GOME-2 polarisation data and products L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1) SRON; (2) KNMI GSAG #42, EUMETSAT,

2 Validation of GOME-2 polarisation data Available techniques: 1)focus on special geometries along the orbit where Q/I = 0 2)limiting atmospheres approach 3)focus on the solar irradiance (sunlight is unpolarised)

3 1)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)

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 (9 months) Every 3 rd day is processed, in total 82 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]

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 (which is varying with time) Data since (21 months) Every 6 th day is processed, in total 94 days PMD band definition v3.1 is active since the beginning of March 2008

6 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?

7 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?

8 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

9 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: )

10 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)

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

12 Limiting atmospheres: results for recent near-real time data Slopes of linear fits O: polarised limit ◊: depolarised limit Data since Every 3 rd day is processed, in total 82 days PMD band definition v3.1 is active since the beginning of March 2008 ? temporal behaviour ?

13 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 ) O: polarised limit ◊: depolarised limit

14 Limiting atmospheres: results for reprocessed data set (v4.0) Slopes of linear fits O: polarised limit ◊: depolarised limit Data since Every 6 th day is processed, in total 94 days PMD band definition v3.1 is active since the beginning of March 2008 ? periodic behaviour ? ? temporal behaviour caused by degradation ?

15 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 ) O: polarised limit ◊: depolarised limit

16 Intercepts of linear fits: near-real time versus reprocessed data set (v4.0) (2008 only) v4.0NRT Strong transition caused by transition from PMD band definition v1.0 to v3.1.

17 3) Degradation: solar irradiance measurements by the PMDs Irradiances of PMDs p and s should be the same; their intensity ratio should be 1 (arbitrary normalisation) 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

18 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 ~25% per year v4.0

19 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

20 Summary  Special geometry and limiting atmospheres analyses show a very clear improvement with every 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  improve (polarisation) key data even further.  Special geometry and limiting atmospheres analyses have fairly consistent results, and point to the influence of instrument degradation.  Behaviour of PMD 15 since processor version 4.0: bug fixed in v4.1.  (Relative) degradation correction for the PMDs may be necessary in the future.  This degradation correction is probably scanner-angle dependent.

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22 Limiting atmospheres: how to determine the limits (1) Some of the practical problems: – filter out rainbow situations (130° ≤ Θ ≤ 150°) – 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, (1 day of data) ~2 million measurements, version 4.0

23 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)