MODIS/Meteosat/MISR Surface Albedo Comparison Exercise

1. 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

2. 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 bothsurface and ambient atmosphericradiative properties and …the Sun angle. All quantities can be defined monochromatic or broadband

3. 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 onsurfaceradiative 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 onsurfaceradiative properties and …the Sun angle. All quantities can be defined monochromatic or broadband

4. 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.

5. 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

6. 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.

7. Comparison of Surface BHRisoproducts 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)

9. Govaerts, pers. Com

11. Spectral Correction for the Meteosat large band effects

12. Spectral conversion to the ‘Ideal’ Band (0.4-1.1μm) Meteosat MISR

14. January 2001

15. MISR low & MODIS high MISR high & MODIS low

16. January 2001

17. Histogram of BHRiso differences

18. (MISR-MODIS) Albedo

19. Using Shunlin’s conversion factors January 2001 June 2001 Using Yves’s conversion factors

20. January 2001

21. Mean BHR values over common area with valid values from one of the two other sensors

22. Ratio of the mean values

23. Primary Eigenvectors

24. Correlation between pairs of samples

25. Results for year 2001 Shortwave domain (0.3-3.0 μm) Africa –Southern Europe

26. Primary Eigenvectors

27. October 2001

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

30. Results for year 2001 Near-infrared domain (0.7-3.0 μm)

32. Results for year 2001 Shortwave domain (0.3-3.0 μm) Northern –Eastern Europe

35. Full inversion Magnitude inversion Backup solution

36. Mean BHR values over common area with valid values from one of the two other sensors Full inversion

39. Primary Eigenvectors Full inversion

40. Northern –Eastern Europe Hexadecad 6: End of March 2001

43. 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.

44. Africa –Southern Europe

45. January 2001 June 2001 Magnitude inversion

46. Northern –Eastern Europe

47. January 2001 June 2001 Magnitude inversion

48. 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.