1 / 29

GOME-2 Polarisation Study First results

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. Overview. Q/I versus (Q/I) ss verification: forward scan versus backward scan pixels verification: PMD readouts versus “187.5 ms subpixels”

portia-benson
Download Presentation

GOME-2 Polarisation Study First results

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

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

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

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

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

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

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

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

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

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

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

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

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

  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. Conclusion Q/I versus (Q/I)ss: • At first sight, the data generally look ok • 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) • Offset-like problems can be seen

  20. 2: Verification: forward scan versus backward scan

  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. Binning: non-homogeneous but otherwise normal scene: 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. 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. 4: Special geometries where Q/I = 0 • Situations where cos(2χss) = 0 [or: χss=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 (!!) • 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–φ0≈180°) Dependencies: pixel number, VZA, SZA, VAZI, SAZI, RAZI, SCAT, CHI_SS To be done: dependency on PMD-p and PMD-s intensity

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

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

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

  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(2χss) = 0 method over the entire mutual pixel number range!

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

More Related