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Comparison of 6 7 amp m radiances computed from aircraft
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21 304 SAI THE AND SMITH COMPUTED AND OBSERVEDRADIANCES. Whilethismayreflecta drybiasin theVaisalathin filmcapacitive. hygrometer,thisinstrument,isconsidered,thebestinusefor. balloonsoundings,andcompareswell with stratospheric. andgasexperiment,SAGE II observations,Larsenetal 1993. The aircraft basedmoisturemeasurements,usedin this study 228. helpeliminate,someof the uncertainties,thatwereunavoid.
able in the earlier balloon basedstudiesand allow a critical exam. inationof the manyotherfactorsinfluencing,the comparison. betweencomputed andobserved,The datausedin thispaperaresoundings. of theatmospheric,temperatureandmoisture,measured fromtheNCARSabreliner. and6 7 IJ mradiances,by the VAS instrument,GOES 7 satellite i I i. 2 i AircraftData Latitude DegreesNorth,Duringtwofieldprojects.
weconducted, of Figure 2 Channel10 brightnesstemperatureobservedby GOES. theuppertroposphere fromtheNCARSa VAS alongthe horizontaltrackof a typicalaircraftsoundingfor. andlowerstratosphere, onRapidlyInten the satelliteimage closestin time solid line and next closest. breliner Thefirstproject partof theExperiment,sifyingCyclones offthe dashedline to the sounding. overtheAtlantic ERICA wasconducted, Maine coastin January1989 The secondwasbasedin Cham. paign Illinois andEdmonton,Alberta duringJanuary, NCAR Sabreliner was instrumented to measure numerous meteo Twelve profileswere extractedfrom the data The soundings.
generallyextendto an altitudeof 12 km into the lower strato. rological chemical andmicrophysical parameters themeasure. mentsof interestin thisstudyaretemperature moisture andcloud sphere Above this altitude thereis very little water vapor Ell. saesser 1983 and hence little contributionto the outgoing. particleconcentration Temperature wasmeasured witha Rose. montplatinum resistancesensor withaccuracy of 1 K resolution radiancenear6 7 IJ m For the purposesof radiativetransfercalcu. of 0 006K andresponse time of 0 1 s Moisturewasmeasured lations theprofilesof temperature andmoisturewereextrapolated. witha newlydeveloped frostpointhygrometer thatusesa cryogen to an altitudeof 100 km The temperature extrapolation followsan. ically cooledmirror pyers Duran 1990 The cryogenic hy exponential thatmatches themean slope at thetop of themeasured. profileandapproaches a value2 K greateror lessthanthehighest. grometer has1 stemporal and0 05 Kfrostpointresolution anda. frostpointrangeof 10 Cto 90 C Ambientfrostpointisderived observedvalue dependingon theslope The specifichumidityde. via a modifiedGoff Gratschformulation Buck 1981 from theva cayslinearlywith pressure Thismethodensures thatthecomputed. weightingfunctionsdecaysmoothlyto zero withoutan artificial. porpressure yieldedbythehygrometer Thecryogenic hygrometer. frequently performs poorlyat low altitudes andthefrostpointis peakatthetopof theobserved profile The computed radiances. substitutedwith the value from a conventionalchilled mirror hy insensitiveto reasonable variationsin theextrapolation. alsoaboardtheaircraft,Cloudparticle,concentration,was 2 2 Satellite Data. by a forwardscattering,spectrometer,probe FSSP that. detectsthe concentration, andsizedistributionof cloudparticles The VAS channel10 or water vapor WV channelfilter re. sponseis centeredat a wavelengthof 6 725 tm wavenumber. cm 1andwidth,150cm 1 inthe6 5 tm,vibrational rota, 0 01 I tionalabsorption bandof water See for example Montgomery. and Uccellini 1985 for a detaileddescription,of theGOES VAS.
