1. A new vector version of the 6S radiative transfer code for atmospheric correction of MODIS data
Svetlana Y. Kotchenova1, Eric F. Vermote1, Raffaella Matarrese2, & Frank J. Klemm, Jr.1
1Department of Geography, University of Maryland, USA; 2Department of Physics, University of Bari, Italy
Web Site:
Second Simulation of a Satellite Signal in the Solar Spectrum
is a basic code used for calculation of LUTs in the MODIS atmospheric correction algorithm.
Wavelength ranges: from 350 to 3750 nm
Input geometrical and spectral options:
a user-defined sun-sensor configuration
configurations peculiar to major satellites (MODIS, POLDER, Meteosat, Goes East, Goes West, NOAA PM, NOAA AM, SPOT, & Landsat)
Molecular atmosphere modeling:
6 code-embedded models (tropical, midlatitude summer and winter, subarctic summer and winter, and US Standard 62)
2 user-defined models (with the user-supplied atmospheric parameters and with the user-supplied concentrations of H2O and O3)
Aerosol atmosphere modeling:
6 code-embedded models (continental, maritime, urban, desert, biomass burning, & stratospheric)
a 4-component model with the user-specified percentage of each component
3 statistical distributions with the user-defined parameters (4-modal Lognormal, modified Gamma, and Junge Power-Law)
AERONET sun-photometer measurements
reading of the previously-calculated parameters from a file
Ground surface simulation:
homogeneous (Lambertian)
non-homogeneous with/without directional effect (10 BRDF models)
After interacting with atmospheric molecules and particles the unpolarized light emitted by the Sun becomes partially polarized.
Real situation accounting for polarization
Vector codes { I, Q, U, V }
Scalar codes { I, 0, 0, 0 }
Artificial situation no polarization
Degrees of polarization
atmosphere:
90-100% - Rayleigh scattering at a 900-scattering angle
30% - small aerosol particles
surface:
90-100% - a sun-glint in the specular direction
5-25% - snow
0-40% - ice
0-15% - sand
2-23% - vegetation
unpolarized
radiation
polarized
vector 6S
DISORT
scalar 6S
MODTRAN
SHARM
Monte Carlo
RT3
A new vector version of the 6S radiative transfer code, 6SV1, which enables accounting for radiation polarization, has been developed and validated against other RT codes and ocean surface reflectance measurements.
As its scalar predecessors, 6SV1 is based on the method of successive orders of scattering (SOS) approximations. The effects of polarization are included through the calculation of the 4 components of the Stokes vector, {I,Q,U,V}, describing the intensity of radiation, and degree, plane and ellipticity of polarization of an electromagnetic wave.
In addition to accounting for polarization, the most recent code updates include:
(1) A more accurate calculation of highly asymmetric aerosol scattering phase functions – the number of scattering angles can be varied up to 1000
(2) An arbitrary variation of a vertical aerosol profile – it can be specified by up to 50 layers in the height range from to 100 km
(3) The ability to change the number of calculation angles and layers (By default, the code uses the “standard accuracy” conditions which provide the user with a relative accuracy of approximately 0.4%.)
(4) The increase in the number of node wavelengths from 10 to 20
A β-version of the vector 6S (6SV1.0B) was publicly released in May 2005 and can be downloaded through ftp://kratmos.gsfc.nasa.gov/pub/eric/6S. A special Web interface which can help an inexperienced user learn how to use the code and to build the necessary input files is available at
Validation
Effects of polarization
Conclusions
- Ignoring the effects of polarization leads to large errors in the calculated TOA reflectances. The maximum relative error is more than 10% for the molecular atmosphere and is up to 5% for the aerosol atmosphere.
- The accounting for polarization is extremely important for atmospheric correction of remotely sensed data, especially those measured over dark targets, such as ocean surfaces or dark vegetation canopies.
A. Molecular atmosphere
To demonstrate the importance of the accounting for radiation polarization in a molecular atmosphere, top-of-atmosphere (TOA) reflectances calculated by 6SV1 have been compared to those calculated by the scalar code SHARM.
The comparison was performed under the following conditions:
- optical thickness  = {0.3445; 0.1} for λ = {400; 530} nm
solar zenith angle SZA = {0.0°; 10.0°; 23.07°; 45.0°; 58.67°; 75.0°}
relative azimuth AZ = {0.0°; 90.0°; 180°}
- view zenith angle VZA varied from 0° to 79°
- background surface reflectance  = 0.0
B. Aerosol atmosphere
TOA calculated with 6SV1 in scalar and vector modes have been compared for a ‘biomass burning smoke’ aerosol model. This type of aerosol is usually produced by forest fires over the Amazonian tropical rainforest region in Brazil. The volume size distribution of aerosol particles, retrieved from AERONET (the Aerosol Robotic Network) measurements, is shown on the right.
The comparison was done under the following conditions:
wavelength λ = {470; 670} nm
- optical thickness  = (hazy atmosphere)
- solar zenith angle SZA = {0.0°; 11.48°; 23.07°; 32.86°; 58.67°}
- relative azimuth AZ = {0.0°; 90.0°; 180°}
Applications
the new version of 6S
vector mode (with polarization)
scalar mode (no polarization)
molecular atmosphere
aerosol atmosphere
molecular + aerosol atmosphere
Coulson’s tabulated values
Monte Carlo
ocean water-leaving reflectances
SHARM
MODTRAN
DISORT
Estimation of ocean water-leaving reflectances
MODIS AQUA data, collected over the Hawaii islands, have been corrected using 6SV1 and AERONET measurements taken at Lanai Island. The estimated water-leaving reflectances were compared with those measured by MOBY (the Marine Optical Buoy System) just above the ocean surface at λ = {412; 443; 490; 530; 550} nm.
The MOBY measurements were conducted during the year of 2003 on January 2, February 1, 10. September 3, 19, and October 6, 22.
A. Molecular atmosphere (some of the results)
The performance of 6SV1 in vector mode has been validated against Coulson’s tabulated values, representing the exact solution of the Rayleigh problem with polarization, and Monte Carlo simulations. For both Coulson’s and MC, ground reflectance  = 0.0 and optical thickness  = {0.1; 0.25} for λ = {530; 440} nm.
The agreement between the corrected MODIS and the MOBY-measured water-leaving reflectances is to for the { }-nm region. A simple regression analysis reveals a slight underestimation (about 2%) of the corrected MODIS reflectances.
Monte Carlo:
- SZA = {0.0°; 23.0°; 50.0°}
photons for each 
Coulson’s:
References
- SZA = {0.0°; 23.07°; 36.87°; 53.13°; 66.42°; 78.46°}
- AZ = {0.0°; 90.0°; 180°}; VZA ranges from 0° to 79°
B. Aerosol atmosphere
B. Aerosol atmosphere (some of the results)
The performance of 6SV1 in vector mode has been validated against Monte Carlo simulations. The atmosphere was represented by a clean maritime aerosol model, consisting mainly of biogenically
produced sulfate (0.457%), and sea-salt particles in nuclei (0.538%) and accumulation (0.005%) modes. This type of aerosol is produced by the oceanic areas of remote maritime environments and occurs mainly in the southern hemisphere between the equator and 600.
The validation was performed under the following conditions:
- λ = 550 nm
- solar zenith angle SZA = {0.0°; 23.0°; 50.0°}
- optical thickness  = 0.2 (clear atmosphere) and 0.7 (hazy atmosphere)
- surface reflectance  = 0
photons were processed for each value of optical thickness
Future directions