The mechanism of rain formation in deep cumulus clouds

Description

The formation of first raindrops in deep convective clouds is investigated. A combination of observational data analysis as well as 2D and 3D numerical bin microphysical simulations of deep convective clouds suggests that the first raindrops form in slightly diluted cloudy volumes at the tops of bubble cores. It is shown that droplet size distributions in the non-diluted and slightly diluted cores are wider and contain more large droplets than in diluted volumes. The results of the study indicate that process of raindrop formation is determined by the basic microphysical processes within ascending adiabatic volumes. It allows one to predict the height of the formation of first rain drops considering the processes of nucleation, diffusion growth, and collisions. The results obtained in the study explain observational results reported by Freud and Rosenfeld (2012) according to which the height of first raindrop formation depends linearly on the droplet number concentration at cloud base. The results also explain why a dynamically simple adiabatic parcel model can reproduce this dependence. The present study provides a physical basis for retrieval algorithms of cloud microphysical properties and aerosol properties using satellites proposed by Rosenfeld et al. ( 2012).

The study indicates that the role of mixing and entrainment in the formation of the first droplets is, supposedly, not of crucial importance. It is also shown that low variability of effective and mean volume radii along horizontal traverses that is regularly observed by in situ measurements can be simulated with cloud models, in which mixing is parameterized by a traditional 1.5 order turbulence closure scheme using the turbulent kinetic energy equation.