Retrieval of cloud top height, effective emissivity, and particle size, from aircraft high-spectral-resolution infrared measurements

RetrievalofCloudTopHeight,EectiveEmissivity,andParticleSize,fromAircraftHighSp ectralResolutionInfraredMeasurementsPaolo Antonelli,StevenA. Ackerman,W.aul Menzel,+Allen Huang,BryanA.Baum,William L.SmithCo op erativeInstitute for Meteorological Satellite Studies,Universityof Wisconsin-Madison,1225 W.DaytonSt.,Madison,WI+NASALangleyResearchCenter,Hampton, VAAbstractIn this study we compare dierent approaches to retrieve Cloud Top Height (CTH), Cloud Eective Emissivity(CEE),andthe CloudParticleSize(CPS)fromaircrafthigh-sp ectralresolutioninfraredmeasurements.Twoindep endentmetho dsareusedtoinferCTH.Oneapproachisbasedonahighsp ectralresolutionversionoftheCO2S l icingalgorithmcharacterizedbyastatisticallybasedselectionofoptimalhannelpairs.An-other approach the Minimum Lo calEmissivityVariance algorithm (MLEV) takes advantage ofhigh-resolutionobservations in the 8-12 micron region to simultaneously derive CTH and CEE. Once CTH has b een retrieveda third metho d, based on the comparison b etween simulated and observed radiances,is used to infer CPS andCEE. Simulated radiances are computed for 18 microwindows b etween 8.5 and 12 microns.The cirrus scatteringcalculationsarebasedonthree-dimensional randomlyorientedicecolumns assumingsixdierent particlesizedistributions.Multiplescatteringcalculationsarep erformedfor26dierentcloudopticalthicknesses(COT)between 0 and 20.The simulated radiances are then compared to the observed radiances to infer COT and CPSfor each sp ectral measurement.We applied these approaches to High-resolution Interferometer Sounder (HIS),NationalPolar-OrbitingOp erationalEnvironmental SatelliteSystem Airb orneSounder Testb ed-Interferometer(NAST-I) and Scanning-HIS (S-HIS) data.The preliminary results, consistentbetween the dierent algorithms,suggest that the high sp ectral resolution measurements improve the accuracy of the cloud prop erty retrievals.bkeywords:Hyp ersp ectral infrared data, Cloud prop erties,CO2Slicing,MLEV,Particle Size retrieval1Intro ductionIn the next few years, new instruments will b e launched to improve observations of atmospheric temp erature, watervap or and winds.In the area of infrared remote sensing a key role is represented by the new generation of very highsp ectralresolution instruments(interferometers/sp ectrometers) liketheGeostationary Imaging FourierTransformSp ectrometer(GIFTS),theAtmosphericInfrared Sounder(AIRS),Cross-Track InfraredSounder(CrIS),andthe Infrared Atmospheric Sounding Interferometer (IASI). In the last decade, prototyp es of these instruments havebeenextensively tested on highaltitude aircrafts, and haveprovided thescienti c community withawide varietyof observations.In particular the High-resolution Interferometer Sounder (HIS) [8], the NPOESS Aircraft SounderTestb ed-Interferometer (NAST-I), the Scanning High-resolution Interferometer Sounder (S-HIS), have b een employedinseveral missionscombination withotherinstrumentsliketheCloudLidarSystem(CLS)andMo derate-Resolution imaging sp ectroradiometer Airb orn Simulator (MAS). Using observations obtained by the HIS instrumentduring the SUbsonic aircraft Contrail and Cloud Eects Sp ecial Study (SUCCESS) mission, this pap er aims to provethat the high sp ectral resolution measuremetns will lead to an overall improement of the cloud prop erties retrieval.The pap er also intro duces a new pro cedure that combines dierent approaches to simultaneously retrieve cloud topaltitude,cloudeectivesp ectralemissivityandparticlesize.Thispro cedure,thatcombinesthreedierent algorithms, represents the foundation of the future cloud detection op erational scheme for GIFTS.The rstapproach used to derive CTH and CEE is based on an extension of the broad bandCO2S l icing[2] tothe High-sp ectral resolution case (Hi-sp ectral resolutionCO2SlicingHiC OS l icing).The algorithm relies on thefact that using the broadCO2absorption band at15m,cloudsatvarious levels of the atmosphere can b e detected.In fact theradiances from strong absorption bands are sensitivetoonlyupperlevels while the radiances from thewings see successively lower levels of the atmosphere.TheHiC O2Slicingslicing algorithm determines b oth cloud