system Figure1 showstheWV channelfilter functionalongwith. 0 008 I 0 8 a high resolutionspectrumof radiancecomputedfrom a typical. aircraftprofile, o o0o Satellitedatawere obtainedfrom the SpaceScienceandEngi. neeringCenterat the Universityof Wisconsinasbrightness tem. o o04 0 4 peraturevaluesalong the satellitescanlines The geographical. positionof individualpixelswas derivedusingthe navigational. t g dataand routinesprovidedwith the imagedata At midlatitudes. o oo the horizontalresolutionof the imagepixelsis about8 km east. I west and 11 km north south, The slopingaircraftsoundings will crossmany satelliteimage. m pixelsasthe aircrafttravelshorizontally Figure2 showsa typical. exampleof the brightness temperature observedoverthetrackof. Fibre 1 Spectrum of radiance,in cgsunitscomput by a line. by lineradiativetransfermodelfroma typic mraft sounding an aircraft. sounding thesolidline shows datafromtheimageclos, solidlinO GOES VAS ch el 10 filter ncfion dash line estin time to thesounding andthedashedline showsdataanhour. e GOES ch nel 10 radi ce is the inte at productof these later The variationin the satellitebrightnesstemperature overthe. two ncfions soundingmakesthe collocationof the aircraftandsatelliteobser. SALATHE AND SMITH COMPUTED AND OBSERVED RADIANCES 21 305. vationsuncertain Typically however the upperportionof the zenith angle0 is computedfrom the aircraftmeasurements of the. sounding above4 km was takenover a smallerhorizontaldis temperatureand moistureprofiles The profilesselectedfrom the. tance but we includedatafrom the returnleg to the airportto fill aircraft data were chosenso that the atmospherewas cloud free. thelowerpartof thesounding Thebrightness temperature is rela abovethe groundor a denselow cloudlayer The lowerboundary. tivelyinsensitiveto theair sampledin thelowerleg thustheob emission. l v zo makesa contribution,to theoutgoingradiance,servedbrightness temperatures.
arebasedonlyontheshorterupper that canbe largewhenthe boundaryis elevated i e a cloud To. leg Thescatter in theobservedbrightnesstemperature overtheup estimateI v zo the lowerboundaryis assumed. to radiateas a, persounding is notsystematic andtheobserved valueis takenas blackbodyat the temperatureobservedby the GOES IR window. theaveragebrightness temperature For sounding E067in Figure channel over the aircraft soundingand is assignedan altitude. 2 theobservedbrightnesstemperature Twv OBS 224 4K was wherethis temperaturematchesthe temperature profile from the. takenfromtheportionnorthof 54 N Thisis compared tothecom aircraft This may yield a different lower boundarytemperature. putedbrightness temperature of 225 3K On the basisof the thanwouldbe derivedfrom the aircraftcloudandtemperatureob. brightness, temperature variabilityfor all cases thereis a random servations but the satellite derived. boundaryconditionis self con,uncertainty, in theobserved brightness temperature of lessthan2 sistentand accountsfor the nonblackemissivityof clouds. 3K The monochromatic radianceis calculatedusingLBLRTM and. In thispaper wewill alsoconsider,theGOES7 channel, 8 orIR theWV channelbrightness temperatureTwv is foundby solving. observations,ofradiances,140cm 1 Since,theatmosphere.
transparent,atthis lt vlvdV,lt vBv Twv dv 1,wavelength. thischannel,givesthetemperature,of theopaque,er boundaryfor theradiativetransfercalculation. where v is the WV channelfilter functionandBv Twv is the. monochromatic emissionof a blackbodyat thetemperature Twv. 3 Radiative Transfer The zenithangle0 is foundgeometricallyfrom thepositionsof. the satelliteand soundinglocation GOES 7 was the only geosta. The third versionof the fastatmospheric signaturecode FAS. tionary meteorologicalsatellite in operationover the western. CODE called the line by line radiative transfermodel LBL. hemisphere duringthe field projects andit wasshuttledbetween. RTM Cloughet al 1981 1992 was usedfor the radiative positionsoverthe eastandwestcoastsof NorthAmerica depend. transfercalculationsin thispaper LBLRTM usesthe HITRAN92 ing uponthe season Thus duringERICA the satellite sviewing. Rothrnanet al 1992 spectraldataand is amongthe mostad anglechangedeachday andits daily positionmustbe takeninto. vanced models for radiative transfer calculations, account The rangeof zenithanglesin thisinvestigationis from44. In the regionof the infraredspectrumcoveredby the GOES to 57. 10filter i e 1413,cm 1 Figure, 1 H20is byfar This methodfor computingthe GOES 7 brightnesstempera. theprincipalgaseous, Rinslandetal 1989 havedemon turesinvolvesseveralassumptions and thereare many sourcesof.
stratedtheimportance, of molecularoxygencontinuum absorption uncertaintythat caninfluencethe results The impactof theseas. in thetransferof radiationin the atmosphere,by observations. sumptionson the comparison of computedand observedbright. lowerstratosphere andit is certainto be discernible. in theupper nesstemperatureswill be discussed,in section6. troposphereaswell To estimate theimpactof methane andmolec. ularoxygenandothertracegasesonthecomputation of theGOES. 6 7 gmbrightness temperatures LBLRTM wasrunwithonlywa 5 Comparison of Computed and Observed. ter vapor with a singleadditionalgas andwith all majorgases Brightness Temperatures. These calculationsindicatethat molecularoxygencontinuumab The brightnesstemperatures observedover the aircraftsound. sorptionreducesthecomputed brightnesstemperatureby lessthan ings and simulatedas describedaboveare summarizedin Table 1. 0 4 K methaneby lessthan0 06 K andall gasestogetherby less Entriesin columnslabeledGOES IR indicatethe lowerboundary. than 0 5 K so that the effect of all othermajor constituents. is un conditionsasestablishedfrom the IR window channelobservation. detectable The contributionof the oxygencontinuum however is. andthoselabeledAircraftindicatethe temperature,andheightof. nontrivial We performedcalculationsusing FASCODE2 and the lower boundaryas observedfrom the aircraft The following. HITRAN86 andfoundthatthe changesmadein theseupdatesap columnidentifiesthe type of lower boundary The next two col. pearto havelittle effecton thecomputed,brightness,temperatures umnslist thebrightnesstemperatures.
