Microscopic approach to cloud droplet growth by condensation. Part I: Model description and results without turbulence

Recent observations have shown that even in adiabatic cloud cores, the cloud droplet size distributions are broader than what is expected if all droplets were exposed to the same supersaturation. This suggests the existence of sources of supersaturation variability independent of mixing with noncloudy air. The authors investigate the hypothesis that nonuniformity in the spatial distribution of the size and position of droplets and/or variable vertical velocity in a turbulent medium may be such a source. A 3D numerical model that solves for the trajectory and growth of individual cloud droplets in an evolving turbulent flow field is presented. In this first article of a series of two papers, results from simple experiments with no turbulent flow and with droplets randomly distributed in space are presented. It is found that the random distribution of the position of droplets creates significant supersaturation perturbations. Whether these results in an increase in the width of the size distribution depends on the width of the initial size spectrum. When sedimentation is included, droplets grow in a variable environment. Sedimentation has the effect of reducing the decorrelation time of supersaturation perturbations to a few seconds, thereby decreasing the standard deviation of the distribution of the supersaturation pertubations by 35%-50% and dispersion of the degree of growth (time integral of supersaturation) by ≈65%. Comparison of these results with observations made in adiabatic cloud cores lead to the conclusion that supersaturation perturbations caused by randomly distributed droplets produce too little broadening to explain the observations.