1. Validation of a new vector version of the 6S radiative transfer code for atmospheric correction of MODIS data AFRL Transmission MeetingJune 16 th, 2005 Svetlana Y. Kotchenova 1, Eric F. Vermote 1 & Raffaella Materesse 2 1 Department of Geography, University of Maryland, USA 2 Department of Physics, University of Bari, Italy

2. Radiation polarization 2 A new version of 6S, which accounts for radiation polarization, has been developed. The real situation account for polarization Vector codes { I, Q, U, V } Scalar codes { I, 0, 0, 0 } The artificial situation no polarization Degrees of polarization 90-100% - molecular atmosphere at 90 0 scattering angle 30% - small aerosol particles 5-25% - snow 0-40% - ice 0-15% - sand 2-23% - vegetation unpolarized radiation polarized radiation

3. Second Simulation of a Satellite Signal in the Solar Spectrum 15 6S is based on the method of successive orders of scattering approximations. Besides its extensive applications in the radiative transfer studies, it is the basic code used for MODIS atmospheric correction. The scalar version of 6S (6SV4.1): accurate simulations of satellite and plane observations accounting for elevated targets use of Lambertian and anisotropic surfaces calculation of gaseous absorption The vector version of 6S (6SV1.0B): all the features of the old version accounting for radiation polarization calculation of highly asymmetric phase functions possibility to change the number of calculation angles and layers

4. 3 Validation of the new version of 6S molecular atmosphere vector mode (with polarization) Monte Carlo (with polarization) the new version of 6S scalar mode (no polarization) molecular atmosphere aerosol atmosphere Coulsons tabulated values DISORTSHARM C O M P A R I S O N aerosol atmosphere MODTRAN C O M P A R I S O N Deutsch

5. 4 Scalar mode – a molecular atmosphere geometry: SZA={0.0, 10.0, 23.07, 45.0, 58.67, 75.0} + a wide range of VZA + AZ={0.0, 90.0, 180} optical thickness: = 0.1 (λ 0.53 μm), = 0.3445 (λ = 0.4 μm) ground reflectance: = 0.0 (black soil), = 0.25 (Lambertian) Agreement is better than 0.015%

6. 5 Scalar mode – an aerosol atmosphere model: continental (70% of dust, 30% of water-soluble, and 1% of soot) geometry: SZA={0.0, 10.0, 23.07, 45.0, 58.67, 75.0} + a wide range of VZA + AZ={0.0, 90.0, 180} waveform: λ = 0.694 μm optical thickness: = {0.210, 0.778} Agreement is better than 0.08%

7. Vector mode – Monte Carlo 6 atmosphere: purely molecular optical thickness: = 0.35 geometry: SZA = {0.0; 23.0; 57.0} + {VZA, AZ} ground surface: black soil Agreement is better than 0.45%

8. Vector mode – Coulsons tabulated values 7 atmosphere: purely molecular geometry: a wide range of {SZA, VZA, AZ} optical thickness: = 0.1 (λ 0.53 μm), = 0.25 (λ 0.44 μm) ground reflectance: = 0.0 (black soil), = 0.25 (Lambertian) Agreement is better than 0.21%

9. Validation of the new version of 6S - Conclusions 8 The new vector version of 6S, which accounts for radiation polarization, has demonstrated good agreement with the Monte Carlo code and Coulsons tabulated values for a wide range of geometrical and atmospheric conditions. The agreement is better than 0.45% for Monte Carlo and 0.22% for Coulsons. The new vector version of 6S, used in scalar mode, has demonstrated good agreement with the scalar codes SHARM and DISORT: better than 0.015% for a molecular atmosphere and better than 0.08% for an aerosol atmosphere. The observed difference is not of concern, as it is much less than the 2% accuracy of raw MODIS top-of-atmosphere reflectance data.

10. Effects of polarization – a molecular atmosphere 9 Error can be more than 10% geometry: SZA={0.0, 10.0, 23.07, 45.0, 58.67, 75.0} + a wide range of VZA + AZ={0.0, 90.0, 180} optical thickness: = 0.1 (λ 0.53 μm), = 0.3445 (λ = 0.4 μm) ground reflectance: = 0.0 (black soil)

11. Effects of polarization – an aerosol atmosphere ( 1 ) 10 dV/dlnr, (μm 3 /μm 2 ) radius, μm SSA = 0.9237 asym = 0.585 Average aerosol volume size distribution (from AERONET measurements) The scalar (no polarization) and the vector (with polarization) versions of 6S have been compared for a biomass burning smoke aerosol model. This model is a typical pattern produced by forest fires over the Amazonian tropical forest region in Brazil. Reference - O. Dubovik et al., J. Atmos. Sci., 59, pp.590-608, 1996

12. Effects of polarization – an aerosol atmosphere ( 2 ) 11 6S (with polarization) vs. 6S (no polarization) biomass burning smoke, λ = 0.67 μm, = 0.72804, SZA = {0.0; 11.48; 23.07; 32.86; 58.67}, AZ = {0; 90.0; 180.0} Error can be up to 5%

13. Retrieval of ocean surface reflectance ( 1 ) 12 MODIS AQUA data, collected over the Hawaii islands, have been corrected using the new version of the 6S code (with polarization) and AERONET measurements collected at Lanai island. The corrected data were compared with surface reflectances measured by MOBY (the Marine Optical Buoy System) just above the ocean surface. The MOBY data dates = {January 2, February 1, February 10, September 3, September 19, October 6, October 22; 2003} λ = {412; 443; 490; 530; 550; 667; 678} nm. * MOBY

14. Retrieval of ocean surface reflectance ( 2 ) 13 measured reflectances AQUA reflectances Corrected MODIS AQUA data vs. MOBY data The agreement between the corrected AQUA and the MOBY surface reflectances was 0.001 to 0.002 for the 400-550 nm region.

15. Effects of polarization - Conclusions 14 Ignoring the effects of radiation polarization leads to large errors in calculated top-of-atmosphere reflectances. The maximum relative error is more than 10% for a purely molecular atmosphere and is up to 5% for a purely aerosol atmosphere. Accounting for radiation polarization is extremely important for atmospheric correction of remotely sensed data, especially those measured over dark targets, such as ocean surface or dark dense vegetation canopies.

16. Release of the new version of 6S 15 Code:ftp://kratmos.gsfc.nasa.gov/pub/eric/6S Web Interface:http://6s.ltdri.org Manual: in preparation (the manual for the scalar version is at ftp://kratmos.gsfc.nasa.gov/pub/6S) Questions:skotchen@kratmos.gsfc.nasa.gov Paper:in preparation (S. Kotchenova, E. Vermote, R. Matarrese, & F. Klemm, Validation of a new vector version of the 6S radiative transfer code for atmospheric correction of MODIS data, to be submitted to IEEE in July 2005.)

17. Future plans 15 Further theoretical validation of the new version of 6S - comparison with Monte Carlo for aerosol and mixed atmospheres - inclusion of anisotropic surfaces - refinement of the code Calculation of new Look-Up Tables for the MODIS (Collection 5) atmospheric correction algorithm Further experimental validation of the new version of 6S - retrieval of ocean surface reflectance and comparison with MOBY data - comparison with ground-measured surface reflectances Use of the new version of 6S to potentially improve the inversion of AERONET measurements (depends on funding). Collaboration with O. Dubovik