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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”
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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
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
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”]
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
BAND 2 : 314 nm • offset problem smaller unphysical regime unphysical regime data from 13-APR-2007
BAND 3 : 319 nm unphysical regime unphysical regime data from 13-APR-2007
BAND 4 : 325 nm unphysical regime unphysical regime data from 13-APR-2007
BAND 5 : 332 nm unphysical regime unphysical regime data from 13-APR-2007
BAND 6 : 354 nm unphysical regime unphysical regime data from 13-APR-2007
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
BAND 8 : 413 nm • similar unphysical regime unphysical regime data from 13-APR-2007
BAND 9 : 482 nm • looks pretty ok unphysical regime unphysical regime data from 13-APR-2007
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
BAND 11 : 621 nm unphysical regime unphysical regime data from 13-APR-2007
BAND 12 : 749 nm • again somewhat out of physical regime… • “outliers” : rainbow? unphysical regime unphysical regime data from 13-APR-2007
BAND 13 : 761 nm • offset • “outliers” unphysical regime unphysical regime data from 13-APR-2007
BAND 14 : 795 nm • offset problem • outliers unphysical regime unphysical regime data from 13-APR-2007
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
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
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)
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.
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).
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
4.1 cos(2χss) = 0 : pixel number (index) [sheet “2”] half-way orbit symmetrical... geometrical effect…?
4.1 cos(2χss) = 0 : VZA [sheet “3”] ? Viewing angle dependence? Or just indirect? noisy + stable branch
4.1 cos(2χss) = 0 : SCATTERING ANGLE [sheet “5”] stable branch turns into noisy branch Indirect dependence on VZA, SZA, …?
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!
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…