Life Cycle Evolution of Mixing in Shallow Cumulus Clouds

Understanding how entrainment and mixing shape the cloud droplet size distribution (DSD) is crucial for understanding the optical properties and precipitation efficiency of clouds. Different mixing scenarios, mainly homogeneous and inhomogeneous, shape the DSD in a distinct way and alter the cloud's impact on climate. However, the prevalence of these mixing scenarios and how they vary in space and time is still uncertain, as underlying processes are commonly unresolved by conventional numerical models. To overcome this challenge, we employ the L3 model, which considers supersaturation fluctuations and turbulent mixing down to the finest relevant lengthscales, making it possible to represent different mixing scenarios realistically. We investigate the spatial and temporal evolution of mixing scenarios over the life cycle of shallow cumulus clouds for varying boundary layer humidities and aerosol concentrations. Our findings suggest homogeneous mixing is generally predominant in cumulus clouds, while different mixing scenarios occur concurrently in the same cloud. Notably, inhomogeneous mixing increases over the cloud life cycle across all analyzed cases. The mean and standard deviation of supersaturation are found to be the most capable indicators of this evolution, providing a comprehensive insight into the characteristics of mixing scenarios. Finally, we show inhomogeneous mixing is more prevalent in drier boundary layers and for higher aerosol concentrations, underscoring the need for a more comprehensive investigation of how these mixing dynamics evolve in a changing climate. Clouds play a crucial role in Earth's climate system by influencing the radiation balance and moisture transfer. When clouds mix with their environment, which occurs during a process known as entrainment and mixing, the number and size of cloud droplets change, affecting cloud optical properties and their ability to precipitate. This mixing can happen in two major ways: either the cloud and environmental air are rapidly mixed (homogeneous mixing) or slowly mixed (inhomogeneous mixing). However, accurately representing these effects, particularly inhomogeneous mixing, is challenging in nearly all atmospheric models, including weather or climate models, as essential scales and processes are unresolved. By applying a high-resolution model that resolves all essential scales and processes, we investigate how mixing scenarios evolve over the cloud life cycle. We show that while homogeneous mixing prevails, older (mature or dissipating) clouds tend to mix more inhomogeneously. Moreover, clouds in drier and more polluted environments are shown to favor inhomogeneous mixing. In these environments, inhomogeneous mixing becomes the predominant mixing scenario for older clouds, emphasizing the need for further investigations on the impact of different mixing scenarios in a changing climate. The evolution of predominant mixing scenarios during the shallow cumulus life cycle is investigated While homogeneous mixing is prevalent, inhomogeneous mixing gradually increases as the cloud ages Inhomogeneous mixing is more favored for higher aerosol concentrations and in drier boundary layers