Turbulence-induced broadening of cloud droplet size distributions: implications for aerosol indirect effects

The optical properties and precipitation efficiency of warm clouds depend on the droplet size distribution and its moments, including the statistical relative-dispersion of the distribution. Cloud droplet growth in a turbulent environment is studied by creating turbulent moist Rayleigh-B{\'{e}}nard convection in a laboratory chamber (the Pi Chamber) and a parallel LES with (bin) cloud-microphysics. Cloud formation is achieved by injecting aerosols into the water-supersaturated environment created by the isobaric mixing of saturated air at different temperatures. A range of steady-state cloud droplet number concentrations is achieved by supplying aerosols at different rates. The results reveal a surprising role of turbulence in cloud droplet formation and growth that can be understood as occurring in two regimes: a polluted cloud regime ($Da \gg 1$) in which thermodynamic conditions are rather uniform and cloud droplet sizes are similar, and a clean cloud regime ($Da \ll 1$) in which thermodynamic conditions are highly variable and cloud droplet sizes are very diverse. The narrowing of the cloud droplet size distribution under polluted conditions introduces a new stabilizing factor by which increased aerosol concentration can suppress precipitation and enhance cloud brightness.