The Role of Supersaturation Fluctuations in Determining the Droplet Spectrum in Turbulent Clouds

An equilibrium droplet spectrum can be achieved in thin clouds if they have steady sources of supersaturation and CCN. Droplet activation and growth is then balanced by droplet fallout. Such clouds have been produced and extensively studied in the turbulent cloud chamber (the Pi Chamber) at Michigan Technological University. The Pi Chamber generates supersaturation by mixing saturated vapor at different temperatures; it produces turbulence by inducing Rayleigh-Bénard convection. We are also using the Explicit Mixing Parcel Model (EMPM) to numerically simulate such laboratory clouds. The EMPM explicitly simulates turbulent mixing and individual droplet activation and growth by representing turbulence in a 1D domain with the linear eddy model. Steady-state turbulent clouds have been produced and measured in the Pi Chamber under a range of supersaturations and aerosol injection rates. Chandrakar et al. (2016) proposed that supersaturation fluctuations play a crucial role in determining the width of the resulting droplet spectra. On the other hand, under non-turbulent conditions, the analytical solution for the droplet spectrum shows that the mean supersaturation determines the droplet spectrum width. We are currently using the EMPM and the Pi Chamber to further investigate the relative roles of mean supersaturation and supersaturation fluctuations, and how these are regulated by the aerosol input rate, in determining the resulting equilibrium droplet spectrum in these clouds, and what these results imply for atmospheric clouds.