<i>Colloquium</i> : Convection-cloud chambers: Experiment and theory

A key problem in atmospheric physics is to quantitatively understand the processes controlling the formation of precipitation. In clouds consisting of only liquid water (no ice), there is a bottleneck between the initial activation and growth of cloud droplets by vapor condensation and the subsequent growth to form drizzle or rain through the collision and coalescence of cloud droplets. The precipitation bottleneck problem is the context for this Colloquium. Convection-cloud chambers are experimental tools that allow aerosol-cloud-precipitation interactions to be investigated under controlled laboratory conditions. The Colloquium explores the governing physics for cloudy Rayleigh-Bénard convection, detailing the thermodynamic, turbulence, and microphysical processes that underlie cloud formation and evolution. Mean-field and stochastic mathematical models are presented to capture the effects of turbulence, water-vapor supersaturation, and droplet growth. Mechanisms that promote broadening of the droplet size distribution are emphasized because they are expected to contribute to overcoming the condensation-to-coalescence bottleneck problem. Connections between laboratory experiments and atmospheric cloud processes are made using computational models that allow chamber observations to be scaled up to real-world conditions. Simulations of cloudy Rayleigh-Bénard convection are compared with measurements to validate the representation of microphysical processes, including fluctuations in supersaturation and droplet growth. Additional topics, such as the study of mixed-phase clouds, aerosol-cloud interactions, and the role of turbulent entrainment, are explored, emphasizing the versatility of the convection-cloud chamber. The Colloquium concludes with a discussion of future directions, including the development of convection-cloud chambers with larger vertical extents, mixed-phase capabilities, and applications to planetary atmospheres. In warm clouds, drops grow into raindrops through both condensation and collision coalescence, but the observed rapid transition between these mechanisms remains difficult to theoretically explain. The convection-cloud chamber, which reproduces key phenomena such as turbulent fluctuations in droplet concentrations and supersaturation under controlled laboratory conditions, offers a promising approach to addressing this long-standing bottleneck in understanding in cloud physics. This Colloquium reviews the physics underlying the precipitation bottleneck, examines how convection-cloud chambers capture the essential processes, and synthesizes insights from experiments, theory, and computational models that bridge laboratory and atmospheric scales.