1. GOME-2 Polarisation Study — Final Presentation L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1) SRON; (2) KNMI EUMETSAT, Darmstadt, 28-11-2008

2. 2 Overview 1)Introduction of validation techniques 2)Evolution of the data processor 3)Task 1 – Special geometries analysis 4)Task 2 – Limiting atmospheres analysis 5)Solar irradiance measured by the PMDs 6)Errors on Stokes fractions and main science channel radiance 7)Task 3 – PMD raw mode 8)Summary 9)Recommendations

3. 3 1) Validation of GOME-2 polarisation data (Stokes fraction Q/I) A.Inspect so-called “butterfly diagrams” of Q/I versus (Q/I) ss B.Focus on special geometries along the orbit where Q/I = 0 C.Limiting atmospheres approach D.Focus on the solar irradiance (sunlight is unpolarised) Available techniques:

4. 4 A. Butterfly diagrams In general, we expect 0 < Q/I < (Q/I) ss for most measurements. Exceptions are:  near-backscattering geometries  sunglint geometries  rainbow geometries (Θ between 130° and 155°) (v4.0)

5. 5 B. Special geometries where Q/I = 0 A.Situations where cos(2χ ss ) = 0 [or: χ ss =45° or 135°] +many situations are found, along virtually the entire orbit (because of the large range of viewing angles and the small pixel sizes in scan direction) +many situations are found, along virtually the entire orbit (because of the large range of viewing angles and the small pixel sizes in scan direction) +very high accuracy (for each day of data) +very high accuracy (for each day of data) – these are special situations where (U/Q) ss is undetermined, and the data processor treats these situations in a special way by setting U/I = 0 (!!) – these are special situations where (U/Q) ss is undetermined, and the data processor treats these situations in a special way by setting U/I = 0 (!!) B.Backscatter situations (Θ = 180°) +rainbow and sunglint situations are automatically filtered out +rainbow and sunglint situations are automatically filtered out – situations are only found “around the equator” (φ–φ 0 ≈180°) – situations are only found “around the equator” (φ–φ 0 ≈180°) – situations occur for a very small range of viewing angles – situations occur for a very small range of viewing angles The results from approach (B) agree completely with those of approach (A) Q/I = P·cos (2χ) (P = degree of polarisation, χ = direction of polarisation)

6. 6 C. Limiting atmospheres PMD 14 (~756 nm): Very low scattering optical thickness (~0.02); mostly single scattering processes. The depolarised limit is reached for clouds. The polarised limit is obtained in the case of a “black surface” (ocean and sea). For the depolarised limit we have Q/I≈0. For the polarised limit we have Q/I≈(Q/I) ss. PMD 2 (~317 nm): Higher scattering optical thickness. Single, but also multiple scattering processes. Black surface: soil, vegetation, ocean, or sea.

7. 7 2) Evolution of the data processor

8. 8 “Butterfly diagrams”: (rainbow geometries removed) (v4.0)  General behaviour ok  Scatter for the shorter wavelengths Scatter is caused by very low signal levels. Happens rarely. Could even be corrected by bringing the Q/I back to their single scattering value.

9. 9 Processor version 3.4 / PMD band definition v1.0 14 June 2007, one orbit 1.Wavelength dependent offset 2.Scan-angle dependence 3.Two branches exist blue points: |cos(2χ ss )| ≤ 0.01 red points: 177° ≤ Θ ≤ 180° green points: (Q/I) ss Branches: Northern and Southern part of the orbit, separated by the principal plane (where the backscattering points are located).

10. 10 Processor version 3.7 / PMD band definition v2.0 9 October 2007, one orbit blue points: |cos(2χ ss )| ≤ 0.01 red points: 177° ≤ Θ ≤ 180° green points: (Q/I) ss Introduction of PMD band definition v2.0 has a large effect, especially in the UV. Offsets have changed, but there is clearly a strong overcorrection.

11. 11 By studying the ratio of the (calibrated) solar irradiance spectra measured by PMD-p and PMD-s, and by shifting PMD-s w.r.t. PMD-p by 0, 1, 2, 3, and 4 detector pixels, we found a misalignment of ~2 detector pixels in the UV more appropriate than the ~4 detector pixels shift in PMD band definition v2.0. Cause of the overcorrection: The wavelength calibration of the PMDs turned out to be incorrect, leading to an overestimation of the spectral misalignment of PMD-p w.r.t. PMD-s, and hence an overcorrecting by PMD band definition v2.0. EUMETSAT improved the wavelength calibration of the PMDs, resulting in a new PMD band definition, v3.1.

