MODISMeteosatMISR Surface Albedo Comparison Exercise

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MODIS/Meteosat/MISR Surface Albedo Comparison
Exercise

• B. Pinty (1), M. Taberner (1),
• S. Liang (2), Y. Govaerts (3), J.V. Martonchik
  (4), Lattanzio (5), C. Barker Schaaf (6),
• M. M. Verstraete (1), R. E. Dickinson (7),
• N. Gobron (1), and J-L. Widlowski (1)
• (1) Institute for Environment and Sustainability
  of EC-JRC, Ispra (VA) Italy
• (2) University of Maryland, College Park, USA
• (3) EUMETSAT, Darmstadt, Germany
• (4) Jet Propulsion Laboratory, Caltech, Pasadena,
  USA
• (5) Makalumedia gmbh, Darmstadt, Germany
• (6) Boston University, Boston, USA
• (7) SEAC, Georgia Institute of Technology,
  Atlanta, USA

2nd CEOS/WGCV/LPV Workshop on Albedo Products,
Vienna, April 27-28, 2005
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Various types of Surface albedo (1)
BHR Bi-Hemispherical Reflectance is the ratio
between the upward and the downward radiant
fluxes, that is, accounting for the downwelling
diffuse intensities from the sky.
Depends on both surface and ambient atmospheric
radiative properties and the Sun angle.
All quantities can be defined monochromatic or
broadband
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Various types of Surface albedo (2)
BHRiso If the downwelling diffuse intensities
from the sky is assumed fully isotropic then the
BHR is equal to the integral of the DHR over all
incoming directions (White sky).
Depends on surface radiative properties only.
DHR Directional Hemispherical Reflectance is the
ratio between the upward flux and the downward
collimated flux coming thus from one single
direction (Black sky).
Depends on surface radiative properties and the
Sun angle.
All quantities can be defined monochromatic or
broadband
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Surface albedo products from space agencies

• MISR delivers DHRs and BHRs as flux ratios but
  under ambient conditions and for the Sun
  illumination conditions at time of observations

and all information needed to reconstruct the
DHRs and BHRiso

• EUMETSAT delivers DHRs for a fixed Sun angle

and all information needed to reconstruct the
DHRs at any other Sun angle as well as the
BHRiso

• MODIS delivers DHRs (Black sky) and BHRiso
  (White sky)

to reconstruct the BHRs may require some
investments or some level of assumption
The albedo products may also differ wrt the
spectral bands of integration they refer to.
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Parameterization of the surface-atmosphere
radiative coupling
Assuming that the field of downwelling diffuse
intensity reaching the surface is PERFECTLY
isotropic yields a convenient parameterization
for the BLUE SKY ALBEDO
ratio of direct to total downward flux
Sun angle
Surface level
Atmospheric optical depth (type of atmosphere)
Surface BRF (amplitude and shape)
ratio of diffuse to total downward flux
with
Pinty et al., JAS, 2005
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Surface albedo comparison

• Perform a comparison between MODIS-Meteosat-MISR
  surface albedo products a user perspective.
• Compare similar physical quantities, e,g., BHRs,
  BHRiso, DHRs.
• Based on year 2001 products latest public
  version available.
• For two large geographical regions
  Africa-Southern Europe and North-East Europe.

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Comparison of Surface BHRiso products from
MODIS/Meteosat/MODIS

• Select the same period of time and identical
  geographical regions

• Identify the product values showing appropriate
  QA

• Achieve the needed transformations (e.g., BHRs,
  spectral conversions) to ensure comparison of
  physical quantities having same meaning

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Albedo comparison for an Ideal Band
(0.4-1.1µm)
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Govaerts, pers. Com
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Spectral Correction for the Meteosat large band
effects
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Spectral conversion to the Ideal Band
(0.4-1.1µm)
Meteosat
MISR
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January 2001
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MISR low MODIS high
MISR high MODIS low
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January 2001
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Histogram of BHRiso differences
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(MISR-MODIS) Albedo
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Using Shunlins conversion factors
January 2001
June 2001
Using Yvess conversion factors
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January 2001
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Mean BHR values over common area with valid
values from one of the two other sensors
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Ratio of the mean values
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Primary Eigenvectors
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Correlation between pairs of samples
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Results for year 2001 Shortwave domain (0.3-3.0
µm)
Africa Southern Europe
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Primary Eigenvectors
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October 2001
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Results for year 2001 Visible domain (0.3-0.7
µm)
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Results for year 2001 Near-infrared domain
(0.7-3.0 µm)
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Results for year 2001 Shortwave domain (0.3-3.0
µm)
Northern Eastern Europe
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Full inversion
Magnitude inversion
Backup solution
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Mean BHR values over common area with valid
values from one of the two other sensors
Full inversion
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Primary Eigenvectors
Full inversion
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Northern Eastern Europe
Hexadecad 6 End of March 2001
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Issues and caveats

• An error was recently identified in the MISR
  processing code BHRs tend to be biased high by
  about 2 to 3 on average (season latitude
  dependent).
• The nominal spectral conversion formulae (from
  Liang and Govaerts) agree well.
• MODIS BHRs are off when estimated from the backup
  algorithm.

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Africa Southern Europe
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January 2001
June 2001
Magnitude inversion
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Northern Eastern Europe
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January 2001
June 2001
Magnitude inversion
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Conclusions and Perspectives

• Albedo (BHRiso) comparison reveals very good
  agreement between MODIS-MISR-Meteosat (high QA)
  products.
• Extend the comparison exercise globally.
• Repeat the exercise for DHRs.