Less Is More: Low Aerosol Concentrations Produce High Variability in Cloud Droplet Sizes

Shaw, R. A., Michigan Technological University

Cloud Processes

Aerosol Processes

Chandrakar KK, W Cantrell, K Chang, D Ciochetto, D Niedermeier, M Ovchinnikov, RA Shaw, and F Yang. 2017. "Aerosol indirect effect from turbulence-induced broadening of cloud-droplet size distributions." Proceedings of the National Academy of Sciences of the United States of America, 113(50), 50, 10.1073/pnas.1612686113.


Inside the chamber the cloud is turbulent, allowing researchers to keep it for as long as they keep feeding it aerosols.


Inside the chamber the cloud is turbulent, allowing researchers to keep it for as long as they keep feeding it aerosols.

Science

Clouds form when water vapor in the atmosphere condenses around airborne particles, or aerosols, but the exact details of the process remain relatively unknown. Researchers used large-eddy simulations (LES) to model cloud droplets that formed in Michigan Technological University’s cloud chamber by injecting various amounts of aerosols. They found that high concentrations of aerosols decreased the variability in cloud droplet sizes, which could reduce precipitation.

Impact

Because a wider range of droplet sizes is known to promote faster precipitation development, understanding how turbulence, aerosols, and cloud droplets interact is important for predicting next month’s weather as well as next decade’s climate. Clouds are a major forcing agent in radiative processes, and precipitation plays a major role in the water cycle. This study provides insight into the role of aerosols in cloud formation and will enable better understanding and modeling of clouds. It also may help to explain how higher aerosol concentrations due to human activity can lead to clouds with lower precipitation.

Summary

The research team used a cloud chamber and LES modeling to find the differences between high- and low-aerosol scenarios. They prepared a cloud chamber with a temperature gradient to generate turbulence and mixing leading to a cloud formation, and added aerosols at a constant rate to ensure a steady concentration. With a low aerosol injection rate, they found that cloud droplets tended to be larger, as expected, but surprisingly the overall variability in droplet size increased relative to that under higher aerosol injection rates. The low-aerosol scenarios also had a higher variability in supersaturation levels and a slower regime of microphysical processes when compared to the high-aerosol scenarios, which could be responsible for the broader distribution of droplet size. By recreating the cloud chamber in LES simulations with disabled collision processes among the droplets, the researchers confirmed that supersaturation variability was the most likely driving force behind droplet size variability in the turbulent cloud.