Quantification of aerosol effects on warm clouds with the cloud-by-cloud approach

• Applicant: Dr. Matthias Tesche
• Subject Area: Atmospheric Science
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 554555011

Atmospheric aerosol particles that act as cloud condensation nuclei (CCN) are needed to form warm liquid-water clouds under atmospheric conditions. Changes in CCN concentration, however, affect the macro- and microphysical properties of those clouds – namely the albedo, development, phase, lifetime, and rain rate – with implications for the Earth’s energy budget. The resulting effective radiative forcing due to aerosol-cloud interactions (ACI) and rapid adjustments, though a focus of atmospheric research for several decades, still causes the largest uncertainty in the IPCC’s assessment of climate change as it is still understood only with medium confidence. Satellite observations with their global coverage and wealth of observed cloud and aerosol parameters have been widely used to quantify the magnitude and importance of different ACI mechanisms – not always leading to unified conclusions. Apart from technical and methodological challenges, the main limitations are (i) that studies based on polar-orbiting observations are confined to snapshot observations at fixed overpass times and (ii) that we are still lacking reliable information on CCN concentrations that could be used for such studies. The recent activity of the applicant provides a new approach for solving both problems. The research proposed here will improve our understanding of ACI and rapid adjustments based on spaceborne observations of aerosols and clouds by combining snapshots of polar-orbiting sensors with temporally resolved observations from geostationary sensors. Recently developed methods to (i) track the position and development of individual clouds in geostationary observations and to (ii) infer height-resolved aerosol-type specific CCN concentrations from polar-orbiting lidar observations have resulted in a novel cloud-by-cloud approach that collates (i) individual clouds with (ii) the surrounding CCN concentrations, and (iii) meteorological re-analysis fields. The project will create a data set of 15+ years for selected target regions that will be the first of its kind to systematically relate the concentration of cloud-relevant aerosol concentrations at cloud level to information on evolving clouds under different meteorological conditions. The data set will be used to study the direct effect that aerosol perturbations have on the brightness, droplet number concentration, and droplet size of liquid-water clouds as well as how the perturbed clouds adjust to these changes. The resulting process-specific sensitivities will help to further bound the role of ACI in the climate system and to improve the fidelity of climate projections.

• DFG Programme: Research Grants