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Recent Advances in SCIAMACHY Near Infrared Nadir Level 2 Algorithm Development

Franz Schreier(1), Michael Hess(1), Adrian Doicu(1), Thomas Schroeder(1) and Albrecht von Bargen(1)

(1) DLR German Aerospace Center, Oberpfaffenhofen, 82234 Wessling, Germany


The near infrared channels of SCIAMACHY onboard ENVISAT can be used to derive information on CO, CH4, N2O, and H2O. A core element of the current operational level 2 data processor for nadir observations is the BIAS (Basic Infrared Absorption Spectroscopy) algorithm using a DOAS-type approach with a standard nonlinear least squares fit. In addition to the column scale factors for molecular densities a one-parameter ``closure term'' is fitted to account for any other effects such as (single or multiple) scattering, surface reflection, or instrumental artefacts. However, recent sensitivity studies have shown the importance of adequate modelling the instrumental slit function; in particular an under- or overestimate of the slit function width can have a significant impact on the retrieved columns. Furthermore scattering can contribute substantially to the near infrared radiation.

Here we present work on recent advances towards a ``Better InfraRed Retrieval Algorithm'', covering both the forward modelling and inversion aspect:

* Nonlinear least squares using a state-of-the-art trust region implementation optionally with inequality bounds (nb. positivity constraints) and exploiting special structure of model function

* Option for 'online' line-by-line absorption cross section computation (including continua, e.g. CKD) giving greater flexibility w.r.t. spectroscopic line parameter database, atmospheric data (pressure, temperature, densities for arbitrary altitude grid), and spectral microwindow

* Fit parameters: molecular columns and surface reflection/albedo. Option to fit the slit function half width (and other instrument parameters, e.g. baseline)

* Improvements with respect to the forward model utilize routines adapted from the MIRART line-by-line code, that computes transmission, radiance, and optionally Jacobians (by means of algorithmic derivatives) and has been verified by extensive intercomparisons for up/down-looking and limb-viewing geometries in the microwave, FIR, and MIR spectral regimes.

The poster shows first results of the improved algorithm applied to SCIAMACHY near infrared nadir observations.


Full paper

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry