The Impacts of Aerosol Particles in Entrained Air on Cloud Microphysics

The impacts of aerosol particles on clouds, precipitation, and climate remain one of the significant uncertainties in climate change. The activation of aerosols is a fundamental microphysical process to understand how cloud microphysical and radiative properties change. Most aerosol particles are activated at cloud base where the cloud is formed. However, aerosols entrained at the cloud top and edge can contribute to the number of cloud droplets above cloud base. The role of entrained aerosols should be considered because it could significantly affect cloud microphysics and lifetime. However, this is still poorly understood.

The Pi convection-cloud chamber is a unique facility that creates a turbulent, supersaturated environment for cloud formation. Aerosols can be activated as cloud droplets, which subsequently grow in the supersaturation field and finally settle out. We modify the basic setup to mimic the entrainment process by using an inlet flange designed to control the temperature, humidity, and flow rate of cloud-free air into the cloudy chamber. In this study, we conduct experiments to investigate the cloud microphysical responses to the entrainment of aerosol-laden air by including aerosol particles in dry-air entrainment flow. The temperature and flow rate of entrained air are fixed for all experiments, but the sizes of aerosol particles in the entrained air differ for each experiment.

We find that cloud droplet number concentration and liquid water content increase significantly regardless of entrained aerosol sizes when entrained air contains aerosol particles. The shape of the droplet size distribution (DSD) is shifted to smaller sizes, and the relative dispersion also decreases because the width of DSD decreases more than the mean radius of droplets (spectral narrowing). These results imply that the droplet residence time increases because the competition for water vapor is more intense. The effect is analogous to an enhancement of liquid water content due to suppression of precipitation in a natural cloud. An increase in aerosol concentration from entrainment reduces the effective radius, ultimately increasing the cloud optical thickness and albedo, which means the clouds are brighter.