observedby GOES andcom,for the GOES WV channel, putedby LBLRTM The final threecolumnsindicatethe upper. Toyieldanexceedingly,fastline by line,computation,LBL tropospherichumidity UTH as definedbelow. RTM optimizesthenumberandplacement of computational. layers In Figure3 the computedbrightness,temperatures. are plotted, basedon a user provided maximumratio of Voigt widths 1 05 against the GOES observations The solid line showsthe linear. andtemperature difference variesfrom 2 0 K nearbaseto 2 5 K trendof theresults andthedottedline indicateswherecomputed. at top betweenadjacentlayers Alteringthe verticalresolution andobservedvaluesare equal To summarizetheseresultsandal. doesnothavea significant effectonanyof themodelresults Since low comparisonto otherstudies Table 2 listsstatisticsfor the com. the brightnesstemperature dependsbothon the watervaporand parison The linearcorrelation. is quitehigh andtheslopeis nearly, temperature profiles the verticalresolutiondoesnot havea sys 1 indicatingthat the computationreturnsthe full rangeof ob.
tematicimpacton the computedresult servedbrightness temperatures The computed brightnesstemper. atures however are on average greater than the observed. brightnesstemperatures This biasimpliesthatthe aircrafthumid. 4 Computed BrightnessTemperatures, ity measurements indicatea drier atmospherethan would be in. Themonochromatic radiance l v Zoo O alongthepathfrom ferred from the satelliteobservationsor that the atmosphereis. the surfaceto Zoo top of the atmosphere, takenas 100km with more opaquethan indicatedby the radiativetransfermodel The. 21 306 SALATHEAND SMITH COMPUTED AND OBSERVEDRADIANCES. Table 1 Summaryof Results,IRa Aircrafta Surface Twv. T C z km T C z km Type GOES LBLRTM GOES LBLRTM Aircraft. E017 34 6 2 40 7 cloud 232 0 232 6 47 5 44 3 30 3,E043 15 2 4 24 4 cloud 233 6 236 8 37 1 25 7 29 1. E044 6 1 8 0 0 ground 242 6 244 6 12 9 10 2 12 5,E054 32 5 0 32 5 cloud 227 4 231 2 48 2 31 1 38 2.
E060 4 1 0 10 2 cloud 235 4 238 6 27 7 19 1 25 6,E067 32 4 0 42 5 cloud 224 4 225 4 61 3 54 9 38 0. E072 24 0 0 22 0 ground 228 1 227 1 44 1 49 6 41 6. E083 3 0 0 4 0 ground 245 6 244 4 9 0 10 3 4 6,E084 1 0 0 5 0 ground 235 5 238 7 26 1 18 0 7 6. C063 19 4 2 10 3 cloud 229 0 236 0 50 3 22 4 27 0,C083 9 2 0 10 1 cloud 233 0 238 8 31 8 16 2 13 9. C103 10 2 5 5 0 cloud 233 0 236 3 31 8 21 7 25 2,Twv is watervaporchannel. brightness,temperature,UTH is uppertropospheric,GOESrefersto satelliteobservations.
refersto computedvalues seesection5,aretemperature. T andheight z ofthelowerboundary, biasin thepresent of 2 6 K ismuchbetter ThefactorPois theratioof thepressure. LBLRTM computations at ambienttemperature, thanthe 5 27 K biasfoundby Hayden 1988 andcomparable to 240 K to 300 mbar The valuesof UTH computedastheweighted. the 2 4 K biasfoundby Sodenet al 1994 B J Soden private meanof the aircrafthumiditymeasurement labeledAircraft and. communication 1995 computedfrom 2 usingboththe observedandcomputedbright. For illustrativepurposes, thebrightness temperatures maybe nesstemperatures GOES andLBLRTM aregivenin Table 1. interpreted asuppertropospheric,humidityaccording toa relation Thisinterpretation.
