Sciamachy Water Vapour Retrieval Using Amc-doas

Measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on ENVISAT have been used to derive global water vapour total column amounts over ocean and land. For this purpose, the Air Mass Corrected Differential Optical Absorption Spectroscopy (AMC-DOAS) approach, which has already been successfully used with GOME data, has been applied to SCIAMACHY’s nadir measurements. Whereas operational SCIAMACHY water vapour products are derived from measurements in the near-infrared (NIR), the AMC-DOAS algorithm is applied to the spectral region around 700 nm where both water vapour and molecular oxygen absorptions are present. AMC-DOAS is an extension of the standard DOAS method. To be applicable to water vapour retrieval, it considers the non-linear relation between absorber amount and absorption depth (saturation effect). Furthermore, an air mass correction factor is derived from O2 absorption features. This air mass correction factor not only compensates deviations of the modelled atmospheric conditions from reality, it also serves as an inherent quality check for the retrieval by which e.g. too cloudy scenes can be identified. Comparisons of the SCIAMACHY total water vapour columns derived with the AMC-DOAS method with corresponding SSM/I and ECMWF water vapour measurements have been performed. Taking into account the large temporal and spatial variability of water vapour, the SCIAMACHY results agree well with these correlative data not only over ocean but also over land, thus showing the capability of SCIAMACHY to provide a new independent global water vapour data set.