12. 12 Processor version 3.8 / PMD band definition v3.1 5 February 2008, one orbit blue points: |cos(2χ ss )| ≤ 0.01 red points: 177° ≤ Θ ≤ 180° green points: (Q/I) ss 1.Offsets ≈ 0 (for exact nadir) 2.Scan-angle dependence 3.Branches have joined together Idea: scan-angle dependence of polarisation key data incorrect. EUMETSAT: new polarisation key data obtain from PMD raw mode.

13. 13 blue points: |cos(2χ ss )| ≤ 0.01 red points: 177° ≤ Θ ≤ 180° green points: (Q/I) ss Processor version 3.9 / PMD band definition v3.1 / New polarisation key data 26 March 2008, one orbit Scan-angle dependence better, but can still be improved. There are still systematic offsets, but only for the longer wavelengths (so, not caused by spectral misalignment PMDs). Processor version 4.0: sign of U/I. Effect on Q/I small, large effect on radiance main science channels.

14. 14 3) Task 1 – Special geometry analysis

15. yellow: 3.7 a red: 3.8 a + b 02/2008 green: 3.9 b + c 03/2008 blue: 4.0 b + c + d 06/2008 Data since 01-01-2008 (10 months) Every 3 rd day is processed, in total 95 days PMD band definition v3.1 is active since the beginning of March 2008 a)PMD band definition v1.0 b)spectral calibration fixed c)PMD band definition v3.1 + new key data d)change in sign Stokes parameter U Special geometries: results [recent near-real time data]

16. Special geometries: results [recent near-real time data] yellow: 3.7 a red: 3.8 a + b 02/2008 green: 3.9 b + c 03/2008 blue: 4.0 b + c + d 06/2008 a)PMD band definition v1.0 b)spectral calibration fixed c)PMD band definition v3.1 + new key data d)change in sign Stokes parameter U Data since 01-01-2008 (10 months) Every 3 rd day is processed, in total 95 days PMD band definition v3.1 is active since the beginning of March 2008

17. Special geometries: results [reprocessed data set v4.0] PMD band definition used: green: v1.0 blue: v3.1 03/2008 There is a (small) dependence on scanner angle (except for PMD 15: large scan- angle dependence) Data since 01-01-2007 (22 months) Every 6 th day is processed, in total 99 days PMD band definition v3.1 is active since the beginning of March 2008

18. Special geometries: results [reprocessed data set v4.0] PMD band definition used: green: v1.0 blue: v3.1 03/2008 There is a (small) dependence on scanner angle (except for PMD 15: large scan- angle dependence) Data since 01-01-2007 (22 months) Every 6 th day is processed, in total 99 days PMD band definition v3.1 is active since the beginning of March 2008

19. 19 PMD 8: Looks ok: no transition occurs when going from data measured with PMD band definition v1.0 to data measured with PMD band definition v3.1. PMD band definition used: green: v1.0 blue: v3.1 Accuracy of the method: 0.001–0.005 Trend due to degradation?

20. 20 PMD 7: Clear transition in going from PMD band definition v1.0 to PMD band definition v3.1 (improvement) ; small scan-angle dependence, increasing with time. PMD band definition used: green: v1.0 blue: v3.1 Trend due to degradation?

21. 21 PMD 12: In this case, the situation seems to have worsened. However, the wavelengths in the two PMD band definitions are very different (744 versus 589 nm). PMD band definition used: green: v1.0 blue: v3.1

22. 22 PMD 1: Transition (improvement). Deviating behaviour (at a scanner angle of about –40 degrees) is caused by measurement 245 in each (backward) scan. Reported. PMD band definition used: green: v1.0 blue: v3.1 Trend due to degradation? (reset pixels: 241-244)

23. 23 PMD 15: The error was reported and the bug was traced down by EUMETSAT. The bug will be fixed in the next version of the GOME-2 data processor (v4.1). PMD band definition used: green: v1.0 blue: v3.1 This large scan-angle dependence was not present in version 3.9. (version 3.9 should give the same results as version 4.0 for the special geometries)

24. 24 4) Task 2 – Limiting atmospheres analysis

25. 25 Limiting atmospheres: results for recent near-real time data Slopes of linear fits O: polarised limit ◊: depolarised limit Data since 01-01-2008 Every 3 rd day is processed, in total 95 days PMD band definition v3.1 is active since the beginning of March 2008 ? temporal behaviour ?

