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Verification of GOME-2 PMD reflectances

Verification of GOME-2 PMD reflectances. L.G. Tilstra, O.N.E. Tuinder, P. Stammes Royal Netherlands Meteorological Institute (KNMI). O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011. Outline Verification approach Intermediate result: PMD 8

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Verification of GOME-2 PMD reflectances

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  1. Verification of GOME-2 PMD reflectances L.G. Tilstra, O.N.E. Tuinder, P. Stammes Royal Netherlands Meteorological Institute (KNMI) O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  2. Outline • Verification approach • Intermediate result: PMD 8 • Intermediate result: PMD 10 ; spatial aliasing effect • Final results for all PMD bands • Summary and conclusions O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  3. 1) Verification approach: MSC reflectance versus PMD reflectance GOME-2 PMD bands: spectral response functions: (PMD band definition 3.1) Wavelength calibration + PMD band definition + slit function  response function • Convolution of MSC reflectance with PMD band spectral response function • Binning of 8 PMD measurements to match MSC footprint O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  4. 1) MSC versus PMD reflectance intercomparison 25–10–2008, 1 measurement PMD-p solar spectrum used Complications: • Overlaps regions • PMD 15 outside spectral range MSC • Two different PMD band definitions (v1.0, v3.1) O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  5. 2) PMD 8: Scatter plot of PMD band reflectance versus MSC band reflectance 25–10–2008, 1 level-1b orbit IT = 187.5 ms, SZA < 80° –60° < latitude < 60° no narrow swath data no nadir static data no PMD raw data PMD-p solar spectrum used At first sight, the correlation between PMD and MSC seems to be good. Issues PMD 8: slope (a) deviates from 1 and intercept (b) deviates from 0. O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  6. 2) PMD 8: Global image of reflectance ratio PMD/MSC No clear latitude dependence. Cloud structures visible. Blue band at eastern side. O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  7. 2) PMD 8: Explanation of cloud structures Create artificial PMD reflectances from the MSC reflectances to simulate the effect of calibration errors (slope, offset) on the intercomparison: Cloud structures can be explained entirely by offset (b). O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  8. 2) PMD 8: Recalibration; remnants of polarisation sensitivity in PMD radiance Recalibration: Blue bands still there! Polarisation sensitivity visible. Possible explanation: the “post-processing” step on the Stokes fraction Q/I is currently not applied to the PMD radiances. O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  9. 3) PMD 10: Scatter plot of PMD band reflectance versus MSC band reflectance 25–10–2008, 1 level-1b orbit IT = 187.5 ms, SZA < 80° –60° < latitude < 60° no narrow swath data no nadir static data no PMD raw data PMD-p solar spectrum used The correlation between PMD and MSC is marginal (compared to PMD 8). Issues PMD 10: slope (a), a small offset (b), and a large amount of scatter. O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  10. 3) PMD 10: Global image of reflectance ratio PMD/MSC Large amount of scatter. O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  11. 3) Explanation of scatter: spatial aliasing Spatial aliasing: The first and last detector pixel of a detector arrays are integrated over different time intervals. This leads to a spatial mismatch between MSC and PMD, mainly depending on the wavelength of the MSC measurement. Image: GOME-2 L1 PGS 6.1 PMD 08: at the beginning of MSC channel 3; required PMD shift ≈ 0 PMD 10: in the middle of MSC channel 3; required PMD shift ≈ 1 O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  12. 3) PMD 10: Scatter plot after application of a PMD shift of 1 measurement 25–10–2008, 1 level-1b orbit IT = 187.5 ms, SZA < 80° –60° < latitude < 60° no narrow swath data no nadir static data no PMD raw data PMD-p solar spectrum used Almost no scatter; the correlation between PMD and MSC seems to be good. Issues PMD 10: slope (a) of 0.96 (!), but the offset (b) is very small. O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  13. 3) PMD 10: Global image after application of a PMD shift of 1 measurement Scatter is (almost) completely gone. So are the cloud structures (offset ≈ 0). No polarisation-related feature at the eastern side of the swath! O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  14. 4) Final results for all PMD bands Slopes and offsets as a function of PMD band, for PMD shifts of 0, 1, 2: The slopes and intercepts are almost not influenced by the PMD shift used. O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  15. 5) Summary and conclusions Results from the MSC versus PMD intercomparison: Scatter plots: slopes deviate from one (errors up to 5%) Scatter plots: offsets (of up to 0.01 in reflectance) for PMD 1–9 Polarisation-related feature for some PMD bands (these may slightly influence slope and offset found from the scatter plots) Spatial aliasing effect not important for resulting slopes, offsets, and the existence of polarisation-related features Remnants of C-shape visible for certain PMD bands (not shown) Time series: only small signs of instrument degradation (not shown) Proposed way forward: Check if the polarisation feature at the east side of the swath can be removed Study the offsets in the UV (PMD 1–9). Are they polarisation-related? Slopes: radiometric calibration might be corrected by a multiplicative factor O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  16. Extra slides O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  17. A) PMD 4: C-shape (recalibrated image) O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

  18. B) Standard deviation w.r.t. linear fit PMD shift = 0 PMD shift = 1 PMD shift = 2 O3M SAF AOD Meeting, EUMETSAT, Darmstadt, Germany, 11-04-2011

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