1. GOME-2 Polarisation Study First results L.G. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1) SRON; (2) KNMI EUMETSAT, Darmstadt, 29-06-2007

2. 2 Overview 1. Q/I versus (Q/I) ss 2. verification: forward scan versus backward scan pixels 3. verification: PMD readouts versus “187.5 ms subpixels” 4. special points where Q/I =P∙cos(2χ)= 0 owing to: - cos(2χ ss ) = 0 - backscatter geometry

3. 3 1: Q/I versus (Q/I) ss GOME-2 polarisation measurements: 256 PMD readouts per scan (in 15 spectral bands). A scan lasts exactly 6 seconds, of which 4.5 seconds in forward scan, and the remaining 1.5 seconds in backward scan. Data: one level-1B orbit (actually consisting of 3-min. “chuncks” of data) of 13-APR-2007. Total number of NADIR scans: 593 scans = 151808 polarisation values per orbit. IDEA: 0 ≤ Q/I ≤ (Q/I) ss Geometry in the data product is given only 32 times per scan (for the 187.5 ms subpixels).  Verification: checked (Q/I) ss on the 32-per-scan grid in the GOME-2 product with our own calculations  Interpolated viewing and solar angles to the 256-readouts-per-scan grid  Calculated (Q/I) ss on the 256-readouts-per-scan grid [sheet “1”]

4. 4 BAND 1 : 311 nm  distribution largely outside “physical regime” (where 0≤Q/I≤(Q/I)ss)  there is an offset problem  ~70% of the data points outside interval [-1,1]  these data points ALWAYS have the same value of –2147.4836  reported wavelengths: same value when wrong  reported errors: ALWAYS equal to 0.065535 unphysical regime unphysical regime data from 13-APR-2007

5. 5 BAND 2 : 314 nm  offset problem smaller unphysical regime unphysical regime data from 13-APR-2007

6. 6 BAND 3 : 319 nm unphysical regime unphysical regime data from 13-APR-2007

7. 7 BAND 4 : 325 nm unphysical regime unphysical regime data from 13-APR-2007

8. 8 BAND 5 : 332 nm unphysical regime unphysical regime data from 13-APR-2007

9. 9 BAND 6 : 354 nm unphysical regime unphysical regime data from 13-APR-2007

10. 10 BAND 7 : 381 nm  distribution again outside “physical regime”, even for (Q/I)ss far from 0 unphysical regime unphysical regime data from 13-APR-2007

11. 11 BAND 8 : 413 nm  similar unphysical regime unphysical regime data from 13-APR-2007

12. 12 BAND 9 : 482 nm  looks pretty ok unphysical regime unphysical regime data from 13-APR-2007

13. 13 BAND 10 : 558 nm  Looks ok: not a lot of measurements in the unphysical regime, and spread around Q=0 is smaller than before unphysical regime unphysical regime data from 13-APR-2007

14. 14 BAND 11 : 621 nm unphysical regime unphysical regime data from 13-APR-2007

15. 15 BAND 12 : 749 nm  again somewhat out of physical regime…  “outliers” : rainbow? unphysical regime unphysical regime data from 13-APR-2007

16. 16 BAND 13 : 761 nm  offset  “outliers” unphysical regime unphysical regime data from 13-APR-2007

17. 17 BAND 14 : 795 nm  offset problem  outliers unphysical regime unphysical regime data from 13-APR-2007

18. 18 BAND 15 : 842 nm  offset problems are quite severe  again a lot of outliers  BAND 15 was not working at all for older versions of the data (06-DEC-2006) unphysical regime unphysical regime data from 13-APR-2007

19. 19 Conclusion Q/I versus (Q/I) ss: At first sight, the data generally look ok At first sight, the data generally look ok Data outside interval [-1,1] : algorithm gives up too soon? Data outside interval [-1,1] : algorithm gives up too soon? BAND 01 (311 nm) : 69.8 % out BAND 02 (314 nm) : 38.5 % out BAND 03 (319 nm) : 27.7 % out BAND 04 (325 nm) : 21.0 % out BAND 05 (332 nm) : 18.5 % out BAND 06 (354 nm) : 17.2 % out BAND 07 (381 nm) : 16.8 % out BAND 08 (413 nm) : 14.0 % out BAND 09 (482 nm) : 12.6 % out BAND 10 (558 nm) : 12.8 % out BAND 11 (621 nm) : 13.1 % out BAND 12 (749 nm) : 12.8 % out BAND 13 (761 nm) : 13.0 % out BAND 14 (795 nm) : 13.4 % out BAND 15 (842 nm) : 13.6 % out (and having a value of –2147.4836) Reported errors on Q/I are always equal to 0.065535 (for all measurements, for all bands) Reported errors on Q/I are always equal to 0.065535 (for all measurements, for all bands) Offset-like problems can be seen Offset-like problems can be seen

20. 20 2: Verification: forward scan versus backward scan

21. 21 Sometimes the forward-scan measurement fails while the backward-scan measurement is ok; sometimes it is the backward-scan that is at fault. Why are there so many errors and where do they come from? (TBD)

22. 22 non-homogeneous but otherwise normal scene: Binning: rather inhomogeneous scene:  Good correlation for mildly inhomogeneous scene; spread probably also determined by improper binning of ratios instead of intensities. Inhomogeneous scene: the higher the wavelength, the worse the correlation.  In conclusion, no problems related to scan direction.

23. 23 3: Verification: PMD readouts versus 187.5 ms subpixels Binning: Conclusion: As far as we can tell, there don’t appear to be any obvious problems in the mapping of the measured polarisation values of the PMD readouts (256 scan -1, 23.4 ms) to 187.5 ms subpixels (32 scan -1 ).

24. 24 4: Special geometries where Q/I = 0 1. 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) – no physical link with data, selection of points determined by choice of reference plane (which is the local meridian plane) – these are special situations where (U/Q) ss is undetermined, and the data processor treats these situations in a special way (!!) 2. Backscatter situations (Θ = 180°) +does not depend on the definition of a reference plane +rainbow and sunglint situations are automatically filtered out – situations are only found “around the equator” (φ–φ 0 ≈180°) Dependencies: pixel number, VZA, SZA, VAZI, SAZI, RAZI, SCAT, CHI_SS To be done: dependency on PMD-p and PMD-s intensity

25. 25 4.1 cos(2χ ss ) = 0 : pixel number (index)[sheet “2”] half-way orbit symmetrical... geometrical effect…?

26. 26 4.1 cos(2χ ss ) = 0 : VZA[sheet “3”] ? Viewing angle dependence? Or just indirect? noisy + stable branch

27. 27 4.1 cos(2χ ss ) = 0 : SCATTERING ANGLE[sheet “5”] Indirect dependence on VZA, SZA, …? stable branch turns into noisy branch

28. 28 4.2 BACKSCATTERING ( Θ = 180°) : pixel number (index)[sheet “6”] Mind the small range in pixel number (compare with sheet “1”…) Complete agreement with cos(2χ ss ) = 0 method over the entire mutual pixel number range!

29. 29 4.2 Θ = 180° : DIRECTION OF POLARISATION CHI_SS[sheet “8”] We find no strange behaviour at the points where χ ss =45° or χ ss =135°.  data having this geometry, where cos(2χ ss ) = 0 and (U/Q) ss does not exist, behave similar to other data, despite alternative treatment of data processor.  therefore, the cos(2χ ss ) = 0 method appears to be a reliable tool. χ ss =45° χ ss =135° Clearly, more analyses are needed to sort out the problems…