The Effect of a Bimodal Droplet Distribution upon the Dispersion Parameter

The size distribution of droplets significantly affects the microphysical processes of clouds. Frequent bimodal distributions of droplets were observed in the condensation regime across a range of fogs and orographic clouds, with a consistent valley at a diameter of ~13 µm. Cloud chamber experiments were conducted to simulate the formation of clouds: when inducing the entrainment process by mixing in less-saturated air, the droplets could grow larger and show a more apparent bimodal distribution. This indicates that the entrainment and additional activation of droplets may promote the bimodal distribution. The bimodal distribution importantly increases the relative dispersion of droplets (ε). The emergence of the second mode increases ε, which reaches a maximum when the volume-mean diameter is ~13 µm and the small-droplet mode comprises ~70% of the total population of droplets. A parameterization scheme is developed to derive the bimodal distribution from bulk number and mass concentration of droplets, which better reproduces the observed ε. The parameterization reduces the RMSE by ~12%–33% and increases R² (coefficient of determination) from ~0.2–0.4 to ~0.8 in independent datasets.