Where Aerosols Become Clouds-Potential for Global Analysis Based on CALIPSO Data

This study evaluates the potential to determine the global distribution of hydrated aerosols based on Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data products. Knowledge of hydrated aerosol global distribution is of high relevance in the study of the radiative impact of aerosol-cloud interactions on Earth’s climate. The cloud-aerosol discrimination (CAD) score of the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) instrument on the CALIPSO satellite separates aerosols and clouds according to the probability density functions (PDFs) of attenuated backscatter, total color ratio, volume depolarization ratio, altitude and latitude. The pixels that CAD fails to identify as either cloud or aerosol are used here to pinpoint the occurrence of hydrated aerosols and to globally quantify their relative frequency using data of August from 2006 to 2013. Atmospheric features in this no-confidence range mostly match with aerosol PDFs and imply an early hydration state of aerosols. Their strong occurrence during August above the South-East Atlantic and below an altitude of 4 km coincides with the biomass burning season in southern Africa and South America.