Overview of SCIAMACHY validation: 2002?2004

International audienceSCIAMACHY, on board Envisat, is now in operation for almost three years. This UV/visible/NIR spectrometer measures the solar irradiance, the earthshine radiance scattered at nadir and from the limb, and the attenuation of solar radiation by the atmosphere during sunrise and sunset, from 240 to 2380 nm and at moderate spectral resolution. Vertical columns and profiles of a variety of atmospheric constituents are inferred from the SCIAMACHY radiometric measurements by dedicated retrieval algorithms. With the support of ESA and several international partners, a methodical SCIAMACHY validation programme has been developed jointly by Germany, the Netherlands and Belgium (the three instrument providing countries) to face complex requirements in terms of measured species, altitude range, spatial and temporal scales, geophysical states and intended scientific applications. This summary paper describes the approach adopted to address those requirements. The actual validation of the operational SCIAMACHY processors established at DLR on behalf of ESA has been hampered by data distribution and processor problems. Since first data releases in summer 2002, operational processors were upgraded regularly and some data products ? level-1b spectra, level-2 O3, NO2, BrO and clouds data ? have improved significantly. Validation results summarised in this paper conclude that for limited periods and geographical domains they can already be used for atmospheric research. Nevertheless, remaining processor problems cause major errors preventing from scientific usability in other periods and domains. Untied to the constraints of operational processing, seven scientific institutes (BIRA-IASB, IFE, IUP-Heidelberg, KNMI, MPI, SAO and SRON) have developed their own retrieval algorithms and generated SCIAMACHY data products, together addressing nearly all targeted constituents. Most of the UV-visible data products (both columns and profiles) already have acceptable, if not excellent, quality. Several near-infrared column products are still in development but they have already demonstrated their potential for a variety of applications. In any case, scientific users are advised to read carefully validation reports before using the data. It is required and anticipated that SCIAMACHY validation will continue throughout instrument lifetime and beyond. The actual amount of work will obviously depend on funding considerations