26. 26 Limiting atmospheres: results for recent near-real time data Intercepts of linear fits Intercepts of polarised and depolarised fit agree nicely v4.0: PMD 15 ? (data since 01-01-2008) O: polarised limit ◊: depolarised limit

27. 27 Limiting atmospheres: results for reprocessed data set (v4.0) Slopes of linear fits O: polarised limit ◊: depolarised limit Data since 01-01-2007 Every 6 th day is processed, in total 99 days PMD band definition v3.1 is active since the beginning of March 2008 ? periodic behaviour ?

28. 28 Limiting atmospheres: results for reprocessed data set (v4.0) Intercepts of linear fits PMD band definition v3.1 yields better results: initial wavelength mismatch between PMD-p and PMD-s smaller v4.0: PMD 15 noisy (data since 01-01-2007) O: polarised limit ◊: depolarised limit

29. 29 Polarised limit: comparison with simulations of limiting atmospheres Blue circles: simulated Q/I for PMD 14, based on input parameters of GOME-2 measurements, including ozone column, viewing and solar angles, for a fixed surface albedo of 0.002 (ocean). Dependence on surface albedo: even very small changes in the surface albedo already cause a different position of the limit. Polarised limit is very sensitive. 756 nm

30. 30 Polarised limit: comparison with simulations of limiting atmospheres Blue circles: simulated Q/I for PMD 3, based on input parameters of GOME-2 measurements, including ozone column, viewing and solar angles, for a fixed surface albedo of 0.005 (ocean). 325 nm Dependence on surface albedo: even large changes in the surface albedo have little effect on the location of the limit. Polarised limit is not very sensitive in the UV. Also soil and vegetation surfaces contribute. 325 nm

31. 31 5) Solar irradiance measured by the PMDs

32. 32 4) Solar irradiance measurements by the PMDs Irradiances of PMDs p and s should be the same; their intensity ratio should be 1 Effect of distance between Earth and Sun is divided out Improvement with new PMD band definition v3.1 Temporal behaviour: different degradation of PMD-p and PMD-s

33. 33 Difference in solar irradiance, for all 15 PMD bands: The shape of the ratio cannot be caused by degradation (alone). Key data issue?

34. 34 Degradation of PMD-p: corrected for distance Earth-Sun, normalised to 1 at the start of the time series Strong degradation for the shorter wavelength PMDs (1-8): Rate of ~20% per year v4.0

35. 35 Degradation of PMD-s: corrected for distance Earth-Sun, normalised to 1 at the start of the time series Again strong degradation for the shorter wavelengths Looks like PMD-s is less stable than PMD-p, and also more responsible for the periodic behaviour of their ratio than PMD-p v4.0

36. 36 6) 6) Errors on Stokes fractions and main science channel radiances

37. 37 Systematic offsets found for the PMD bands: (average over 2½ months)  very consistent results  depolarised limit agrees with polarised limit  polarised limit less accurate for long wavelengths Error bars:  limiting atmospheres: no error assumed in slope (statistical error)  special geometries: error larger because of scan-angle dependence

38. 38 Effect of errors in the Stokes fractions on the radiances of the main science channels  Errors are below ~0.5% for all wavelengths  Shorter wavelength: smaller errors despite higher sensitivity to polarisation Sensitivity of the main science channels to polarisation (Stokes fraction Q/I) : mu_2

39. 39 7) Task 3 – PMD raw mode  Earth reflectance  Stokes fractions

40. 40 PMD raw mode: In PMD raw mode, GOME-2 measures the spectrum at the full spectral resolution, using all 256 detector pixels, at the expense of the spatial coverage, which is 1/16 th of the normal coverage. The first 15 measurements in a scan are skipped, and the 16 th is recorded (at the full spectral grid). This is repeated 16 times in each scan. Simulations: The simulations are performed by the polarised radiative transfer model “DAK” (v3.1). The ozone columns are determined from SCIAMACHY assimilated total ozone columns. Surface albedo values are obtained from a LER database. Above 400 nm, a very high accuracy of the LER values is required, which is not available.