of Twv asUTH showsthatthe2 6 K biasin, shipderived bySoden andBretherton 1993 1996 Theweighted the LBLRTM computedbrightnesstemperatureis equivalentto. meanuppertropospheric humidityis relatedto theWV channel the satelliteUTH being 10 percentage pointsmoremoistthanthe. brightness temperatureTwv by UTH derivedfrom the computedbrightness temperature Another. way to expressthe magnitudeof theradiancebiasis to determine. assumingour radiancecalculations are perfect the amountof ad. 31 5 0 115Twv 2 ditionalwatervaporneededto reducethe calculatedradiancesto. theobservedvalues A full doublingof the specifichumidityis re. quired This adjustmentlieswell outsidethe hygrometererrorand. thusdoesnot helpto explainthebiasbut rather indicatesits sig. 250 nificanceandthepotentialproblemsin determining moisturefrom. radiance observations,6 Discussion,Thereareseveraluncertainties. in computing thesatellitebright, nesstemperatures from atmosphericprofiles Someare deficien. ciesin the observations suchas the lack of cloudparameters to. characterizethe lower boundary or errorsin the humiditymea. surement Other uncertainties arise from limitations in the current. understanding andtheoryof radiativetransferin theuppertropo. sphereat thesewavelengths suchas water vaporcontinuumab. sorption We shall discusstheseuncertaintiesto illustratetheir. contributionto the total discrepancy,betweenthe computedand. o observedbrightnesstemperatures, 220 230 240 250 Table 2 Summaryof ComparisonStatistics.
ObservedBrightness,Temperature,Parameter Value, Figure 3 Scatterplotof computedandobservedbrightness. tem Correlation 0 92,peratures Circlesdenotecalculations. from LBLRTM The solid Slope 0 9 0, line is the linear tend and the dotted line indicateswhere the com. Bias K 2 6, putedand observedvaluesare equal The correlation slope and. Std Dev of Bias 2 4, biasof thecomputedandobserved resultsareshownin Table2.
AND SMITH COMPUTEDAND OBSERVED,RADIANCES 21 307,6 1 Evaluation of Water Vapor Measurement 12. The accuracyof the hygrometeris criticalto this studyandan. attemptwas made verify the NCAR ResearchAviation Facility. RAF specifications givenabove Underice saturated conditions. the frostpoint shouldmatchthe ambienttemperature sothe frost. point observedin deepcold cloudsprovidesan in situ calibration 8. of the hygrometeragainstthe temperatureprobe Figure4 shows. datacollectedduringan ascentthrougha thickcloudthatextended. to the tropopause at 12 km The presenceof cloudis indicatedby. the FSSP and the cloudis quitethick throughout the midtropo. sphere Figure4 right The frostpointtemperature firmly follows. the ambienttemperature throughthecloud Figure4 left The ra 4. tio of theambientwatervaporpartialpressure to thevaporpressure. at saturationwith respectto ice is near1 0 withinthecloud Figure. 4 fight aswouldbe expected The saturation ratiofluctuatesabout. 1 0 by 0 1 in thethintopof thecloud thismaybe theresultof dis. equilibriumdueto radiativecoolingandmixing Supersaturation is. observednear6 km wherethe cloudis likely a mixtureof ice and o. 80 60 40 20 0, waterdroplets At temperatures belowabout 40 C watermayex. ist in the liquid phase producingsupercooledcloud droplets Temperature. Heymsfieldet al 1991 Under suchconditions the frostpoint Figure 5 Simultaneous. atmospheric,fromtheNCAR, temperatureexceedsthe ambienttemperatureas observedin the Sabreliner solid line and a balloon borneVaisala instrument. lowestlevelsin Figure4 The abilityof the frostpointhygrometer package dashedline. to accuratelyindicatesaturated conditionsis alsoobservedduring. descentthrougha cloud not shown illustratingthe negligibleef. fectof the slowerresponse time of thehygrometerrelativethether uppertroposphere Larsenet al 1993 Only oneof the Vaisala. soundingswas suitablefor comparison of the moisturemeasure. We conducteda seriesof soundingswith the Sabrelinerin con ments andtheresultsareshownin Figure5 The aircraftpassed. junctionwith the releaseof Vaisalaradiosondes, The thin film hy within 4 km of the balloon launch site and the balloon was re. gristor in the Vaisala instrumentpackage is consideredmore leasedmidwayin the aircraftdescent The regionsmarkedas. reliable for upper troposphericmoisturemeasurements than the cloudyweredetermined fromtheaircraft bothbytheFSSPandby. carbonhygristorusedoperationallyby the U S NationalWeather sight Assuming theatmosphere wassaturated. withrespect to ice, Service andit compareswell to SAGE II derivedmoisturefor the withinthecloudregions theVaisalasondeappears moreaccurate.
