Aerosols and play important but uncertain roles in influencing the radiation budget and hence the circulation of the atmosphere. In order to better understand the role of aerosols-clouds on Earth's radiative budget, a space borne observing system, ‘Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations' (CALIPSO) was launched in June 2006. CALIPSO is part of the A-Train multi-satellite observing system and provides high temporal and spatial resolution lidar data that is designed to shed light on both aerosol and cloud physical properties and the interactions between them.

We use CALIPSO Level-2 data to characterize the optical depth (AOD) and Angstrom exponent of elevated biomass burning aerosol layers advected over low altitude (< 5km) water clouds over the tropical South-East Atlantic Ocean. Using the observed AOD and angstrom exponents from CALIPSO together with cloud optical properties from MODIS, we estimate the direct radiative effect (DRE) by the elevated aerosol layers. We demonstrate that the DRE per unit AOD is a sensitive function of the cloud fraction, and therefore that the DRE of aerosols in models is sensitive to the model representation of clouds. The results highlight the synergy of using multiple sensors to determine important climatological variables.