Laboratory Studies of the Effect of Turbulence on Aerosol-Cloud Interactions

Michigan Technological University, Houghton MI

Investigators

Abstract

The optical properties of warm clouds depend on the droplet size distribution and its moments. In turn, these are influenced by the aerosol particles that act as nuclei for the formation of cloud droplets. This project will investigate on how the aerosols and cloud droplets interact within a turbulent environment, in the context of both the aerosol first and second indirect effects (albedo and lifetime effects). The studies will be performed in the Pi Chamber, in which clouds are generated through isobaric mixing within a turbulent environment. The laboratory environment allows for aerosol-cloud interaction in the presence of constant thermodynamic forcing and turbulence properties. The laboratory system does not attempt to reconstruct the full complexity of the atmosphere, but rather aims to isolate specific aspects of the aerosol-cloud system. Objectives of the project are to investigate the transient response of clouds as they interact with and cleanse the surrounding aerosol population; To investigate supersaturation variability in a turbulent environment; To investigate the growth of cloud droplets in a steady-state, turbulent environment, specifically to understand the relative roles of the phase relaxation time and the turbulence correlation times; To continue development of a Large Eddy Simulation (LES)/bin-microphysics model for comparison with and interpretation of laboratory observations, with the eventual purpose of serving as a bridge to the full atmospheric context; And to perform initial exploratory work on the ability to form and measure mixed-phase clouds. Intellectual Merit: Results from the Pi Chamber have already led to the verification that cloud droplet growth by condensation is fundamentally different when the phase relaxation time is small or large compared to the turbulence correlation time; and to the illustration that this provides a mechanism for accelerating aerosol activation and cloud collapse. Both of these processes are fundamentally linked to aerosol-cloud interactions and their influence on indirect effects. This research builds directly on those results and will lead to a deeper understanding of cloud cleansing of aerosols, supersaturation variability in turbulent flows, and how that supersaturation variability influences the broadening of cloud droplet size distributions, key aspects of the precipitation formation problem. The results also will be compared with a LES/bin microphysics cloud model that can serve as a link to studying the phenomena identified in the laboratory in the context of the more complex atmosphere. Finally, the study will provide further insight into the conditions necessary to generate steady-state mixed-phase clouds. Broader Impacts: Precipitation formation and radiative properties of clouds are critical aspects of the climate and weather forecasting problems. They are therefore issues of immediate societal relevance and have potential to translate to tangible economic benefits and improvements in the human condition. The Pi Chamber uses Rayleigh-Benard convection as the mechanism for producing supersaturation for cloud formation. The research will contribute to the diverse community of researchers studying Rayleigh-Benard convection. It will have relevance to the vast range of applied problems studied by that community, from stellar atmospheres to the earth's mantle to applications of myriad engineering problems involving turbulent heat transfer. Finally, the Pi Chamber provides valuable opportunities for education: from outreach at the K-12 levels, to undergraduate internships and senior projects, and finally to graduate student and postdoctoral research. The current project includes support for two full-time graduate students who will become well versed in the operation of the cloud chamber as well as with the wide range of associated instrumentation. Outreach events in the cloud chamber lab will be continued on a regular basis. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.