at low levels thetwoagreeat midtroposphere andthecryogenic. hygrometer is moreaccurate in theuppertroposphere Thecryo. Saturation ratio wrt ice genichygrometer generallydoesnotrespond well at frostpoints. 0 4 0 8 1 2 above about 20 C where another instrumentis used as noted. above also thelowestsegment,of theaircraftsounding. duringan unusuallyrapiddescentdueto deterioratingweather. 12000 1200O,conditions,and hencethe cryogenic hygrometer may not have. beenabletorespond quicklyenoughtotherapidmoisteningasthe. aircraftpassedthroughthe cloud, Thehygrometer hasalsobeenchecked against a cryogenic va. 10000 por trap, 1O00O collectiontechnique during a field project in October. 1993 The accuracyof the vapor trapcollectionis estimated to be. 20 andthecryogenic hygrometer agreeswiththecollected water. vapor concentrations to within theselimits The cryogenichy. grometerhasalsobeenusedrecentlyby Heyrnsfield et al 1991. who reportgoodaccuracy We estimatethatthe cryogenic hy. grometerprovideswatervaporconcentrations massmixingratio. to well within10 whichyieldsanuncertainty in thecomputed. brightnesstemperature of lessthan0 4 K, Sodenet al 1994 founda similardiscrepancy to what we.
80 40 0 0 5 10 presentabove 2 4 K comparedwith 2 6 K for a comparison of. Temperature,Point C Cloud,Concentration, cm GOES brightness temperatures to computations usingVaisala. soundings Figure5 showsthatthethin filmcapacitance hygrom. Figure 4 left Atmosphericsoundingof temperatureand frost. point in degreesCelsius conductedfrom the National Centerfor eterreportstoodry values in moistlayersbut thatit agrees withthe. AtmosphericResearch NCAR Sabrelinerasthe aircraftascended cryogenic hygrometer in drylayers Whilethissingleprofilecan. througha thick cloud This soundingwasusedto checkthe cryo not establishthe relativeperformance of the two instruments it is. genichygrometer fight Corresponding profileof cloudparticle importantto notethat they agreeunderthe conditionsmostrele. concentration andtheratioof the ambientvaporpressure to theva vantto this study thatis in cloud free middleto uppertropo. por pressureat saturationwith respectto ice sphere Thus deficiencies in the moisture measurementare. 21 308 SALATHEAND SMITH COMPUTEDAND OBSERVEDRADIANCES. factorin creatingthedisparity spectraldifferencesfor computedand observedupwardradiances. unlikelyto be themostsignificant, betweenobservedand computedbrightnesstemperatures at high altitudeof Revercombet al 1990 Figure 3 however. shows the differences become negative between 1400 and. 6 2 Horizontal Variability 1500cm 1 where,theGOESwater. spectralregion the FASCODE2 computationsexceedthe ob. As discussedin section2 2 the slantedaircraft soundingsex. byupto 1 0mW m,forhigh altitude, tendhorizontallyoverseveralsatellitepixels We haveassumed. nadir viewingHIS observationsfrom a U2 over the ocean The. thatthehorizontalvariabilityin themoistureandcloudfieldsdoes. 2 6 K brightness,temperaturediscrepancywe find is approximate.
not significantly,affecttheresults,The flightsusedin thisstudy. wereintended toprobetheundisturbeduppertroposphere andlow. ly equivalent to a 0 5mW m 2strcm l 1 radiance difference. which is in qualitativeagreementwith the HIS results This dis. er stratosphereandthuswere conducted away from regionsof. crepancymay reflect a dry bias in the water vaporobservations. strongbaroclinicity,Horizontal,flightlegswereusuallyavailable. usedfor that study but it is consistentwith our results Neverthe. to confirmthe lack of stronghorizontalgradients One way to. less while thereis someuncertaintyin the spectroscopy at 1400. quantifythis effectis to considerthe variationsin the GOES. 1500cm l it isnotobvious,thatproblems,withthecontinuum. brightnesstemperatures overtheflighttrack whichare on aver. sorptionaccountfor the discrepancyin computedand observed. age 2 K Figure2 Interpreting,theerrorin thiswayassumesit to. brightnesstemperatures, be a normallydistributed errorin theobservedbrightness.
