Atmospheric effect in the solar spectrum

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Atmospheric effect in the solar spectrum

• Vermote et al.
• University of Maryland/ Dept of Geography
• and
• NASA/GSFC Code 614.5

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Solar Energy Paths
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atmospheric contribution
direct direct
diffuse direct
direct diffuse
multiple scattering
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Solar (reflective) spectral domain
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Observation Geometry
Solar zenith angle
View zenith angle
Relative azimuth angle
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Solution of the Radiative Transfer in the
reflective domain for non absorbing atmosphere
and lambertian ground
Ground reflectance ( albedo for lambertian)
Atmospheric reflectance
Atmospheric Transmissions
Apparent reflectance at satellite level
Atmosphere spherical albedo
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Perfect Lambertian Reflector
Radiance of the Perfect Lambertian Reflector
Es
Isotropic radiation
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Simple Radiative Transfer Equation
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SRTE (cont.)
Absorbing ground
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SRTE (cont.)
Ei
Et
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SRTE (cont.)
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SRTE (cont.)
?
v
E0
Er
40
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SRTE 1 interaction (cont.)
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SRTE 2 interactions
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SRTE Multiple Interactions
?groundSatm lt 1 so when n-gt8 then (?groundSatm)n
-gt0 Therefore
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STRE for non absorbing atmosphere and lambertian
ground
Ground reflectance ( albedo for lambertian)
Atmospheric reflectance
Atmospheric Transmissions
Apparent reflectance at satellite level
Atmosphere spherical albedo
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The composition of the atmosphere
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Gaseous Absorption (H2O)
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Modified SRTE to account for absorption
In case of a pure molecular atmosphere (no
aerosol) we can write
m is the air mass 1/cos(?s)1/ cos(?v) Ugaz is
the gaz concentration
10
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Final SRTE approximation
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Water vapor effect for different sensors in the
near infrared
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Scattering angle ,

• The scattering angle, ??? is the relative angle
  between the incident and the scattered radiation

Incident Radiation
Particle
?
scattered radiation
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Phase function

• The phase function, P(?)?? describe the
  distribution of scattered radiation for one or an
  set of particles. It is normalized such as

since
we have
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Rayleigh/molecular scattering 1/4

• Rayleigh or molecular scattering refers to
  scattering by atmospheric gases, in that case

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Rayleigh/molecular scattering 2/4

• The concentration in scatterer is better
  described by the efficiency they scatter at a
  certain wavelength or the proportion of direct
  transmission which is related to the spectral
  optical thickness ????

E0(??
Et(?)/ E0(?)e- ????
Et(??

• For Rayleigh ?????is proportional to ?-4 and for
  standart pressure is 0.235 at 0.45 ?m

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Rayleigh/molecular scattering 3/4

• The rayleigh reflectance, ?R, could be crudely
  approximated by

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Rayleigh/molecular scattering 4/4

• Compute the reflectance of the sky (assumed clear
  no aerosol) at solar noon at 45degree latitude at
  vernal equinox looking straight up at 0.45?m,
  0.55?m, 0.65?m

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Aerosol scattering 1/5

• aerosol scattering refers to scattering by
  particles in suspension in the atmosphere (not
  molecules). The MIE scattering theory could be
  applied to compute the aerosol phase function and
  spectral optical depth, based on size
  distribution, real and imaginary index.

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Aerosol scattering 2/5
Continental aerosol phase function
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Aerosol scattering 3/5
single scattering albedo (0.2-1.0) to account for
absorbing particles
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Aerosol scattering 4/5
Continental aerosol optical thickness spectral
variation
60
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Aerosol scattering 5/5
Continental aerosol single scattering albedo
spectral variation
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Atmospheric effect Vegetation 1/3
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Atmospheric effect Vegetation 2/3
No absorption, Continental aerosol
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Atmospheric effect Vegetation 3/3
Absorption tropical atmosphere, Continental
aerosol
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Atmospheric effect Ocean 1/2
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Atmospheric effect Ocean 2/2
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Perfect Lambertian Reflector
Isotropic radiation
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Different Types of Reflectors
diffuse reflector (lambertian)
Specular reflector (mirror)
Nearly Specular reflector (water)
nearly diffuse reflector
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Sun glint as seen by MODIS
Gray level temperature image
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MODIS data illustrating the hot-spot over dense
vegetation
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BRDF atmosphere coupling correction
Lambertian infinite target approximation
BDRF atmosphere coupling approximation
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Adjacency effect correction
Lambertian infinite target approximation
adjacency effect approximation
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Adjacency effect correction (practical
implementation)
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Adjacency effect correction (testing)
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Adjacency effect correction (testing)
Reflectances observed over a horizontal transect
on the checkerboard. The red bars are the true
surface reflectance, the blue bars correspond to
the top of the atmosphere signal including
adjacency effect. The green bars correspond to
the corrected data using the infinite target
assumption. The open square correspond to the
data corrected for the adjacency effect using the
operational method developed.
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Adjacency effect correction (validation)
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Conclusions

• Review of atmospheric effect including the
  BRDF-coupling and the adjacency effect
• Current version of MOD09 (Collection 4) does not
  include the BRDF-coupling or adjacency effect
  correction
• Collection 5 (start scheduled in Jan06), will
  include adjacency correction for 250m bands.
  BRDF-coupling correction is still being evaluated.