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WP 3 : Absorbing Aerosol Index (AAI) WP 10 : Level-1 validation

WP 3 : Absorbing Aerosol Index (AAI) WP 10 : Level-1 validation. L.G. Tilstra 1 , I. Aben 2 , and P. Stammes 1 1 Royal Netherlands Meteorological Institute (KNMI) 2 Netherlands Institute for Space Research (SRON). SCIAvisie Meeting, SRON, Utrecht, 10-07-2009.

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WP 3 : Absorbing Aerosol Index (AAI) WP 10 : Level-1 validation

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  1. WP 3 : Absorbing Aerosol Index (AAI)WP 10 : Level-1 validation L.G. Tilstra1, I. Aben2, and P. Stammes1 1Royal Netherlands Meteorological Institute (KNMI) 2Netherlands Institute for Space Research (SRON) SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  2. WP3: scientific AAI (SC-AAI) and operational AAI (L2-AAI) Algorithm improvements: SQWG YR 1/2 SQWG YR 3/4 • correction for calibration offset at t=0 • correction for the obstruction in the FOV for westernmost scan mirror positions (problem affects data until April 2003) • correction for (scan-angle dependent) instrument degradation (on top of the standard m-factor correction) • look-up tables (LUTs) calculated by RTM taking polarisation into account • completely new algorithm approach: more accurate + allows negative albedos • improved surface height calculation method + database • proper flagging of (potential) sunglint situations • flagging of solar eclipse events • viewing and solar angles calculated w.r.t. sea-level (instead of w.r.t. 100 km) • ozone column dependency of AAI taken into account • LUTs calculated by RTM taking the atmosphere’s sphericity into account • O2-O2 absorption included in LUTs SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  3. (10) Taking ozone absorption into account in the simulated reflectances / LUTs  ozone column fixed to 334 DU Effect of neglecting ozone on the AAI  • New LUTs + ATBD + documentation sent to DLR (end of June 2009) • Total (SCIAMACHY L2) ozone columns are required (and available) as input • DLR are working on changing the algorithm code of the operational processor • Testing is needed with SC-AAI as a reference SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  4. (11) Pseudo-spherical treatment of the atmosphere’s sphericity in the LUTs: LUTs of the reflectances at 340 and 380 nm were originally calculated for plane parallel atmospheres (using the RTM “DAK”). We proposed to improve this. Impact on the AAI: LUTs recreated using DAK v3.1, which offers pseudo-sphericity. Improvement is relatively large for solar zenith angles above 75°. Proposed change requires modest changes in the L2-AAI retrieval code. SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  5. (12) Taking O2–O2 absorption into account: Effect of including O2–O2 absorption in the AAI LUTs: [O2–O2 absorption bands at 360 and 380 nm] • non-negligible effect: 0.1–0.5 index points for thick clouds • offset of ~0.1 index points for thin/no clouds • same offset for positive residues Effect cannot be neglected. Including O2–O2 absorption in the LUTs improves matters at little or no cost. SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  6. Plans for WP3: • Keep improving the scientific AAI product (SC-AAI) • Assist DLR with the implementation of the proposed changes to improve the quality of the L2-AAI (“SQWG year 3/4 activities”) • Continue validation of SC-AAI and L2-AAI • Maintain SC-AAI data archive at the TEMIS site • Further study the scan-angle dependent degradation for support of WP10 SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  7. WP10: level-1 validation Validation techniques for the reflectance: • comparison with radiative transfer model “DAK” (in the UV) • comparison with other satellite instruments (GOME-1, MERIS, POLDER-2, …) • qualitative analysis of the spectra (spectral properties) calibration offset spectral features SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  8. Validation techniques for the polarisation: • comparisons with single scattering model • validation via special geometries • comparison with POLDER-2 PMDs 1–4 improved a lot. Degradation has become a problem. Key data improvements: • SRON is working on a new set of key data which is expected to improve the radiometric calibration significantly • This should also solve the presence of spectral features in the spectra • Also polarisation key data will be improvement Validation techniques for the reflectance + polarisation will be used to study the influence of the new key data. (We are repeating the validation work for every new level-1 data version any way.) SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  9. Degradation monitoring using the scientific AAI product: global mean AAI Without m-factors - increase of ~4 index points With m-factors SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  10. Plans for WP10: • Verify improvement in calibration brought about by the new key data using the various tools we developed • Monitor degradation and analyse quality of applied degradation correction using the Absorbing Aerosol Index (AAI) in the UV • Monitor and validate polarisation product using special geometries • Analyse the reflectance over specific stable Earth targets SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  11. Extra slides (R) SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  12. R1: The “Global Dust Belt” SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  13. R2: Introduction of the Absorbing Aerosol Index (AAI) and the residue – The AAI represents the scene colour in the UV – • Definition of the residue: where the surface albedo A for the simulations is such that: (A is assumed to be wavelength independent: A340 = A380) no clouds, no aerosols : r = 0 clouds, no absorbing aerosols : r < 0 absorbing aerosols : r > 0 B. Definition of the AAI: AAI = residue > 0 (and the AAI is not defined where residue < 0) SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

  14. R3: Typical global aerosol distribution: The “Global Dust Belt”: Desert Dust Aerosols (DDA) (dust storms, all year) Biomass Burning Aerosols (BBA) (dry season, anthropogenic) AAI from other UV satellite instruments: TOMS, GOME-1, GOME-2. Combined with SCIAMACHY there are more than three decades (1978–2009) of AAI data available for studies of trends in desert dust and biomass burning aerosol. SCIAvisie Meeting, SRON, Utrecht, 10-07-2009

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