Coalescence Scavenging Drives Droplet Number Concentration in Southern Ocean Low Clouds

• Published in: Geophysical research letters Vol. 49; no. 7
• Main Authors: Kang, L., Marchand, R. T., Wood, R., McCoy, I. L.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.04.2022
• Subjects: aerosol; Aerosols; Aircraft; Aircraft observations; Biology; Cloud condensation nuclei; Cloud droplet concentration; cloud droplet number concentration; Cloud droplets; Cloud formation; Clouds; Coalescence; coalescence scavenging; Coalescing; Condensation nuclei; Correlation coefficient; Correlation coefficients; Drizzle; Droplets; Low altitude; Low clouds; Model testing; Modelling; Oceans; Precipitation; Precipitation processes; Precipitation rate; Reflectance; Scavenging; Sea spray; Southern Ocean; Spray; Stratiform clouds; Stratocumulus clouds; Summer; Sunlight; Water droplets; Water drops; Southern Ocean
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL097819

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. Plain Language Summary Low altitude stratiform clouds are ubiquitous over the Southern Ocean (SO) and have a profound climate impact through reflecting sunlight back to space, cooling the earth. The number of water droplets in a given volume (the concentration) is a key variable that determines the reflective ability of the cloud. The cloud droplet number concentration (Nd) is largely set by the number of aerosols (tiny airborne particles) on which water can condense and is controlled by the balance between sources that generate aerosols (e.g., ocean biology, sea spray) and sinks which remove aerosols from the atmosphere. The formation of liquid precipitation in clouds (drizzle) is a strong sink of aerosols because each precipitation droplet is created through the merger of many cloud droplets (i.e., coalescence) where each cloud droplet contains at least one aerosol particle. Here, we test a simple source‐and‐sink model to predict SO Nd using aircraft measurements from a recent aircraft campaign in the SO during austral summer. We find this model can predict cloud droplet number well and that liquid precipitation processes are important in controlling droplet number concentration (and thus cloud reflectance) over the SO. Key Points A simple source‐and‐sink budget model accurately predicts in situ observations of Southern Ocean (SO) cloud droplet number concentration (Nd) Coalescence scavenging by liquid droplets is a dominant sink of SO cloud condensation nuclei Free tropospheric aerosol controls Nd more strongly than surface aerosol during austral summer