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Validation of Solar Backscatter Radiances Using Antarctic Ice

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Validation of Solar Backscatter Radiances Using Antarctic Ice. Glen Jaross and Jeremy Warner Science Systems and Applications, Inc. Lanham, Maryland, USA. Outline Justification for using ice surfaces The technique, including necessary external information - PowerPoint PPT Presentation
  • Validation of Solar Backscatter Radiances Using Antarctic IceGlen Jaross and Jeremy WarnerScience Systems and Applications, Inc.Lanham, Maryland, USAOutline Justification for using ice surfaces The technique, including necessary external information Error budget where do we focus attention? Results for OMI, TOMS, MODIS, and SCIAMACHY

  • What products benefit most from scene-based calibration ?Cloud Fractions (-independent radiance errors)Cloud studiesEnergy balance and UV irradianceCloud height: errors are directly proportional to cloud fraction (low cloud amounts)Gas vertical column amounts: Air Mass Factor errors directly related to cloud fraction. ( 3% - 5% column NO2 error per 5% cloud error; low cloud amounts)Aerosol Properties (-independent and -dependent radiance errors)0.015 error in single-scatter albedo per 1% radiance errorOptical Depth error 0.06-0.12 per 1%/100 nm -dependent radiance errors

  • Where is scene-based calibration less effective ?Spectral fitting algorithms (e.g. DOAS)Insensitive to low-order-in- calibration errorsConversion from slant to vertical column still sensitiveGas abundances (slant column)Need knowledge of abundance to calculate expected radiances, but gas abundance depends upon calibrationLimb scattering and OccultationNormalizing radiances at a reference height nearly eliminates sensitivity to underlying scene reflectanceMost instruments do not have a nadir view

  • TOMS Earth Probe 360 nm Reflectivity (1996)Antarctica is a good radiance calibration target High Reflectance direct / diffuse TOA radiance ratio greatest radiances least affected by clouds and aerosols Low Aerosol Loading Uniform Reflectance Over a Large Area Highly Repeatable (stable) ReflectanceR (Lambertian) > 0.9590 %100 %

  • TOMS Earth Probe 360 nm Reflectivity (1996)Areas with Slope
  • Are clouds an issue ?Either the cloud model is wrong,or Clouds are statisticallyunimportant

  • Time Dependence of radiometric calibrationSeasonal Cycle: Neglecting terrain height variations Surface reflectance non-uniformityTOMS Nimbus 7 380 nmTOMS Earth Probe 360 nmOMI (Aura) 360 nmGreenlandAntarctica

  • Comparison between sensorsGOME / TOMS-EP Radiance RatioVery early GOME calibrationComparisons need not be over the same time period360 nm331 / 360 nm

  • Validation of Absolute RadiometryDevelop a 2 steradian directional reflectance (BRDF) model for the Antarctic surface; independent of wavelength.Combine BRDF with surface measurements of total hemispheric reflectance measurements; wavelength-dependentCreate a look-up table of sun-normalized Top-of-the-Atmosphere (TOA) radiances for all satellite observing conditions using a radiative transfer modelProcess sensor sun-normalized radiance data from a region of Antarctica chosen for uniformity and low surface slopeCompute ratio between each measurement and table entries; average results

  • Warren et al. Reflectance anisotropy derived from 1986-1992 dataSpectral Albedo Measuremnts at South Pole, 1986 = 600 nmSol. ZA = 80BRDF probably the same: 300-800 nmSurface properties based upon reflectance measurements by Warren et al. BRDF derived from parameterization of measured reflectance anisotropy

  • New Reflectance Measurements by Warren et al. Funded by U.S. National Science Foundation and CNES will support radiometric validation for SPOT4 (Laboratoire de Glaciologie et Gophysique de lEnvironnement) data not yet published Measurements at Dome C, 2003-2005 Spectral BRDF of surface 0.35 2.5 m Solar Zenith Angles 52 - 87 Measure spectral transmission of sunlight into snow Measurements used for inputs to models for effect of clouds on TOA radiances

  • Error BudgetSurface BRDF model represents single largest error source

    100 ( Rel. std. uncertainty




    Ground measurements



    Satellite measurements

    < 0.01


    Atmospheric effects



    Surface BRDF






  • Surface BRDF model vs. Solar Zenith Angle

  • BRDF is most important at longer wavelengthsSimulated Nadir-scene albedos Solar Zenith Angle = 75Column Ozone = 325 DUBRDF plays bigger role asdiffuse / direct ratio decreases

  • OMI ResultsOMI L1b Data:7 Dec 4 Jan, 2004 Perfect model would yield flat SolZA dependence Perfect calibration would yield values = 1 at all wavelengths

    Plot suggests probable radiative transfer errors surface BRDF model treatment of atmosphere We believe that results obtained below SolZA = 70 fall within our 2.2% uncertainty estimate

  • OMI Full spectral range ice radiance resultsFlat spectral result gives us confidence that result is resonable62 < SolZA < 6883 < SolZA < 86Spectral dependence is not realistic consistent with BRDF errorApparent error increases at long as predicted

  • Shadowing ErrorsLarge scale structures (snow dunes) not captured by ground characterizationsFrom Radarsat-1

  • Simple linear shadow model for testing errorsTune barrier height and separation to yield flattest SolZA dependence in data

  • Shadow study using MODIS / AquaComparison to RTM, without correctionComparison to RTM, with shadow correctionConsistent with ~2% uncertainty estimate

  • RTM handles ozone poorly at < 330 nmComparison between MODIS, OMI, TOMS and model radiancesRTM does not include Ring Effect or O2-O2 abs.

  • Preliminary SCIAMACHY ResultsSCIAMACHY Level 1b ( v5.04 )18 24 Dec., 2004Provided by R. van Hees, SRON}Ozone Absorption ignoredComparison with RTM over Sahara (from G. Tilstra, KNMI)

  • Summary Model calculations of TOA radiances over Antarctica are good to approximately 2% at low solar zenith angles (i.e. near Dec. 21) Radiometric characteristics of nadir-viewing sensors can be validated from ~330 nm to ~750 nm Wavelength-to-wavelength radiometry is better than 2%, but not useful for absorption spectroscopy We derive the following sensor calibration errors (preliminary)OMI / Aura: -2.5% (330 < < 500 nm)MODIS / Aqua : -0.5% ( < 500 nm)TOMS / Earth Probe : 0% (331 nm), -1% (360 nm)Future Work : Evaluate more sensors. SCIAMACHY, GOME 2 ? Refine BRDF for improved performance at high SolZA and SatZA

  • Spares

  • X-track dependence is mostly Lambertian near SolZA = 50Results near 50 are least affected by BRDF errorsBRDF surface slices at 67

  • Same time and geographic locationOMI radiances compared directly to MODIS / Aqua band 3MODIS has broad bandwidth (459 < < 479 nm) which includes O2-O2 absorptionOMIMODIS

  • Developed a 2 steradian BRDF model

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