The Pi Cloud Chamber offers a unique opportunity to study aerosol-cloud microphysics interactions in a steady-state, turbulent environment. In this work, an atmospheric large-eddy simulation (LES) model with spectral bin microphysics is scaled down to simulate these interactions, allowing comparison with experimental results. A simple scalar flux budget model is developed and used to explore the effect of sidewalls on the bulk mixing temperature, water vapor mixing ratio, and supersaturation. The scaled simulation and the simple scalar flux budget model produce comparable bulk mixing scalar values. The LES dynamics results are compared with particle image velocimetry measurements of turbulent kinetic energy, energy dissipation rates, and large-scale oscillation frequencies from the cloud chamber. These simulated results match quantitatively to experimental results. Finally, with the bin microphysics included the LES is able to simulate steady-state cloud conditions and broadening of the cloud droplet size distributions with decreasing droplet number concentration, as observed in the experiments. The results further suggest that collision-coalescence does not contribute significantly to this broadening. This opens a path for further detailed intercomparison of laboratory and simulation results for model validation and exploration of specific physical processes.

Aerosol indirect effects are one of the leading contributors to cloud radiative properties relevant to climate. Aerosol particles become cloud droplets when the ambient relative humidity (saturation ratio) exceeds a critical value, which depends on the particle size and chemical composition. In the traditional formulation of this problem, only average, uniform saturation ratios are considered. Using experiments and theory, we examine the effects of fluctuations, produced by turbulence. Our measurements, from a multiphase, turbulent cloud chamber, show a clear transition from a regime in which the mean saturation ratio dominates to one in which the fluctuations determine cloud properties. The laboratory measurements demonstrate cloud formation in mean-subsaturated conditions (i.e., relative humidity <100%) in the fluctuation-dominant activation regime. The theoretical framework developed to interpret these measurements predicts a transition from a mean- to a fluctuation-dominated regime, based on the relative values of the mean and standard deviation of the environmental saturation ratio and the critical saturation ratio at which aerosol particles activate or become droplets. The theory is similar to the concept of stochastic condensation and can be used in the context of the atmosphere to explore the conditions under which droplet activation is driven by fluctuations as opposed to mean supersaturation. It provides a basis for future development of cloud droplet activation parameterizations that go beyond the internally homogeneous parcel calculations that have been used in the past.

No abstract

Clouds play a crucial role in the Earth's energy balance by reflecting solar radiation and absorbing terrestrial radiation. They are also important to the hydrological cycle through precipitation and water redistribution in the atmosphere. A better representation of clouds in atmospheric models can improve weather forecasts and climate projections. One challenge of cloud simulation is to accurately represent cloud properties and processes under various conditions. The challenge arises from limited understanding of complicated cloud microphysical processes at the fundamental level, as well as from imperfect translation of this understanding to the numerical algorithms used to resolve or parameterize microphysical processes (e.g., Khain et al., 2015;Morrison et al., 2020).In general, there are three approaches to simulate microphysical properties of hydrometeors and related processes in atmospheric models: bulk representation of particle properties, bin microphysical schemes, and Lagrangian particle methods. Bulk microphysics schemes use one or more attributes (e.g., moments of the hydrometeor size

The Pi Chamber is a laboratory convection-cloud chamber in which "fully resolved" by nature aerosol and cloud microphysical processes take place within a turbulent medium (Chang et al., 2016). This is a fundamentally different approach than the traditional expansion cloud chamber, which takes its inspiration from the parcel viewpoint, that is, all particles are exposed to the same environment and have the same lifetime. In contrast, a convection-cloud chamber is inspired by a turbulent mixed-layer viewpoint, in which microphysical processes exist in a dynamic steady state with aerosols being continuously introduced, and cloud droplets settling to the bottom. The aerosols and cloud particles are exposed to fluctuating velocity, temperature and water vapor fields, resulting in both positive and negative supersaturations, leading to corresponding activation and deactivation of cloud condensation nuclei (MacMillan et al., 2022;Prabhakaran et al., 2020), as well as the growth and evaporation of cloud droplets (Chandrakar et al., 2016) and ice particles (Desai et al., 2019).The Pi Chamber volume is a cylinder with height and radius both equal to 1 m (the name denoting the volume of π m 3 ). The thermodynamic conditions in a convection-cloud chamber, including the supersaturation forcing