floating frozen particle orientation inferred from global...

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).