Coalescence Scavenging Drives Droplet Number Concentration in Southern
Ocean Low Clouds
Abstract
Cloud droplet number concentration (Nd) is a key microphysical property
that is largely controlled by the balance between sources and sinks of
aerosols that serve as cloud condensation nuclei (CCN). Despite being a
key sink of CCN, the impact of coalescence scavenging on Southern Ocean
(SO) cloud is poorly known. We apply a simple source-and-sink budget
model based on parameterizations to austral summer aircraft observations
to test model behavior and examine the relative influence of processes
that determine Nd in SO stratocumulus clouds. The model predicts Nd with
little bias and a correlation coefficient of ~0.7
compared with observations. Coalescence scavenging is found to be an
important sink of CCN in both liquid and mixed-phase precipitating
stratocumulus and reduces the predicted Nd by as much as 90% depending
on the precipitation rate. The free tropospheric aerosol source controls
Nd more strongly than the surface aerosol source during austral summer.