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GOME-2 polarisation data and products L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1) SRON; (2) KNMI GSAG #42, EUMETSAT, 14-10-2008

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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)

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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)

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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 (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]

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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 01-01-2007 (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

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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?

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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?

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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

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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: 241-244)

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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)

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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.

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12 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 82 days PMD band definition v3.1 is active since the beginning of March 2008 ? temporal behaviour ?

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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 01-01-2008) O: polarised limit ◊: depolarised limit

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14 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 94 days PMD band definition v3.1 is active since the beginning of March 2008 ? periodic behaviour ? ? temporal behaviour caused by degradation ?

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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 01-01-2007) O: polarised limit ◊: depolarised limit

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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.

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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

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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

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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

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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, 14-07-2008 (1 day of data) ~2 million measurements, version 4.0

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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)

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