Growth of cloud droplets by turbulent collision-coalescence

An open question in warm rain process and precipitation formation is how rain forms in warm cumulus as rapidly as it has sometimes been observed. In general, the rapid growth of cloud droplets across the size gap from 10 to 50 microns in radius has not been fully explained. In this talk, we focus on the growth of cloud droplets by collision-coalescence taking into account both the gravitational mechanism and various enhancements of the collision-coalescence rate due to airflow turbulence. Based on recent direct simulation results of collection rates of settling droplets in atmospheric turbulence, several effects of airflow turbulence on the collection kernel are considered, including (1) the enhanced relative motion due to differential acceleration and shear effects, (2) enhanced average pair density due to local clustering of droplets, and (3) enhanced collision efficiency due to turbulent fluctuations. The kinetic collection equation (KCE) is solved with an accurate bin-integral method and newly developed parameterizations of turbulent collection kernel. At fine bin resolution, the bin-integral method is shown to produce an identical, converged solution as other known integration methods for KCE. Therefore, the method allows for a precise study of the collision-coalescence growth in terms of any initial size distribution and a prescribed form of the collection kernel.

Starting with a narrow initial size distribution centered at 10 microns in radius, our objective is to understand how the time for drizzle formation may be shortened by the various effects of turbulence, when compared with the base case in which only gravitational coagulation operates. Another objective is to study which aspect of turbulent enhancements is most effective in promoting the growth of droplets. Results for the droplet size distribution as a function of time as well as the local mass conversion rate across the size spectrum will be presented. These results will also be compared with other studies using different parameterizations of the turbulent collection kernel.