# Role of turbulent fluctuations in aerosol activation and cloud formation

### Submitter:

Cantrell, Will H. — Michigan Technological University

Shaw, Raymond A — Michigan Technological University

### Area of research:

Cloud-Aerosol-Precipitation Interactions

### Journal Reference:

*Proceedings of the National Academy of Sciences*, 117(29), 10.1073/pnas.2006426117.

## Science

We examine the effects of fluctuations produced by turbulence. Our measurements, from the Pi chamber, show a clear transition from a regime in which the mean saturation ratio dominates to one in which the fluctuations determine cloud properties. We show 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.

## Impact

Our results show that fluctuations in the saturation ratio must be considered, as well the average, when considering which aerosol particles will become cloud droplets.

## Summary

Aerosol particles become cloud droplets when the ambient 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, and see a clear transition from a regime in which the mean saturation ratio dominates to one in which the fluctuations determine cloud properties. The theory developed to interpret these measurements indicates 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 provides a basis for development of activation parameterizations that go beyond the internally homogeneous parcel calculations that have been used in the past.