Broadening of cloud droplet size distributions and warm rain initiation associated with turbulence: an overview

In the study of warm clouds, there are many outstanding questions. Cloud droplet size distributions are much wider, and warm rain is initiated in a shorter time and with a shallower cloud depth than theoretical expectations. This review summarizes the studies related to the effects of turbulent fluctuations and turbulent entrainment-mixing on the broadening of droplet size distributions and warm rain initiation, including observational, laboratorial, numerical, and theoretical achievements. Particular attention is paid to studies by Chinese scientists since the 1950s, since most results have been published in Chinese. The review reveals that high-resolution observations and simulations, and laboratory experiments, are needed because knowledge of the detailed physical processes involved in the effects of turbulence and entrainment-mixing on cloud microphysics still remains elusive. The effects of turbulent fluctuations and entrainment-mixing processes have been unrealistically separated in most theoretical studies. They could be unified by further advancement of a systems theory into a predictive theory. Developing parameterizations for the effects of fluctuations and entrainment-mixing processes is still in its infancy, and more studies are warranted.