Untangling the Broadening of Adiabatic Cloud Droplet Spectra through Eddy Hopping in a High-Resolution Cumulus Congestus Simulation

Abstract Turbulence has long been considered an important source of droplet spectral broadening in adiabatic volumes of warm convective clouds. The key idea is that, in a turbulent environment, droplets follow different trajectories, and this leads to wide droplet spectra for droplets arriving at a given location inside a cloud. This has been referred to as eddy hopping. Past theoretical studies and idealized turbulence simulations applying direct numerical simulation (DNS)-like approaches suggested that eddy hopping can potentially explain the difference between the observed droplet spectra and those predicted from adiabatic ascent in a nonturbulent volume. This paper considers droplet spectra in an adiabatic volume not far from the cloud base in an unprecedented high-resolution (7.5-m grid length) three-dimensional (3D) simulation of a warm turbulent cumulus congestus cloud applying Lagrangian particle–based microphysics. The spectral width approaches several tenths of 1 µ m in the 3D simulation versus only up to 0.2 µ m in a reference nonturbulent adiabatic parcel. We apply an idealized one-dimensional stochastic cloud updraft model that either excludes or includes turbulent vertical velocity fluctuations to show how the fluctuations affect cloud condensation nuclei (CCN) activation and subsequent growth of cloud droplets. Droplet spectra are significantly wider when effects of turbulence are included. The more complete droplet growth equation that includes kinetic, surface tension, and solute effects above the cloud base significantly adds to the variability of cloud droplet growth in the turbulent flow and thus to the adiabatic spectral width at a given height within the simulated cloud. Significance Statement The width of cloud droplet spectra has a significant impact on radiative transfer through a cloudy atmosphere and on drizzle and rain formation in natural clouds. Aircraft observations show that droplet spectra in undiluted or weakly diluted cloudy volumes are often wider than expected in an adiabatic parcel rising from the cloud base. In turbulent clouds, cloud droplets growing by the diffusion of water vapor experience different growth histories while following different turbulent eddies. This is what we mean by the eddy-hopping mechanism. This paper discusses eddy hopping as seen in a turbulence-resolving three-dimensional cloud simulation and in idealized one-dimensional cloud updraft simulations that provide context for the three-dimensional cloud model results.