For further author information - mjcosta@uevora.pt; phone +351 266 745300; fax +351 266 702306; asilva@uevora.pt; phone +351 266 745300; fax +351 266 702306; v.levizzani@isao.bo.cnr.it; phone +39 051 6399578; fax +39 051 6399649 http://www.isao.bo.cnr.it/~meteosat/; http://www.dfis.uevora.pt Aerosol radiative forcing assessment from polar and geostationary satellite measurements Maria João Costa 1,2 , Ana Maria Silva 1 , and Vincenzo Levizzani 2 1 Department of Physics, University of Évora, Évora, Portugal 2 Institute of Atmospheric and Climate Sciences (ISAC-CNR), Bologna, Italy ABSTRACT Aerosols direct and indirect effects on the Earth’s climate are widely recognized but have yet to be adequately quantified. Aerosol particles scatter and absorb the radiation while at the same time acting as cloud condensation nuclei and thus entering the cloud formation process, influencing their microphysics and eventually the precipitation processes. Therefore the assessment of the aerosol optical properties is of greatest importance. Difficulties arise due to the very high spatial and temporal variability of aerosol concentration, which is the major cause of uncertainties in quantifying the atmospheric radiative forcing. A method to exploit the synergy between the polar orbiting Global Ozone Monitoring Experiment (GOME) onboard ERS-2 and the METEOSAT geostationary system was proposed 1 , aiming at increasing the accuracy of the aerosol characterization and monitoring of the optical thickness. Results of the ongoing validation are presented for relevant transport events of desert dust and biomass burning aerosol over the Atlantic and Indian Oceans during year 2000. Retrieved aerosol optical properties are combined with radiative transfer calculations to assess the direct short wave aerosol radiative forcing in selected regions over the ocean, where strong aerosol events are detected. Retrievals are compared with space-time co-located measurements from the Clouds and the Earth’s Radiant Energy System (CERES) TOA flux product. Keywords: aerosol, satellite, validation, short wave flux, direct radiative forcing 1. INTRODUCTION The uncertainty in aerosol radiative forcing is still considerably large, especially for strong aerosol events, therefore an accurate aerosol characterisation in terms of their optical properties is important to improve estimates of the forcing induced by these particles. The suggested approach using different sensors is expected to improve aerosol characterisation over methods based on a single instrument. New generation polar orbiting sensors present in general good spectral resolution for aerosol studies allowing for the selection of spectral regions mostly avoiding gas absorption, in particular GOME 2 with its high spectral resolution. Nevertheless their inadequacy to follow the evolution of a phenomenon is evident since they overpass the same area once a day at the most. The synergy between polar and geostationary satellite measurements aims at extending the accuracy of aerosol characterisation from polar orbiting satellite sensors to the temporal frequency of a geostationary satellite, thus assuring accuracy and monitoring of a certain event. The method is applied to several case studies using the geostationary meteorological satellite METEOSAT. However it‘s versatility allows it to be used with any geostationary satellite measurements, hence over any Earth region covered by a geostationary satellite, enabling global monitoring of aerosol events. METEOSAT data is used to characterise aerosol properties during strong transport events of dust and biomass burning. These properties are subsequently used to estimate the direct short wave aerosol radiative forcing over the geographical area where the events occur.