Turbulent and Sedimentation Fluxes of Aerosols and Droplets in a Convection Cloud Chamber

A laboratory convection cloud chamber, such as the Pi Chamber at Michigan Technological University, can produce a steady-state cloud of water droplets by mixing between a warm and wet lower boundary and a cool and wet upper boundary. The mixing is driven by turbulent Rayleigh-Benard convection. Cloud droplets form (are activated) by condensation on hygroscopic aerosols (such as NaCl) when the saturation ratio of water vapor exceeds a critical value. Conversely, droplets can evaporate and shrink (deactivate) into haze particles when the saturation ratio falls below the critical value. To form and maintain a steady cloud in the chamber, dry aerosols are injected at a constant rate. In a steady state, the rate of droplet fall out balances the rate of aerosol injection. The sedimentation speed of the particles in the chamber is much less than the typical turbulence velocity, so particle motion is largely random, yet it is droplet loss by sedimentation fluxes that balances the injection rate. This aspect, in combination with inhomogeneity of the saturation ratio, leads to regions of preferred droplet activation (sources) and deactivation (sinks), which produce surprising profiles of turbulent and sedimentation fluxes of aerosols and droplets in numerical simulations of a convection cloud chamber.