ature As such thiseffectcannotexplainthebias onlythescatter. 6 4 Cloud Scattering,of the data ThusGOES brightness temperatures. canbe chosenso, that all but two cases C063 andE083 lie alongthe bestfit line Comparingthe entriesin Table 1 indicatesthat caseswith. Likewise valuescanbe chosensothatsevencasesarewithin 1 K agreementin the IR and aircraft derivedlower boundary E017. of thecorresponding LBLRTM value butonlythreecases E083 E054 E072 E083 E084 give on average betteragreementbe. E072 andE067 allow the choiceof valueslessthanor equalto tweencomputedandobservedbrightnesstemperatures The aver. the computed brightness temperatures Choosingthe observed age difference between the LBLRTM and GOES brightness. brightness temperatures in thiswaycorruptsthedata whichwere temperaturesfor these five casesis only 1 1 K comparedwith. chosen independentof thecomparison withcomputed valuesby an 2 6 K for all cases The biasshowsno clearrelationshipto bound. objectivemethod Nevertheless observed valuescannotbechosen ary height however thusit is unlikelythaterrorsin the boundary. so asto eliminatethe bias Thus while muchof the scatteris attrib emission are at fault since these errors become small when the. utableto misalignment,of satelliteandaircraftobservations. boundaryis low andits radiativecontributiondiminishesrelative. notexplainthedeviation, of all casesfromthebestfit line to the atmosphericemission Rather it appearsthat thereis a cor. respondencebetweendiscrepancies in the IR and WV channels. 6 3 Water Vapor Continuum This may be the resultof misalignmentof satelliteandaircraftob. servationsor mesoscalevariability which would affectboth chan. LBLRTM employs a continuum modelbasedonself broaden. nels but thisexplanationis not consistentwith the caseanalysisin. ingandforeign broadeningof thefarwingsof theabsorption. section6 2 Very thin cirrusice particlesor aerosollayerswould. Cloughet al 1992 The shapeof the far wingsof absorption. lines is determined, by foreign broadening water nitrogen oxy alsoreduceboththe IR andWV channelbrightness temperatures.
whichis a likely explanationof the correspondence in discrepan. gen andself broadening water water interactions,andtheab cies for the two channels. sorptionfromthesuperpositionof thefarwingsof manylinescan. The contributions,of theseeffectsare alsoindicatedby consid. be treatedas a separate,contributionfrom the individualnearby. of eachabsorp eringtheUTHvalues, lines In LBLRTM the line by linecontribution derived. fi omtheaircraft, tionlineistheVoigtprofile l fromthelinecenterSoundingE083 is amongthe driestsoundingsand was a caseof.
outto 25cm, withthevalueat 25cm 1subtracted The very little horizontalvariability The LBLRTM andGOES bright. fromtheVoigtprofile,nesstemperatures,agreequite well for this case SoundingE043. remaining contributionsarethewatervaporcontinuum Clough et. wastakenundermoistconditionsdownwindof a largecirrussys. al 1992 Foreignbroadening of thefar wingsof absorption lines. tem Despitetherebeinglittle horizontalvariabilityin the satellite. is a significant,contributionto atmospheric absorption in theupper. image thiscasedoesnotyield goodagreement betweencomputed. troposphere andwithinthe6 5 1am band wherethelinesarewell. andobservedbrightnesstemperatures, separated andthecontinuum absorption fillsin between thelines. While thereis not a greatdifferencein theabsorptive properties. Cloughet al 1992 Sincethecontinuum usedin LBLRTMin. of ice at 11 gm and6 7 gm a thin cirruscloudnearthe weighting. cludesabsorption fromthelinecenters it is difficultto assess the. isolatedeffectof far wingabsorption ontheresults Nevertheless function peak of the WV channelwould causescatteringand. eliminating continuum absorption in thecomputation results in an lengthenthe optical path Thus a thin cirruscloud could have a. increase in brightness temperature by 2 K whichindicates the largeimpacton the WV channelbrightnesstemperaturesyet notbe. obviousin the IR channelobservations Unfortunately withoutde. importance oftakingthewatervaporcontinuum intoaccount when. tailed cloud microphysicalobservations it is impossibleto satis. computing radiancesin thewatervaporvibrational band. The resultsof severalrecentobservational Revercombet al factorilyaccountfor the effect of thin cloudson our results. 1990 Thdriaultet al 1994 and theoretical Ma and Tipping. 1991 studies, suggest thatthemagnitude of thecontinuum absorp 7 Conclusions.
