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Retrieval of cloud fraction, optical thickness and top-height from GOME and SCIAMACHY

Bastiaan van Diedenhoven(3), Otto P. Hasekamp(1) and Jochen Landgraf(2)

(1) SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584CA Utrecht , Netherlands
(2) SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584CA Utrecht, Netherlands
(3) SRON Netherlands Institute for Space Research, sorbonnelaan 2, 3584 CA Utrecht, Netherlands

Abstract

We present an algorithm to simultaneously retrieve cloud fraction, optical thickness and top-height from reflectance measurements of spaceborn nadir viewing spectrometers as SCIAMACHY, GOME and GOME-2. Knowledge of these cloud parameters is needed to accurately retrieve trace gas profiles and columns from these instruments. However, none of the present cloud retrieval algorithms retrieves all three cloud parameters from the measured reflectances simultaneously. The here presented algorithm makes use of the measurements in and around the Oxygen A band between 755 and 775 nm and additionally of measurements in the UV between 350 and 400 nm. Since clouds shield the underlying atmosphere and reflect light back into space, the relative depth of the Oxygen A band and the continuum around it are sensitive to the cloud fraction, optical thickness and top-height. However, retrieval of all three parameters from the Oxygen A band alone is ill-posed. Extra information about the cloud fraction is contained in the spectral range between 350 and 400 nm, where the Rayleigh scattering optical thickness is large. This is because most Rayleigh scattering takes place below the cloud top and therefore the relative contribution of Rayleigh scattered light to the measurements depends on the cloud fraction. In this paper, a retrieval algorithm based on this concept is presented and its accuracy and main error sources are discussed. Additionally, we present results of the algorithm applied to GOME data. These results are validated with independent data. The retrieved cloud parameters will be used to improve the retrieval of tropospheric ozone concentrations from GOME and SCIAMACHY data.

 

Workshop presentation

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