Frequent bimodal distribution of droplets importantly increases the dispersion

The size distribution of droplets importantly determines 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 diameter of ~13 µm. Cloud chamber experiments were conducted to simulate the formation of clouds: when inducing the entrainment process by mixing less-saturated air mass, the droplets could grow to be 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 maximum when the volume-mean diameter is ~13 µm and the small-droplet mode takes ~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 from ~0.2-0.4 to ~0.8 in independent datasets.