Dr. Graham Feingold
NOAA Environmental Technology Laboratory
Decades of cloud microphysical research still have not provided conclusive understanding
of the physical processes responsible for droplet spectral
broadening. Numerous mechanisms have been proposed, including entrainment
mixing, vortex shedding, giant cloud condensation nuclei (CCN), chemical processing
of CCN, and radiative cooling, all of which are likely candidates under select
conditions.
In this talk it is suggested that variability in the composition of CCN, and
in particular, the existence of condensation inhibiting compounds, is another
possible candidate. The inferred potential abundance of these amphiphilic film-forming
compounds (FFCs) suggests that their effect may be important. Using a cloud
parcel model with a simplified treatment of the effect of FFCs, it is shown
that modest concentrations of FFCs (on the order of 5% of the total aerosol
mass) can have a marked effect on drop growth and can cause significant increases
in spectral dispersions. Moreover, it is shown that FFCs may in some cases reduce
the number concentration of cloud droplets, with implications for cloud-climate
feedbacks. This trend is at least in qualitative agreement with results from
the ACE-2 field campaign.