Coalescence Scavenging Drives Droplet Number Concentration in Southern Ocean Low Clouds

Cloud droplet number concentration (N-d) 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 N-d in SO stratocumulus clouds. The model predicts N-d with little bias and a correlation coefficient of similar to 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 N-d by as much as 90% depending on the precipitation rate. The free tropospheric aerosol source controls N-d more strongly than the surface aerosol source during austral summer.

Automated summary

In this study, high-altitude aircraft observations and a simple source-and-sink budget model were used to investigate the formation and control mechanisms of cloud droplet number concentration (N-d) in Southern Ocean stratocumulus clouds during the austral summer. The results showed that coalescence scavenging is an important sink of cloud condensation nuclei (CCN) in Southern Ocean clouds, and the free tropospheric aerosol source has a stronger impact on N-d than the surface aerosol source.