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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 (1,2) , I. Aben (1) , P. Stammes (2) (1) SRON; (2) KNMI. SRON, 16-07-2007. WP3: scientific AAI (SC-AAI) and operational AAI (L2-AAI). Status SC-AAI (“advantage over L2-AAI”):

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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. Tilstra (1,2), I. Aben (1), P. Stammes (2) (1)SRON; (2)KNMI SRON, 16-07-2007

  2. WP3: scientific AAI (SC-AAI) and operational AAI (L2-AAI) Status SC-AAI (“advantage over L2-AAI”): • correction for calibration offset at t=0 • correction for instrument degradation (still being improved) • look-up tables calculated by RTM taking polarisation into account • algorithm more accurate + allows negative albedos • proper sunglint filter Available SC-AAI data: • daily level-2 data (ASCII format) • ‘gridded’ level-3 data for users familiar with the TOMS data format

  3. SCIAMACHY SC-AAI data are available on the TEMIS website http://www.temis.nl Available data: GOME SCIA : 1995–2000 : 2002–2007

  4. Improving L2-AAI: progress of work on new look-up tables • Look-up tables currently used in the operational algorithm were calculated using a RTM neglecting polarisation. This results in large errors in the L2-AAI. • New LUTs calculated by the RTM “DAK” (“Doubling-Adding KNMI”). • Visit by Michael Hess (DLR) to KNMI on 8 and 9 February 2007. Fruitful meeting. Discussed strategy, work to be done, planning for validation, etc. • Work on new LUTs was finished at the end of February 2007; • LUTs delivered to DLR • Format description of files and contents written and delivered to DLR • ATBD for SC-AAI written and delivered to DLR • DLR is changing the operational algorithm to use the new LUTs, and to implement parts of the SC-AAI algorithm.

  5. Degradation: AAI in 2004:(monthly mean)Normal distribution of UV-absorbing aerosols. Similar to observations by TOMS. AAI in 2006:(monthly mean)Degradation of the opticsleads to anomalously highvalues of the residue / AAI.

  6. SCIAMACHY degradation in the UV WLS: White Light Source, internal calibration lamp “Light Path Monitoring” results for 340 and 380 nm: Status: 340 : -25.9% 380 : -17.6% (27-06-2007) Status: 340 : -11.3% 380 : -8.2% (27-06-2007) Status: 340 : -16.5% 380 : -10.3% (27-06-2007)

  7. Effect of instrument degradation on the AAI/residue: (The daily global average of the residue should have a constant value) green points: daily residue, averaged over the globe. blue curve: calculated effect of instrument degradation (from LPM) on residue/AAI The trend in the residue can be more or less explained by degradation as found from “Light Path Monitoring”. Lately, the agreement has become worse…

  8. Scan-angle dependent analysis of the AAI/residue: • Clear effect of scan-angle dependent degradation in the AAI/residue after the end of 2004. • The effect is already quite large (0.9 index points ~ 2% in relative reflectance) and seems to be growing at an increasing rate. • The future LPM-based correction factors (the m-factors) are obtained for only one position (=angle) of the scan-mirror, not for each viewing angle!  The introduction of m-factors will not solve the problem of (scan-angle dependent) instrument degradation

  9. Status radiometric calibration (version 6.02): Errors used to be 10-20% for software versions ≤ 5.04. However, there is still quite a large calibration problem with the reflectance in the UV-visible wavelength range, with errors up to 6%. (data from 2002-2004; degradation in the visible is small)

  10. Plans WP 3/10: • Further study the (scan-angle dependent) degradation • Improve degradation correction of SC-AAI • Validate improvements in L2-AAI (part of the “SQWG work”) • Further validate and improve the SC-AAI (which will benefit the L2-AAI as well) • Further validation of level-1 polarisation correction and product via satellite intercomparisons and comparisons with RTMs • Validation of reflectance calibration for the entire wavelength range (channels 1 to 8) using stable Earth targets

  11. Extra slides

  12. Absorbing Aerosol Index (AAI) and the residue • Definition of the residue: • where the surface albedo A for the simulations is such that: • no clouds, no absorbing aerosols : r < 0 • clouds, no absorbing aerosols : r < 0 • absorbing aerosols, no clouds : r > 0 • B. Definition of the AAI: • AAI = residue > 0 (where residue < 0 the AAI is not defined)

  13. Effect of instrument degradation (c340 ; c380) on residue/AAI: • Trend in residue can be explained by degradation as found from “Light Path Monitoring” results. • Residue/AAI is very sensitive to (absolute) reflectance calibration. • Degradation correction is really important for the AAI. • Residue is recording its own degradation correction

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