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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

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


Wp3 scientific aai sc aai and operational aai l2 aai
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


(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


(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


(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


Plans for wp3
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


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


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


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


Plans for WP10: global mean AAI

  • 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


Extra slides (R) global mean AAI

SCIAvisie Meeting, SRON, Utrecht, 10-07-2009


R1: The “Global Dust Belt” global mean AAI

SCIAvisie Meeting, SRON, Utrecht, 10-07-2009


R2 introduction of the absorbing aerosol index aai and the residue
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


R3: Typical global aerosol distribution: the residue

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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