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The Determination of the Cloud Fraction in the SCIAMACHY Pixel using MERIS:

Alexander Kokhanovsky(1), Wolfgang von Hoyningen-Huene(1), John P. Burrows(1), Olivier Colin(2), Jean-Michel Rosaz(2) and Emmanuel Mathot(2)

(1) University of Bremen, O. Hahn Allee 1, D-28334 Bremen, Germany
(2) ESA, via Galileo Galilei, BP64, 00044 Frascati, Italy


The cloud amount in a given satellite pixel is an important atmospheric parameter needed for an accurate determination of trace gas, cloud, and aerosol properties from space-borne optical measurements. For the aerosol retrievals, only completely cloud free pixels can be used. For trace gas retrievals the cloud coverage of a given pixel is needed. The subpixel clouds constitute a major problem for the SCIAMACHY trace gas retrievals. The cloud remote sensing in the framework of the independent pixel approximation requires the information on the cloud fraction in the SCIAMACHY science pixel as well. In this paper we develop a technique to determine the cloud fraction in the SCIAMACHY pixels (30x30km and even larger ground footprints depending on the channel and the location of measurements) using simultaneous MERIS observations (1x1km ). The results of newly developed SCIAMACHY cloud fraction product are compared with correspondent MERIS browse imagery. An excellent accuracy of the technique is found. In addition to several thresholds applied to MERIS top-of-atmosphere reflectances, also SCIMACHY PMDs and reflectances are used to enhance the retrievals in the case of bright surfaces such as snow, ice fields, and deserts. The algorithm has been implemented in the framework of ESA G-POD project and it is planned that it will be applied to all SCIAMACHY pixels for the entire time span of ENVISAT mission. The results of retrievals are available over the ESA G-POD website located at


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