Soft Matter and Complex Systems Seminar

This talk will discuss spectral broadening of droplet size distributions in turbulent clouds through a mechanism referred to as the eddy hopping. The key idea, suggested a quarter century ago, is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus experience different growth histories, and that this leads to a significant spectral broadening. In this study, the adiabatic parcel model with super-droplets is used to contrast droplet growth with and without turbulence. Turbulence inside the parcel is described by two parameters: i) the dissipation rate of the turbulent kinetic energy, and ii) the linear extent of the parcel. As expected, adiabatic parcel without turbulence produces extremely narrow droplet spectra. In the turbulent parcel, a stochastic scheme is used to account for vertical velocity fluctuations that lead to local supersaturation fluctuations for each super-droplet. These fluctuations mimic the impact of droplets hopping turbulent eddies in a natural cloud. The representation of eddy hopping developed here can be included in a straightforward way in the subgrid-scale scheme of a Lagrangian large-eddy simulation cloud model and may lead to a significant acceleration of simulated rain development through collision/coalescence.