Exploring the transition from condensation to coalescence growth in convective clouds and fogs

Growth by aggregation or coalescence is fundamental to a multitude of natural and engineered processes, from the precipitation of colloids to the formation of planets. The rate at which droplets collide and coalesce to form precipitation is one of the fundamental controlling processes for cloud stability, lifetime, and optical properties. To explore the transition from cloud droplet growth by condensation to growth by coalescence, we take cloud formation in moist Rayleigh-Benard convection as a minimal model. We first explored the conditions favorable to collision-coalescence growth using a Monte Carlo model. Next we developed analytic expressions for the characteristics of the droplet size distribution that account for the onset of growth by coalescence. A dimensionless parameter emerges from the analysis, describing the functional form of the coalescence tail of the size distribution. This allows us to identify cloud conditions that are conducive to drizzle initiation. We conclude by extending the theory to atmospherically relevant conditions and by suggesting that the model may serve as the basis for a physically-based parameterization of drizzle formation in stratiform clouds and fogs.