Drop clustering and drop size correlations from holographic imagery suggest cloud droplet spectral broadening via entrainment-mixing

Abstract. The question of how droplets rapidly grow large enough to initiate collision-coalescence has persisted for decades. Many theories suggest drop clustering on millimeter scales can produce sufficiently large drops (i.e., those in the “bottleneck” size range; ~25 to 50 µm diameters). A novel method is introduced to evaluate drop clustering trends particle-by-particle (i.e., the number/proximity of neighboring drops for given droplets, defined as drop clustering fields)—in contrast to previous studies which determine drop clustering metrics of given sample volumes. Specifically, this study evaluates the statistical likelihood that drops of a given size will either be associated with a significant number of neighboring drops, or be significantly isolated from neighboring drops. Observations are acquired from the HOLODEC during the Cloud System Evolution in the Trades campaign, which sampled subtropical marine warm clouds. The HOLODEC measures drop size distributions and the 3D spatial coordinates of droplets. Results show drops within the bottleneck size range (diameters of ~25–50 µm) are most likely to be significantly isolated from neighboring drops. This “isolated large drop trend” is primarily observed at subsaturated conditions, suggesting entrainment is the contributing factor. Holograms associated with this trend are more likely to have greater mean diameters, suggesting smaller drops are preferentially evaporating. However, these holograms are also more likely to have broader drop size distributions, larger maximum drop sizes and overly regions where precipitation reaches the lowest altitudes from the sampled cloud, suggesting entrainment-mixing drop size distribution broadening is a relevant precipitation-initiation mechanism.