Mie scattering

1
Validation of the 6S Radiative Transfer Code for
Atmospheric Correction of MODIS data Svetlana Y.
Kotchenova (skotchen_at_kratmos.gsfc.nasa.gov)
Eric F. Vermote (eric_at_kratmos.gsfc.nasa.gov)
6S (no polarization) - Second Simulation of the
Satellite Signal in the Solar Spectrum This code
is based on the method of successive orders of
scattering approximations. It enables accurate
simulations of satellite and plane observations,
accounting for elevated targets, use of
anisotropic and Lambertian surfaces, and
calculation of gaseous absorption. References
(1) The 6S User Guide (2) Vermote et al., IEEE
T. Geosci. Remote, 35(3), pp.675-686,
1997. Availability by request from E. Vermote
Rayleigh scattering is the scattering from
molecules. The intensity of scattered light
isproportional to the ( 4) degree of the
incident waveform ? themain reason why the
cloudless sky is blue. This type of scattering is
characterized by a symmetric phase
function Accurate modeling of molecular
scattering is especially important for satellite
monitoring of ocean color and the inversion of
aerosols over land, which require measurements in
the blue spectral range.
Comparison study 6S (no polarization) has been
tested against SHARM for several different
wavelengths over a wide range of geometrical
combinations of view (VZA) and solar (SZA) zenith
angles and relative azimuths (AZ). The selected
values of atmospheric optical thickness included
0.1 (? 0.53 µm), 0.3445 (? 0.4 µm), 0.5 (?
0.36 µm). The ground surface was assumed to be
Lambertian with reflectance ? 0 (black soil)
and ? 0.25. Some results of the comparison are
presented on the right. Both codes calculated the
reflectance at the top of the atmosphere as an
output parameter. The relative difference was
calculated using SHARM as a reference.
n o p o l a r i z a t i o n
s c a l a r c o d e s
SHARM-1D This code is designed to compute
monochromatic radiance / fluxesin the shortwave
spectral region over Lambertian and anisotropic
surfaces. The atmospheric properties can be
changed arbitrarily in the vertical dimension.
The code is based on the modified method of
spherical harmonics. References (1) Manual
Code SHARM-1D (2) Muldashev et al., J. Quant.
Spectrosc. Radiat. Transfer, 61(3), pp.393-404.
1999. Availability by request from A.
Lyapustin, NASA GSFC.

• Results
• The maximum relative difference between the
  outputs of 6S (no polarization) and SHARM does
  not exceed 0.3. In general, the absolute
  difference between the codes slightly increases
  with the increase of VZA or optical thickness.
  The use of ? 0.25 leads to a decrease of the
  maximum difference to 0.2.
• The models difference is not of concern, as it is
  more than 6 times less than the 2 accuracy of
  raw MODIS top-of-atmosphere reflectance data

where ? is the scattering angle
A polarized electromagnetic wave is described by
a set of 4 Stokes parameters, which are related
to the amplitudes of the components ( and
) of the electric field If an
electromagnetic wave is not polarized, then
DISORT Discrete Ordinates It is one of the
most heavily tested codes available for
plane-parallel atmospheric models. The simulated
physical properties include thermal emission,
scattering, gaseous absorption, and bidirectional
reflection and emission at the lower boundary.
The code is based on the discrete ordinate method
for radiative transfer. Reference (1) The
DISORT Manual (2) Stamnes et al., Appl. Optics,
27(12), pp.2502-2509, 1988. Availability
ftp//climate1.gsfc.nasa.gov/users/ftp/wiscombe
Comparison between 6S (no polarization) and
SHARM purely aerosol atmosphere, 70 of dust
30 of water-soluble particles, optical thickness
0.72804, SZA 0.0 11.48 23.07 32.86
58.67, AZ 0 90.0 180.0
Mie scattering This type of scattering
predominates for particle sizes largerthan the
wavelength ?. It is characterized by an
asymmetric phase function P(?), which produces a
scattering pattern similar to an antenna lobe,
with a sharper and more intense forward lobe for
larger particles. This scattering is not strongly
?-dependent the reason of the appearance of
almost white glare around the sun when a lot of
particular (e.g. dust, soot) material is present
in the air.
p o l a r i z a t i o n
Monte Carlo (with polarization) In this
computation one photon at a time is followed on
its three-dimensional path through the
atmosphere. The various events which may happen
to the photon at various heights are defined by
suitable probability distributions. This code is
considered a benchmark for any other RT code,
as it does not have any limitations except for
large amounts of calculation time and angular
space discretization. Reference F.-M. Bréon, J.
Atmos. Sci., 49, pp.1221-1232, 1992. Availability
by request from F.-M. Bréon, CEA/DSM/LSCE Gif
sur Yvette, France.
v e c t o r i a l c o d e s
Comparison study 6S (no polarization) has been
tested against SHARM at ? 0.694 µm for a wide
range of geometrical combinations of SZA, VZA and
AZ. The selected aerosol model was similar to the
standard continental model 70 of dust 30 of
water-soluble particles. The aerosol phase
function was calculated as in SHARM. Two
valuesof optical thickness, 0.072804 and
0.72804, werechosen to simulate clear and
hazy atmosphericconditions. 6S (no
polarization) and SHARM have also been compared
with DISORT to make sure of a correctuse of
SHARM. Some results of the comparison are
presented onthe right. In comparison between
SHARM and 6S,SHARM was used as a reference. In
other cases,DISORT was the reference.
6S (with polarization) Availability by request
from E. Vermote, UMd

