SCIAMACHY total ozone product

Henk Eskes and Ronald van der A, KNMI

The Sciamachy total ozone retrieval algorithm (TOSOMI) is an application of the TOGOMI algorithm to SCIAMACHY. TOSOMI is still in the development phase (current version 0.31, November 2003), and updates of the code can be expected in the future.

The new GOME algorithm TOGOMI [Valks and Van Oss, 2003] is based on the total ozone DOAS (Differential Optical Absorption Spectroscopy) algorithm developed for the OMI instrument [Veefkind and De Haan, 2001]. With respect to total ozone column retrieval using the DOAS method, the OMI, SCIAMACHY and GOME instruments are very similar. The main improvements of the new algorithm are:
(i) treatment of the atmospheric temperature sensitivity by using effective ozone cross-sections calculated from ECMWF temperature profiles,
(ii) improvements in the calculation of the air mass factor, using the so-called empirical approach,
(iii) using the Fast Retrieval Scheme for Clouds from the Oxygen A-band (FRESCO) algorithm for the cloud correction,
(iv) a new treatment of Raman scattering in DOAS [De Haan, 2003]. This new formulation of DOAS explicitly accounts for the smearing of the solar Fraunhofer lines as well as the atmospheric tracer absorption structures, and
(v) air-mass factors based on semi-spherical polarization-dependent radiative transfer (KNMI DAK model).

The SCIAMACHY total ozone retrieval algorithm TOSOMI combines a Sciamachy level-1 product reading module with the TOGOMI DOAS modules. The Fresco algorithm [Koelemeijer, 2001] is applied to the Sciamachy spectra to obtain cloud fraction and cloud top height estimates.

It is important to note that the TOSOMI total ozone product is influenced by the quality of the SCIAMACHY calibration procedure. No additional calibration corrections are applied to the ozone fit window (325-335nm), but a crude correction factor of 1.25 is applied to the radiances in the Fresco window (758-772nm) because especially the cloud fraction retrieval is sensitive to the absolute reflectivity (status October 2003).

The total ozone data is available in the form of ASCII data files (one file per day). A description of the format can be found here.
Total ozone images and a list of available data is publicly available from the TEMIS project web site.

References:

• Veefkind, J.P. and J.F. de Haan, OMI Algorithm Theoretical Basis Document, Barthia, P.K (ed), Volume II - Chapter 3, DOAS Total Ozone Algorithm, ATBD-OMI-02, Version 1.0, September 2001.
• Valks, P., and R. van Oss, TOGOMI Algorithm Theoretical Basis Document, KNMI, November 2003.
• De Haan, J.F., Accounting for Raman Scattering in DOAS, SN-OMIE-KNMI-409, KNMI, 2003.
• Koelemeijer, R.B.A., P. Stammes, J.W. Hovenier, and J.F. de Haan, A fast method for retrieval of cloud parameters using oxygen A-band measurements from GOME, J. Geophys. Res., 106, 3475-3490, 2001.