Droplets to Drops by Turbulent Coagulation

This study addresses two central problems in cloud microphysics. The first is the source of large droplets, which initiates the rapid production of warm rain. The second is the broadening of the cloud droplet spectrum at both tails of the spectrum. The study explores how in-cloud turbulence can help to close the gaps in our understanding. With box model simulations, the development of cloud droplet spectra is calculated using a coagulation kernel that recently has been derived from direct numerical simulations. This kernel includes both the effect of turbulence on the relative velocities of the droplets and on the local increases in droplet concentration, the so-called accumulation effect. Under the assumption that this kernel can be extrapolated to atmospheric Reynolds numbers, the results show that for typical atmospheric conditions, the turbulent coagulation kernel is several orders of magnitude larger than the sedimentation kernel for droplets smaller then 100 m. While for calm air after 30-min simulation time, only 7% of the total mass is found in droplets with sizes over 100 m, this increases to 79% for a dissipation rate of 100 cm s 3 and 96% for 300 cm s 3 if a combined sedimentation and turbulent kernel is employed that assumes that the sedimentation and turbulent kernel can be added. Hence, moderate turbulence can enhance significantly the formation of large droplets. Furthermore, a time-scale analysis shows that broadening at the upper end of the spectrum is caused by turbulent coagulation whereas thermodynamic effects are responsible for broadening at the lower end.