tionremains in question TMriaultetal 1994 andRevercomb et. al 1990 foundpositivediscrepanciesbetweencomputed andob There is a positivebiasbetweenobservedand computedWV. near1300cm 1thatindicate,theforeign, continu channelradiances A numberof assumptionsand sourcesof error. um absorptionis too strongin FASCODE2 Reducingthe enterthecomphtation thatmaycontributeto thisbias Theseare. would in fact increasethe differencewe startingfrom the leastimportanterrorsourcesandendingwith the. continuumabsorption, find betweencomputedand observedradiances Examiningthe mostimportant zenithangle satellitecalibration lowerboundary. SALATHE AND SMITH COMPUTED AND OBSERVED RADIANCES 21 309. alignmentof observations,andmesoscale, variability cryogenichy fidently move forward in modeling and monitoring upper. grometer continuumabsorption and scatteringby thin high troposphericmoistureand radiationand their effects on the cli. clouds Each error source is summarized below mate, 1 The zenith angleto the satellitevarieddaily duringERICA. soit may be a sourceof uncertainty However at midlatitudeval Acknowledgments Paul Gluhoskyat Yale Universityplayedan in. uesof the zenithangle the sensitivityof the computedbrightness valuablerole in supportingourresearchduringthefield projectsandin de. temperature is only about0 1 K for a degreeof angle velopingsoftwareto view and analyzethe aircraftand satellitedata J. Sun searlier work at Yale Universityin radiativetransfermademany as. 2 The GOES satelliteradiometeris calibratedagainsta hot. pectsof this work easier The NCAR RAF pilots and researchstaff ably. source and a spacereferenceis usedto removea very largeand. supportedour field projects J Warnockandhiscolleaguesat NOAA ERL. variable baselineradiationfrom the telescope Chesterset al conductedthe Vaisalasoundings The GOES VAS datawere providedby. 1982 Both the spacereference 3 K andthe hot source 320 K the SpaceScienceandEngineeringCenterand the Universityof Wiscon. are considerablyoutsidethe range of Earth observations 220 sin This paperbenefitedfrom commentsby B Soden D Chesters and. 260 K and there may be someuncertaintyin the calibration anonymous reviewers This work was supported underNSF Atmospheric. While the 6 7 tmchannelis essentiallyimpossible to operational ScienceDivision grant ATM 912390 and DOE NIGEC grant DE FC03. ly groundtruth the otherchannelsare verified Chesterset al 90ER61010 andthefirstauthorwassupported by a NASA GlobalChange. 1985 Hayden 1988 andsincethesameradiometeris usedfor all ResearchFellowship. channelsit is unlikelyto be severelyin error, 3 The lowerboundaryis assumed to radiatelike a blackbodyat References.
theIR windowchannelbrightness temperature The resultsarenot. Buck A L New equationsfor computingvaporpressureand enhance. sensitiveto boundaryheight indicatingthis is not an important mentfactor J Appl Meteorol 20 1527 1532 1981. factor Chesters D L W Uccellini andA Mostek VISSR atmospheric. 4 Errorsarisingfrommisalignment, of thesatelliteobservations VAS simulationexperimentfor a severestormenvironment Mon. Weather Rev 110 198 216 1982, and the aircraft soundingsmay be skewed However the errors. Chesters D W P Menzel H E Montgomery andW D Robinson VAS. wouldnot be emirelysystematic andare lessthan2 K Thusthis instrumentperformance appraisal. in VASDemonstration,Description, uncertainlycannotproduceto observedbias and Final Report editedby H E Montgomeryand L W Uccellini. 5 The cryogenichygrometerhasbeendiscussed above andit NASARef Publ 1151 1985. is highlyunlikelyto be reportingmoisturelevelsin errorby the Clough S A F X Kneizys L S Rothman andW O Gallery Atmo. sphericspectraltransmittance andradiance FASCODE1B Proc SPIE. amountindicatedby the bias,Soc Opt Eng 277 152 166 1981. 6 The theoryandmeasurement of watervaporcontinuumab Clough S A M J Ianono andJ L Moncet Line by linecalculations of. sorptionare the subjectof ongoingresearch and thereis much atmospheric fluxesand coolingrates Applicationto water vapor J. variation in how the continuum is modeled in radiation codes De Geophys Res 97 15 761 15 785 1992. ficienciesin modelingcontinuumabsorptionare unlikelyto ac Ellingson R G J Ellis and S Fels The intercomparison. of radiation, codesusedin climatemodels Longwaveresults J Geophys Res 96.
count for the full bias in computedradiances but they may 8929 8953 1991. contribute The resultsin thispaperdo not allow an assessment. of Elliott W P and D J Gaffen On the utility of radiosondehumidityar. themodelingof continuum absorption, buttheydomakeclearthat chives for climate studies Bull Am Meteorol Soc 72 1597 1520. thispr ocessis essentialto properlymodelinguppertropospheric. radiation Ellsaesser H W Stratospheric,water vapor J Geophys Res 88 3897. 7 The effectsof scattering, by thinhighcloudsonradiationhave Gaffen D J T P Barnett andW P Elliott Spaceandtimescalesof glo. beenneglected in computing thebrightnesstemperatures in Table bal tropospheric moisture J Clim 4 989 1008 1991. 1 sincethe cloudsdetectedin the soundingspresentedwere as Hayden C M GOES VAS simultaneous temperature moisture. sumedto be opaqueandto formthelowerboundary As discussed algorithm J Appl Meteorol 27 705 733 1988. Heymsfield A J L M Miloshevich A Slingo K Sassen and D O C. above the possibilityof thin highcloudsescaping detectionmay Starr An observational andtheoreticalstudyof highlysupercooled al. likely havean impacton ourresults Thuscloudeffectsmayhave tocumulus J Atmos Sci 48 923 945 1991. an importantimpactin the remotesensingof uppertropospheric Larsen J C E W Chiou W P Chu M P McCormick L R McMaster. water evenwherethey arenot obviousin theuppertroposphere S Oltmans and D Rind A comparison. of the SAGE II uppertropo, Using excellentquality moistureprofile observations and a spheric. watervaportoradiosonde,measurements,4897 4918 1993.