• Results
• The maximum relative difference between
  reflectances calculated by 6S (no polarization)
  and SHARM does not exceed 0.35. As in the
  caseof the purely molecular atmosphere, the
  absolute difference between the codes slightly
  varies in dependence of VZA and optical
  thickness.
• DISORT and SHARM demonstrate better agreement
  with the maximum difference within 0.1, which
  can be explained by the equivalency of the
  methods for the azimuthally independent
  component.
• Again, the models difference is negligible
  compared to the 2 accuracy of raw MODIS
  top-of-atmosphere reflectance data

Aerosol model parameters(phase function, single
scattering albedo (SSA) and asymmetry factor (g))

• Validation of the new version of 6S which
  accounts for light polarization against Monte
  Carlo (with polarization) and K.L. Coulsons
  tabulated values (Coulson et al., Tables Related
  to Radiation ..., 1960) for a purely molecular
  atmosphere.
• Testing of the new version of 6S in the scalar
  mode. Comparison with SHARM and DISORT for purely
  molecular and aerosol atmospheres under different
  atmospheric and surface boundary conditions.
• Demonstration of the importance of the effects of
  polarization

Objectives
SSA 0.9398g 0.6969
Experimental applications of the new version of
6S (with polarization)
Comparison study 6S (with polarization) and SHARM
have been tested against Coulsons tabulated
values (which, according to S. Chandrasekhar,
represent the complete solution for the Rayleigh
problem) for the case of a purely molecular
atmosphere with optical thickness of 0.1 and 0.5,
for a wide range of geometrical conditions. 6S
(with polarization) has also been tested against
Monte Carlo for a purely molecular atmosphere
with optical thickness of 0.35 (? 0.4 µm), for
SZA 0.0, 23.0, 57.0
Ocean surface reflectance 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 graph below shows
the results of the comparison of surface
reflectances measured by MOBY (the Marine Optical
Buoy System) just above the ocean surface with
AQUA estimated reflectances at ?412 443 490
530 550 667 678 nm. The MOBY measurements
were conducted during the year of 2003 on January
2, February 1, February 10, September 3,
September 19, October 6, October 22.
The agreement between the corrected
AQUA and the MOBY surface reflectances was 0.001
to 0.002 for the 400-550 nm region.
Biomass burning smoke Surface reflectances
calculated with the scalar (no polarization) and
the vectorial (with polarization) versions of 6S
have been compared for a biomass burning smoke
aerosol model to study the effects of
polarization for an aerosol atmosphere. The
selected aerosol model is a typical pattern
produced by forest fires over the Amazonian
tropical forest region in Brazil (O. Dubovik et
al., J. Atmos. Sci., 59, pp.590-608, 1996). The
comparisonwas made for ? 0.67 µm. The results
of the comparison showedthat the relative
difference between the atmospheric
reflectances,calculated with and without account
for light polarization, can beas large as
5.

• Conclusions
• Ignorance of the effects of light 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.
• The new vectorial version of 6S, which accounts
  for polarization, has demonstrated good agreement
  with Monte Carlo and Coulsons tabulated values
  for a wide range of geometrical and atmospheric
  conditions. The agreement is better than 0.5 for
  Monte Carlo and 0.3 for Coulsons.
• The new vectorial version of 6S, used in the
  scalar mode, has demonstrated good agreement with
  the scalar code SHARM better than 0.3 for a
  purely molecular atmosphere and better than 0.35
  for a purely aerosol atmosphere.
• Account for light 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.
• We would like to thank F.-M. Bréon for providing
  the Monte Carlo code and A. Lyapustin for
  providing the SHARM-1D code. We would also like
  to thank both of them for helpful discussions and
  suggestions during this study. This work was
  supported by NASA contract NNG04HZ17C.

• How to account for light polarization?
• To make a scalar code account for polarization,
  one needs
• to modify the molecular phase function to account
  for dipole moments of molecules
• to add new components to the aerosol scattering
  phase function to describe the polarization state
  of scattered light.It means the replacement of
  the MIE subroutine.
• to add 3 new components ( ) describing
  polarization to the radiative transfer part of
  the code. It means the replacementof the
  radiative transfer part (or the core) of the
  code.

easy
relatively easy
not easy at all