  The effect of aerosol emissions on the properties and distribution of clouds
is a large source of uncertainty in predictions of weather and climate. These
aerosol-cloud interactions depend critically on the ability of aerosol
particles to activate into cloud condensation nuclei (CCN). A key challenge in
modeling CCN activation and the formation of cloud droplets is the
representation of interactions between turbulence and cloud microphysics.
Turbulent mixing leads to small-scale fluctuations of water vapor and
temperature that are not resolved in large-scale atmospheric models. We used
Lagrangian parcel simulations driven by a high-resolution Large Eddy Simulation
of a convective cloud chamber to quantify the impact of these small-scale
fluctuations on CCN activation. We show that small-scale fluctuations in
environmental properties strongly enhance CCN activation, which suggests that
conventional Large Eddy Simulations and Earth System Models that neglect these
fluctuations underestimate cloud droplet formation.
