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.