassthroughthesamecloudwithslightlydifferentopticaldepths(τV andτH).Atlargeτ,eventuallyTBsaturatesatthesameτ.
v WecancalculatethatPDhasamaximumof 𝑇" − 𝑇$ 𝑒&'( − 𝑒&)*+'( atln 𝐴𝑅 /(𝐴𝑅 − 1),wheretheAspectRatioAR ≡ 𝜏9/𝜏:.Themaximumamplitudeisonlyweaklydependentonfrequency.
Fig.4:Schematictwo-layermodelwithaliquid/rain/surfacelayeratT1 andicecloudlayeratT2toexplaintheobservedbellcurveinthePD-TBrelationship.
Fig.5:ARbetween1.2and1.4(black)bestsimulatestheobservedPD- TBrelationships(redcurves,pinkindicatingthestandarddeviation,clear-skyremoved).
v FrozenParticlesaredominantlyhorizontallyaligned.
v CylindricalColumn(Fig.6b)withAR=1.46for𝐷<= =250𝜇𝑚matchestheobservationsthebest.
Fig.6:MaximumofPD- TBcurveat640GHzissensitivetoicehabitat(diff.panel)andparticlesize(diffcolor)aswell.
v AuniversalPD- TBrelationshiphasbeenidentifiedfromGMI’s89and166GHz,andCoSSIR’s 640GHzpolarimetric measurements.Sucharelationshipisnearlyindependentofchannelfrequency,latitudeandsurfaceconditions,alwayspeakingat~PD=10KatTB=200- 220K.
v Byassumingallfrozenparticlesarehorizontallyaligned,wecanexplaintheobservedPD- TBrelationshipsusingtheAspectRatio(AR)factor.ThisfactorbestreproducestheobservationatAR~1.3.
v Tumblingandmixinginsidethedeepconvectivecore(i.e.,randomorientation)providesanotherplausibleexplanation,whilestableairflowinsideanvilsandstratiform precipitationregionsfosterthehorizontalalignmentofnon-sphericalparticles,resultingthelargestPDsignal.
v PartofthePDsignalcomesfromthestratiform precipitation,especiallyfor89GHz.ButfloatingiceandsnowdominatethePDsignal(notshown).