Validation of SCIAMACHY’s reflectance and polarisation measurements

1. Validation of SCIAMACHY’s reflectance and polarisation measurements Juan Acarreta, Martin de Graaf, Gijs Tilstra, Piet Stammes (KNMI, De Bilt, NL), Matthijs Krijger (SRON, Utrecht, NL)

2. Overview of talk • Introduction and approach • Selected SCIAMACHY nadir state • Comparison SCIA vs. model reflectance in 300-400 and 400-800 nm ranges • Effect of UV calibration on AAI • Conclusions

3. Introduction • R. Siddans et al. (RAL) have found a radiometric calibration offset of the SCIAMACHY nadir reflectances in the UV (320-390 nm cluster) by comparison with GOME, amounting to about –20 %. • Is this due to the polarisation sensitivity of SCIAMACHY and/or to another calibration problem? • NOTE: reflectance (or: reflectivity) R =   radiance / (0  solar irradiance)

4. Approach • For cloudfree scenes with small aerosol loadings the reflectance and polarisation at TOA can be predicted quite accurately using a radiative transfer model, given the spectral albedo of the surface. Especially in the UV, where most surfaces are dark. • We use the polarised Doubling-Adding KNMI (DAK) model. • The spectral surface albedo is taken from the database of Koelemeijer et al. (JGR, 2002, in press), based on GOME data for 1995-2000. This database contains the Lambertian equivalent reflectivity of the surface at 11 wavelengths between 335 and 772 nm, at a spatial resolution of 1°x1°.

5. Sketch of the situation sun SCIAMACHY O3 absorption Rayleigh scattering no clouds no aerosols Sahara surface albedo

6. Selected orbit: 2509 (from verif. database)

8. MODIS image

9. SeaWifs image + selected state (no. 11)

10. Four selected areas of 60 x 280 km²(per area 7 pixels along track)4 (West) 3 (CenterW) 2 (CenterE) 1 (East)

11. SCIA’s solar irradiance spectrum (ch.1-6)

12. SCIA’s earth reflectance spectrum (ch.1-6)

13. SCIAMACHY – DAK comparison Input parameters for DAK: • Surface pressure from meteorological observations • Ozone column from the GOME fast delivery product • Surface albedo from GOME LER database (1995-2000) • Sun-viewing geometry from level 1b data product Output: Reflectance (I/0E), Q/I, and U/I

14. DAK spectrum 300-800 nmof R (‘’I”), Q/I and U/IEast pixelWest pixel

15. SCIAMACHY – DAK comparison • Comparison for 320-390 nm (cluster 9), with polarisation correction off. • Comparison with calculated polarisation effect, using instrumental sensitivity and calculated Q and U. • Comparison for 300-800 nm range (clusters 10, 9,15, 17, 24, 26), with polarisation correction on/off.

16. Pixel 1 (East) – SCIA vs. DAK

17. Relative difference

18. Pixel 2 (CenterEast) – SCIA vs. DAK

19. Relative difference

20. Pixel 3 (CenterWest) – SCIA vs. DAK

21. Relative difference

22. Pixel 4 (West) – SCIA vs. DAK

23. Relative difference

24. SCIA’s polarisation sensitivity for scan mirror position of East pixel

25. Calculated effect of polarisation – East pixel

26. Actual relative difference

27. SCIA’s polarisation sensitivity for scan mirror position of West pixel

28. Calculated effect of polarisation – West pixel

29. Actual relative difference

30. Effect of polarisation correction in SCIAL1Cfor 320-390 nm

31. Pixel 1 (E) for 300-800 nm

32. Pixel 2 (CE) for 300-800 nm

33. Pixel 3 (CW) for 300 – 800 nm

34. Pixel 4 (W) for 300-800 nm

35. SCIA’s polarisation sensitivity for scan mirror position of East pixel

36. Calculated effect of polarisation – East pixel

37. SCIA’s polarisation sensitivity for scan mirror position of West pixel

38. Calculated effect of polarisation – West pixel

39. Conclusions of polarisation correction: Polarisation correction at 320-390 nm: • Polarisation correction goes in the right way for the east pixel. • Polarisation correction does not remove the negative offset of the reflectance spectrum. • Polarisation correction should not have an effect for the west pixel, because light is almost unpolarised there. However, in the product there is a polarisation effect for west pixels. Polarisation correction at longer wavelengths: • Small effect; it cannot explain a large offset.

40. Effect of level 1 calibration offset on AAI • The Absorbing Aerosol index (TOMS residue method) uses the reflectance pair at 340/380 nm • The AAI is normally in the range 0 (no absorbing aerosol) to 4 (high absorbing aerosol load) • A negative offset in the reflectance will affect the AAI: less reflectance means a higher AAI

44. Correlation betweenLevel 2and own AAI

45. Correlation betweenLevel 2and own AAI

46. Conclusions • Comparison of the clear sky TOA reflectance measured by SCIA with a polarised model, shows that SCIA has a UV polarisation correction problem for west pixels and an offset of around –20 % for all pixels. • At the longer wavelengths the surface albedo of the Sahara is dominating the TOA reflectance. This albedo may be variable. It is not clear what the offset is there. • There are some strange spikes in the reflectance.