state of the art, radiationcode computations of outgoingradiances Ma Q andR H Tipping A farwinglineshapetheoryanditsapplication. at 6 7 I tmdo not satisfactorily, matchobservations from spaceby to the watercontinuumabsorption in the infraredregion I J Chem. the GOES satellite The brightness temperatures computedusing Phys 95 6290 6301 1991. LBLRTM are on average 2 6 K too high whichcanbe interpret Menzel W P W L Smith andL D Herman Visibleinfraredspin scan. ed asbeingtoo dry by aboutl0 percentage pointsin relativehu radiometeratmospheric sounderradiometric calibration. An inflight, evaluationfrom intercomparisons with HIRS andradiosondemeasure. midity In analyzing the results it does not appear that any ments Appl Opt 20 3641 3644 1981. assumptions madein the computationare responsible for the dis Montgomery H E andL W Uccellini VASDemonstration Description. pancy Rather spectroscopic,uncertainties, i e watervapor and Final Report NASARef Publ 1151 1985. continuum andscattering by undetected cloudsarelikely to be the Poc M M M Roulleau N A Scott andA Chedin Quantitativestudies. of METEOSAT water vapor channeldata J Appl Meteorol 19 868. sourceof the discrepancy The inabilityto accuratelyperformthe 876 1980. forwardcomputation of satellite observedradiances from quality Revercomb H E R O Knuteson W L Smith H M Woolf and H B. atmospheric observations makesuncertaintheaccuratemonitoring Howell Spectroscopicinferencesfrom HIS measurements of atmo. of uppertropospheric moisturefromspace While theobservations sphericthermalemission in Optical RemoteSensingof the Atmo. usedin thisstudydo not allow a resolutionof the problem the re sphere OSA Tech Dig Ser Vol 4 pp 590 593 Opt Soc of Am. Washington D C 1990, sultssuggestmechanisms that yield greateropacityof the upper Rinsland C P R Zander J S Namkung C B Farmer andR H Norton.
troposphere Uncertaintiesin theseprocesses andin uppertropo Stratosphericinfraredcontinuumabsorption observedby the ATMOS. sphericradiation in general mustbe resolvedbeforewe cancon instrument J Geophys Res 94 16 303 16 322 1989. 21 310 SALATHE AND SMITH COMPUTED AND OBSERVED RADIANCES. Rothman L S andcollaborators, The HITRAN moleculardatabase Edi Starr D O C andS H Melfi The role of watervaporin climate A stra. tionsof 1991and 1992 J Quant Spectrosc, Radiat Transfer 48 469 tegicplanfor theproposed GEWEX watervaporproject GVaP NASA. 507 1992 Conf Publ 3120 50 pp 1991,Smith W L and collaborators. GAPEX A ground based atmospheric Thtriault H M P L Roney D St Gennain H E Revercomb R O. profilingexperiment Bull Am Meteorol Soc 71 310 318 1990 Knuteson and W L Smith Analysisof the FASCODE modeland its. Soden B J andF P Bretherton Uppertroposphericrelativehumidity H20 continuum basedon long pathatmospheric transmissionmea. from the GOES 6 7 gtmchannel Methodand climatologyfor June surements in the 4 5 11 5 gtmregion Appl Opt 33 323 333 1994. 1987 J Geophys,Res 98 16 669 16 688,Soden B J andF P Bretherton Interpretation. of TOVS watervaporra, diancesin termsof layer average relativehumidities.
Methodandcli E P Salatht Jr Departmentof AtmosphericSciences. matologyfor the upper middle andlowertroposphere, J Geophys Universityof Washington Box 351640 SeattleWA 98195 1640. Res 101 9333 9349 1996,email salathe atmos washington edu. Soden B J S A Ackennan D O C Starr S H Melfi andR A Ferrare. R B Smith Departmentof GeologyandGeophysics, Comparisonof uppertroposphericwatervaporfromGOES ramanli Yale University P O Box 208109 New HavenCT 06520 8109. dar andcross chain,loranatmospheric sounding,systemmeasurements email ronald smith yale edu. Res 99 21 005 21 016 1994,Spyers Duran,P Airbornecryogenic.
frostpointhygrometer, NCAR Tech Note NCAR TN 347 lA 31 pp Natl Cent for Atmos ReceivedOctober25 1995 revisedJune4 1996.


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