Środowiskowe Seminarium Fizyki Atmosfery

This presentation will discuss spectral broadening of the droplet size distribution through a mechanism referred to as eddy hopping. The key idea, suggested in late 1980ies by Al Cooper, is that droplets arriving at a given location within a turbulent cloud follow different trajectories and thus have 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 ε, and ii) the linear extent of the parcel L. 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 are argued to mimic the impact of droplets hopping turbulent eddies in a natural cloud. For L smaller than a few meters, noticeable spectral broadening is possible only for strong turbulence, say, ε > 100 cm2 s-3. For L typical for grid lengths of large eddy simulation (LES) models (say, L between 10 and 100 m), the impact is significant even with relatively modest turbulence intensities. The impact increases with both L and ε. The representation of eddy hopping presented in this talk can be included in a straightforward way in a subgrid-scale scheme of a Lagrangian LES cloud model. Preliminary results show a noticeable impact on the width of the droplet spectra. This effect has likely a significant influence on simulated rain formation through collision/coalescence, an aspect that needs to be investigated in the future.