The aerosol activation process closure experiment

Sarah Guibert, Meteo-France

The CLOUDY-COLUMN project, as part of the second Aerosol Characterization Experiment (ACE-2), was focused on the experimental study of the indirect effect of aerosols on climate in marine boundary layer clouds. The aerosol indirect effect refers to the impact of anthropogenic aerosols on the radiative properties of clouds. It involves various physical processes and interactions between aerosols, cloud microphysics and dynamics, and cloud radiative properties. Central to this topic are the particles that act as cloud condensation nuclei (CCN). This seminar will discuss the aerosol/cloud droplet number concentration (CDNC) interaction, which we refer to as the CCN activation process.

The CLOUDY-COLUMN data analysis converge to the conclusion that CDNC values predicted with numerical modeling of the aerosol activation process, initialized with the measured aerosol properties, were systematically overestimated with respect to CDNC values measured in cloud. In contrast direct measurements of the CCN activation spectra were found to reproduce the observed CDNC. The inputs to the CCN activation model are the physical and chemical properties of the aerosols, and the vertical velocity at cloud base. CDNC, as measured within the cloud layer, is the output or control parameter. The consistency of this diverse data set is examined, including spatial and temporal variability of the measurements, instrumental errors, and the methodology for the characterization of the relevant parameters. The last point concerns the properties of the sub-micrometric aerosols and their possible effect on the aerosol activation process.