Document Type

Data

Publication Date

8-20-2026

Abstract

The spatial distribution of cloud droplets is relevant for various microphysical processes, such as condensational growth, collision-coalescence, riming and aggregation, and radiative transfer. Cloud droplets and other hydrometeors exhibit clustering at dissipation scales and above due to turbulence-induced preferential concentration. Here, we investigate the role of entrainment in modulating the scale-dependent clustering of cloud droplets and explore how this mechanism differs from inertial clustering. We have performed experiments in the Pi Convection-Cloud Chamber, and imaged 2-D droplet fields in the cloud-top region where dry-air entrained through an open cylindrical flange leads to an entrainment-mixing process. Captured images are processed to retrieve instantaneous 2-dimensional velocity fields using particle image velocimetry (PIV), scale-dependent clustering is explored using the radial distribution function (RDF), and spatial non-uniformity is quantitatively identified through the Kolmogorov-Smirnov test. The analysis reveals that the observed droplet clustering is scale-dependent and occurs intermittently with enhanced magnitude in the region influenced by entrainment. Furthermore, the strength of clustering and spatial non-uniformity increases with mean entrainment flow. Using a novel bias-mitigation method, we estimate that the strength of clustering (pair-correlation function) roughly doubles in the presence of entrainment (downdraft), compared to no-entrainment (updraft) regimes. Such an entrainment-clustering relationship could be used to help identify and decipher the role of entrainment in modulating localized clustering in natural atmospheric clouds.

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