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.