41. 41 Selection of cloud-free scenes using AVHRR data (provided by EUMETSAT) (left:) AVHRR cloud fraction Two orbit parts: right part θ 0 > 90°, left part mostly over sea (right:) cloud fraction < 0.05 Yellow circle in Alaska: suitable GOME-2 PMD raw measurement with cloud fraction ~0.00

42. 42 Scene 1 – Alaska (land) (west-viewing geometry) Clear signatures of vegetation. Fair agreement, even for the longer wavelengths. In the UV ~3% off. Below ~320 nm the values are too high (probably straylight). Red points: GOME-2 Blue points: simulations Green lines: Q/I=0 and Q/I=(Q/I) ss Good agreement, keeping in mind that we know that there should be offsets. Offsets roughly as large as expected. Noise at the shorter wavelengths: low signal levels.

43. 43 Poor agreement for the longer wavelengths. Surface albedo clearly not correct. Shape looks reasonable, but the deviation is not as expected for the longer wavelengths. A too high value for the surface albedo leads to enhanced depolarisation. Scene 2 – Canada (land) (east-viewing geometry)

44. 44 Scene 3 – Alaska II (land) (west-viewing geometry) Fair agreement for the longer wavelengths. In the UV ~4–7% off. Very similar to Scene 1. Good agreement, offsets are similar to offsets found for Scene 1 (and those were roughly similar to the offsets for the PMD bands found from the special geometry and limiting atmospheres analyses).

45. 45 Scene 4 – Special geometry scene (cloudy, ocean) There are oscillations at the longer wavelengths. Also seen in Scenes 1–3. Reasonable agreement, offsets follow the shape of the offsets found before. All in all: error on Q/I for the PMD raw mode is smaller than 0.05 (above 320 nm), and most likely similar to the errors found for the PMD band data.

46. 46 8) Summary (1/2)  Special geometry and limiting atmospheres analyses have consistent results (Stokes fraction).  Both analyses show a very clear improvement with every new data processor version.  In particular, there was a large improvement with the introduction of PMD band definition v3.1 and new (polarisation) key data.  The scan-angle dependence has been reduced, but is still there to some degree  see recommendations.  Behaviour of PMD 15 since processor version 4.0: bug will be fixed in v4.1.  Solar PMD data show periodic oscillations suggesting problems in the (diffuser) calibration.  The solar irradiance spectra measured by PMD-p and PMD-s show a rate of degradation ~20% per year in the UV.

47. 47 8) Summary (2/2)  At the moment, the degradation of PMD–p and PMD-s is very similar  The effect on the Stokes fractions (so far) is small. The decision to use two orthogonal PMDs (instead of one in combination with the main science channels for GOME-1) is paying off.  (Relative) degradation correction for the PMDs may be necessary to obtain reliable Stokes fractions in the future.  This degradation correction is probably scanner-angle dependent.  Some time-dependent periodic behaviour is also visible in the Stokes fractions.  PMD raw mode Stokes fraction and Earth reflectance look reasonable, and copy the validation results of the PMD band data.  Systematic offsets in Stokes fractions are below 0.02 for wavelengths below 520 nm, and below 0.04 for the longer wavelengths.  Effect of errors in Stokes fractions on main science channel radiance / reflectance is below the 0.5%.

48. 48 9) Recommendations:  The scan-angle dependence, which was reduced by already a lot, can and needs to be further reduced (ongoing work at EUMETSAT).  Implement a correction for the Stokes fractions (i.e., for the ratio of PMD-p and PMD-s signals) based on the special geometry approach (scan-angle dependent). This then automatically corrects for instrument degradation, temporal variations and scan-angle dependencies. Acknowledgements : We greatly acknowledge support from EUMETSAT (R.Lang, Y.Livschitz, R.Munro). In particular also the very quick analyses, testing and implementation of improvements in the operational data processor.

49. 49

50. 50 Limiting atmospheres: how to determine the limits (1) Some of the practical problems: – filter out rainbow situations (130° ≤ Θ ≤ 155°) – filter out “suspicious” pixel 245 for each scan:  note that this is the pixel following reset pixels 241–244  PMD radiance is approximately a factor 5 too high (why?)  Q/I looks ok, but sometimes “saturation-like” effects occur – outliers for short wavelength PMD bands: signals too low? PMD 1, 14-07-2008 (1 day of data) ~2 million measurements, version 4.0

51. 51 Limiting atmospheres: how to determine the limits (2) Approach: – horizontal bins with cell size 0.02 (optimum) – per horizontal cell, determine the histogram using vertical bins of 0.01 – determine the two edges of this distribution function – assign a weight to these edges, based on the number of measurements – fit a linear function through the points, using the weights depolarised limit polarised limit PMD 5, all limits found in 2008 (some more fine tuning may further improve the results)