Title: MODISMeteosatMISR Surface Albedo Comparison Exercise
1MODIS/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
2Various 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
3Various 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
4Surface 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.
5Parameterization 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
6Surface 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.
7Comparison 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
8 Albedo comparison for an Ideal Band
(0.4-1.1µm)
9Govaerts, pers. Com
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11Spectral Correction for the Meteosat large band
effects
12Spectral conversion to the Ideal Band
(0.4-1.1µm)
Meteosat
MISR
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14January 2001
15MISR low MODIS high
MISR high MODIS low
16January 2001
17Histogram of BHRiso differences
18(MISR-MODIS) Albedo
19Using Shunlins conversion factors
January 2001
June 2001
Using Yvess conversion factors
20January 2001
21Mean BHR values over common area with valid
values from one of the two other sensors
22Ratio of the mean values
23Primary Eigenvectors
24Correlation between pairs of samples
25Results for year 2001 Shortwave domain (0.3-3.0
µm)
Africa Southern Europe
26Primary Eigenvectors
27October 2001
28Results for year 2001 Visible domain (0.3-0.7
µm)
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30Results for year 2001 Near-infrared domain
(0.7-3.0 µm)
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32Results for year 2001 Shortwave domain (0.3-3.0
µm)
Northern Eastern Europe
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35Full inversion
Magnitude inversion
Backup solution
36Mean BHR values over common area with valid
values from one of the two other sensors
Full inversion
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39Primary Eigenvectors
Full inversion
40Northern Eastern Europe
Hexadecad 6 End of March 2001
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43Issues 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.
44Africa Southern Europe
45January 2001
June 2001
Magnitude inversion
46Northern Eastern Europe
47January 2001
June 2001
Magnitude inversion
48Conclusions 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.