MODIS/Meteosat/MISR Surface Albedo Comparison Exercise

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MODIS/Meteosat/MISR Surface Albedo Comparison ExerciseB. 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

Various types of Surface albedo (1)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

Various types of Surface albedo (2) Various types of Surface albedo (2)

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

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.

Surface albedo products from space agencies

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

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

• EUMETSAT delivers DHRs for a fixed Sun angle

and all information needed to reconstruct the DHRs and BHRiso

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

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.

Parameterization of the surface-atmosphere radiative coupling

),;,();,(

);,();,(),;,(

000000

sfcdiff

sfciso

dirsfcsfc

zfzBHR

zfzDHRzBHR

Assuming that the field of downwelling diffuse intensity reaching the surface is PERFECTLY isotropic yields a convenient parameterization for the BLUE SKY ALBEDO

Surface level

Sun angle

Atmospheric optical depth (type of atmosphere)

Surface BRF (amplitude and shape)

ratio of direct to total downward flux

ratio of diffuse to total downward flux

1),;,();,( 0000 sfc

diffdir zfzf with

Pinty et al., JAS, 2005

Surface albedo comparisonSurface 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.

Comparison of Surface BHRiso products from MODIS/Meteosat/MODIS

• Select the same period of time and identical geographical regions

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

• Identify the product values showing appropriate QA

Albedo comparison for an ‘Ideal’ Band (0.4-1.1μm)

Govaerts, pers. Com

Spectral Correction for the Meteosat large band effects

Spectral conversion to the ‘Ideal’ Band (0.4-1.1μm)

Meteosat MISR

January 2001

MISR low & MODIS high

MISR high & MODIS low

January 2001

Histogram of BHRiso differencesHistogram of BHRiso differences

(MISR-MODIS) Albedo

Using Shunlin’s conversion factors

Using Yves’s conversion factors

January 2001

Mean BHR values over common area with valid values from one of the two

other sensors

Ratio of the mean valuesRatio of the mean values

Primary EigenvectorsPrimary Eigenvectors

Correlation between pairs of samplesCorrelation between pairs of samples

Results for year 2001 Shortwave domain

(0.3-3.0 μm)

Africa –Southern Europe

October 2001

Results for year 2001 Visible domain (0.3-0.7 μm)

Results for year 2001 Near-infrared domain

(0.7-3.0 μm)

Results for year 2001 Near-infrared domain

(0.7-3.0 μm)

(0.3-3.0 μm)

Northern –Eastern Europe

Full inversion

Magnitude inversion

Backup solution

other sensors

Full inversion

Hexadecad 6: End of March 2001

Issues and caveatsIssues 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.

Africa –Southern Europe

Magnitude inversion

January 2001

June 2001

Magnitude inversion

January 2001

June 2001

Conclusions and PerspectivesConclusions 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.

MODIS/Meteosat/MISR Surface Albedo Comparison Exercise

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