Variability and trends in global cloud parameters from Oxygen A-band measurements by GOME and SCIAMACHY

Clouds are the most important factor in determining the Earth's albedo. A change of only about 1 % in global cloudiness can either mask or double the radiative effect of a decade of greenhouse-gas emissions. Recent studies by Palle et al. and Wielicki et al. published in Science report either an increase or a decrease in the Earth's albedo in the period 2001-2004. To assess this controversy in an independent way, we derive for the first time a global time series of cloud parameters from satellite measurements in the Oxygen A-band around 760 nm.

Usually, visible and infrared thresholding methods are being used to detect clouds with meteorological satellite imagers, e.g. the ISCCP and similar algorithms. Thermal infrared methods, however, depend on an assumed atmospheric temperature profile. With the spectrometers GOME on ERS-2 and SCIAMACHY on Envisat we have a unique opportunity for an independent measurement of clouds, namely via the absorption of oxygen. Since oxygen is a well-mixed gas, the reflectance in and around the oxygen A-band is a direct measurement of the amount and altitude of clouds. We use the FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A-band) algorithm for deriving the radiometric-effective cloud fraction and cloud top pressure, and apply this to GOME as well as SCIAMACHY Oxygen A-band measurements for the period 1995-2005. We compare with available cloud products from both ISCCP and the ECMWF Re-Analysis project.

We find a clear seasonal cycle in cloud parameters, dominated by clouds over the oceans. Furthermore, an increase is found in the global effective cloud fraction since 2001. The results are compared to and corroborated with ISCCP cloud measurements over this period.