Validation of a new vector version of the 6S radiative transfer code for atmospheric correction of M

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Validation of a new vector version ofthe 6S
radiative transfer code for atmospheric
correction of MODIS data
Svetlana Y. Kotchenova1, Eric F. Vermote1
Raffaella Materesse2 1Department of Geography,
University of Maryland, USA 2Department of
Physics, University of Bari, Italy
AFRL Transmission Meeting
June 16th, 2005
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Radiation polarization
A new version of 6S, which accounts for radiation
polarization, has been developed.
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Second Simulation of a Satellite Signal in the
Solar Spectrum

• 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

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Validation of the new version of 6S
Deutsch
C O M P A R I S O N
C O M P A R I S O N
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Scalar mode a molecular atmosphere

• geometry SZA0.0, 10.0, 23.07, 45.0, 58.67,
  75.0 a wide range of VZA AZ0.0, 90.0, 180
• optical thickness ? 0.1 (? 0.53 µm), ?
  0.3445 (? 0.4 µm)
• ground reflectance ? 0.0 (black soil), ?
  0.25 (Lambertian)

Agreement is better than 0.015
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Scalar mode an aerosol atmosphere

• model continental (70 of dust, 30 of
  water-soluble, and 1 of soot)
• geometry SZA0.0, 10.0, 23.07, 45.0, 58.67,
  75.0 a wide range of VZA AZ0.0, 90.0, 180
• waveform ? 0.694 µm

• optical thickness ? 0.210, 0.778

Agreement is better than 0.08
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Vector mode Monte Carlo
Agreement is better than 0.45
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Vector mode Coulsons tabulated values

• atmosphere purely molecular
• geometry a wide range of SZA, VZA, AZ
• optical thickness ? 0.1 (? 0.53 µm), ?
  0.25 (? 0.44 µm)
• ground reflectance ? 0.0 (black soil), ?
  0.25 (Lambertian)

Agreement is better than 0.21
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Validation of the new version of 6S - Conclusions

• 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.

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Effects of polarization a molecular atmosphere

• geometry SZA0.0, 10.0, 23.07, 45.0, 58.67,
  75.0 a wide range of VZA AZ0.0, 90.0, 180
• optical thickness ? 0.1 (? 0.53 µm), ?
  0.3445 (? 0.4 µm)
• ground reflectance ? 0.0 (black soil)

Error can be more than 10
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Effects of polarization an aerosol atmosphere
(1)
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
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Effects of polarization an aerosol atmosphere
(2)
6S (with polarization) vs. 6S (no
polarization) biomass burning smoke, ? 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
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Retrieval of ocean surface reflectance (1)
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 ? 412 443 490 530
550 667 678 nm.
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Retrieval of ocean surface reflectance (2)
The agreement between the corrected AQUA and the
MOBY surface reflectances was 0.001 to 0.002 for
the 400-550 nm region.
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Effects of polarization - Conclusions

• 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.

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Release of the new version of 6S

• 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_at_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.)

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Future plans

• 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

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