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WP10 – polarisation (only work done at KNMI)

WP10 – polarisation (only work done at KNMI). I will only show 3 of the main results: Direction of polarisation versus a theoretical model value Alternative polarisation retrieval in the UV Comparison between polarisation of SCIAMACHY and POLDER. NLSCIAVAL 22 December 2005 at KNMI.

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WP10 – polarisation (only work done at KNMI)

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  1. WP10 – polarisation (only work done at KNMI) • I will only show 3 of the main results: • Direction of polarisation versus a theoretical model value • Alternative polarisation retrieval in the UV • Comparison between polarisation of SCIAMACHY and POLDER NLSCIAVAL 22 December 2005 at KNMI

  2. 1. Direction of polarisation Χ (POLDER measurements vs theoretical s.s. value) • the direction of polarisation is equal to the theoretical single scattering value: Χ≈Χss • this is true for at least the 443, 670, and 865 nm POLDER bands. • theoretical value is now being used in the data processor (instead of measured value) • after this change, the polarisation product improved quite a lot (more stable, less polarisation features in reflectance) POLDER measures polarisation at 3 spectral bands around 443, 670, and 865 nm Schutgens et al., JGR 109, D09205, doi:10.1029/2003JD004081, 2005.

  3. 2. Alternative polarisation retrieval in the UV (PMD 1) • polarisation retrieval not based on PMD input, but on inversion of polarisation features in the reflectance • only works around 350 nm (PMD 1) because the algorithm uses the polarisation feature around 350 nm • Figure: Stokes parameter Q of alternative algorithm versus data processor (SCIA/4.02 and SCIA/5.00) • polarisation product of software version 4.02 is systematically off. • polarisation of software version 5.00 agrees well with alternative algorithm between 30°S and 60°N but: • only for verification orbits (which receive special attention). For normal orbits the agreement is “reasonable”. • Overall, PMD 1 does reasonably well. ACP 5, 2099–2107, 2005

  4. 3. POLDER versus SCIAMACHY (polarisation) measures polarisation at 443 nm (~PMD 2), 670 nm (~PMD 3), 865 nm (~PMD 4) • agreement is already very poor for PMD 2 (~443 nm) • for the longer wavelengths the agreement becomes worse: • - PMD 2 : systematic factor off • - PMD 3 : same systematic factor off + rubbish • - PMD 4 : rubbish • result not surprising: polarisation features in reflectance point to errors in the polarisation retrieval

  5. WP11 – (ir)radiance • Reflectance = (π× radiance) / (μ0× irradiance) • - 5 methods were developed to validate SCIAMACHY reflectance. • Testing different key data versions. • Outlook. NLSCIAVAL 22 December 2005 at KNMI

  6. (only cloud-free scenes can be modelled) (old key data) 1. Comparison with radiative transfer model in the UV orbit 2509, 23-08-2002, SV 5.00 orbit 11251, 25-04-2004, SV 5.04 orbit 13028, 27-08-2004, SV 5.04 Wavelength dependent / no scan-angle dependency! JGR 110, D18311, doi:10.1029/2005JD005853, 2005.

  7. 2. Large-scale comparison with RTM in the UV (Gijs van Soest) 270–290 nm • SAA clearly visible below 300 nm • no scan-angle dependency • wavelength dependency • no latitude/longitude dependency! • no dependence on scene  is the correction factor we need a multiplicative factor? 320–340 nm Van Soest et al., ACP 5, 2171–2180, 2005.

  8. 3. Comparison with MERIS (Juan Acarreta) Acarreta & Stammes, GRSL 2, 31–35, doi:10.1109/LGRS.2004.838348, 2005. Acarreta et al., KNMI Internal Report, 3 November 2004. • no scan-angle dependency • wavelength dependency • no latitude/longitude dependency • correction is a multiplicative factor! MERIS spectral bands at 442, 510, 665, 708, and 885 nm

  9. 4. Comparison with GOME: (old key data) • strong (scan-angle dependent) degradation of GOME in the UV up to 500 nm • above 500 nm no scan-angle dependency + result accurate • result above 500 nm agrees with MERIS comparison Applied Optics, submitted, 2005.

  10. POLDER spectral bands: 443, 443 (P), 490, 565, 670 (P), 763, 765, 865 (P), 910 nm 5. Comparison with POLDER(reflectance) • no scan-angle dependency • wavelength dependency • no latitude/longitude dependency • correction is a multiplicative factor!

  11. In summary: radiometric calibration of SCIAMACHY Overall uncertainty: 2–4% Everything points to a 15–25% underestimation of the reflectance

  12. Key data analysis:Old key data, NASA sphere key data, Spectralon key data… Old key data: Yes, there is an offset of 15–25% in the reflectance. Spectralon key data: Offset is less, but now there are spectral features in the reflectance  Better not use it for DOAS retrievals!

  13. Key data analysis:Spectralon key data… Black curve: “correction” introduced by applying new (Spectralon) key data

  14. Outlook polarisation and reflectance:- Limb polarisation is of unknown quality. Investigation needed.- Nadir polarisation: PMDs 3–6 are of bad quality. 0-1 processor improvement needed.- Reflectance: use of other Earth targets to validate the reflectance using RTMs at higher wavelengths (e.g. Antarctica).- Reflectance: extend analysis to near-IR channels (beyond 900 nm). AATSR?

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