Droplet Clustering in Shallow Cumuli: The Effects of In-Cloud Location and Aerosol Number Concentration.

Aerosol-cloud interactions are complex, including albedo and lifetime effects that cause modifications to cloud characteristics. With most cloud-aerosol interactions focused on the previously stated phenomena, there has been no in-situ studies that focus explicitly on how aerosols can affect droplet clustering within clouds. This research therefore aims to gain a better understanding of how droplet clustering within cumulus clouds can be influenced by in-cloud droplet location (cloud edge vs. center) and aerosol number concentration. The pair-correlation function (PCF) is used to identify the magnitude of droplet clustering from data collected onboard the Center for interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft, flown during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS). Time stamps (at 10^-4 m spatial resolution) of cloud droplet arrival times were measured by the Artium Flight Phase-Doppler Interferometer (PDI). Using four complete days of data with 81 non-precipitating cloud penetrations organized into two flightsof low (L1, L2) and high (H1, H2) pollution data shows more clustering near cloud edge as compared to cloud center for all four cases. Low pollution clouds are shown to have enhanced overall clustering, with flight L2 being solely responsible for this enhanced clustering. Analysis suggests cloud age plays a larger role in the clustering amount experienced than the aerosol number concentration, with dissipating clouds showing increased clustering as compared to growing or mature clouds. Results using a single, vertically developed cumulus cloud demonstrate more clustering near cloud top as compared to cloud base.