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 clou d droplet size distributions are broader than what is expected if all dropl ets were exposed to the same supersaturation. This suggests the existence o f sources of supersaturation variability independent of mixing with nonclou dy air. The authors investigate the hypothesis that nonuniformity in the sp atial distribution of the size and position of droplets and/or variable ver tical velocity in a turbulent medium may be such a source. A 3D numerical m odel 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 create s significant supersaturation perturbations. Whether these result in an inc rease in the width of the size distribution depends on the width of the ini tial size spectrum. When sedimentation is included, droplets grow in a variable environment. Se dimentation has the effect of reducing the decorrelation time of supersatur ation perturbations to a few seconds, thereby decreasing the standard devia tion of the distribution of the supersaturation perturbations by 35%-50% an d the dispersion of the degree of growth (time integral of supersaturation) by approximate to 65%. Comparison of these results with observations made in adiabatic cloud cores lead to the conclusion that supersaturation perturbations caused by random ly distributed droplets produce too little broadening to explain the observ ations.