OMI BSDF Validation Using Antarctic and Greenland 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 Error budget – where do we focus attention? Results for OMI, TOMS, MODIS, and SCIAMACHY

Time Dependence of radiometric calibration Seasonal Cycle: Neglecting terrain height variations Surface reflectance non-uniformity TOMS Nimbus nm TOMS Earth Probe 360 nm OMI (Aura) 360 nm Greenland Antarctica

Validation of Absolute Radiometry 1.Develop a 2  steradian directional reflectance (BRDF) model for the Antarctic surface; independent of wavelength. 2.Combine BRDF with surface measurements of total hemispheric reflectance measurements; wavelength-dependent 3.Create a look-up table of sun-normalized Top-of-the-Atmosphere (TOA) radiances for all satellite observing conditions using a radiative transfer model 4.Process sensor sun-normalized radiance data from a region of Antarctica chosen for uniformity and low surface slope 5.Compute ratio between each measurement and table entries; average results

Warren et al. Reflectance anisotropy derived from data Spectral Albedo Measuremnts at South Pole, 1986 = 600 nm Sol. ZA = 80  BRDF probably the same: nm Surface properties based upon reflectance measurements by Warren et al. BRDF derived from parameterization of measured reflectance anisotropy

Error Budget Surface BRDF model represents single largest error source

Surface BRDF model vs. Solar Zenith Angle SolZA=40  SolZA=60  SolZA=50  SolZA=85 

BRDF is most important at longer wavelengths Simulated Nadir-scene albedos Solar Zenith Angle = 75  Column Ozone = 325 DU BRDF plays bigger role as diffuse / direct ratio decreases Lambertian Non-Lambertian Non-Lambertian / Lambertian Radiance Ratio

OMI Results OMI 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 results Flat spectral result gives us confidence that result is resonable 62  < SolZA < 68  83  < SolZA < 86  Spectral dependence is not realistic – consistent with BRDF error Apparent error increases at long  as predicted

Shadowing Errors Large scale structures (snow dunes) not captured by ground characterizations From Radarsat-1

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

Shadow study using MODIS / Aqua Comparison to RTM, without correction Comparison to RTM, with shadow correction Consistent with ~2% uncertainty estimate

RTM handles ozone poorly at < 330 nm Comparison between MODIS, OMI, TOMS and model radiances OMI / Aura MODIS / Aqua TOMS / EP O 2 O 2 Absorption RTM does not include Ring Effect or O 2 -O 2 abs.

X-track and wavelength error surfaces UV2 VIS Noise in UV2 masks X-track dependence

MODIS / Aqua OMI / Aura TOMS / Earth Probe Shared wavelengths do not behave the same for different sensors X-track errors at selected wavelengths

Comparison of OMI to Predicted Albedos (470 nm) Longer wavelengths  larger errors due to BRDF Why should we trust the solid curve ?

X-track dependence also supports smaller solar zenith angles

Methods agree that there is little wavelength dependence in the BSDF error Evaluation of Aerosol Index wavelengths OMTO3 method utilizes land surface reflectances; cannot evaluate absolute

Solar Irradiance for Ice taken near reference azimuth Comparison of ice and land X-track dependence (360 nm) OMTO3 Ice Ice minus OMTO3 dIrrad error in OMTO3 results can account for much of the difference OPFv8 IRR error

Common structure suggests it’s geophysical UV2 – VIS overlap region looks good Noise in longwave UV2 probably from irradiance

Structure most likely from UV irradiance UV2 – VIS overlap has slight X-track dependence

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 OMI Day 1 BSDF errors Nadir: -2.5% (330 < < 500 nm) Positions 1 & 60 : -4.0% (approximately) Future Work : Re-derive ice and land X-track errors using GDPS 9.15 Confirm shadowing error

Spares

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

Same time and geographic location OMI radiances compared directly to MODIS / Aqua band 3 MODIS has broad bandwidth (459 < < 479 nm) which includes O 2 - O 2 absorption OMI MODIS