Effect of nucleus size and vapor fluctuations on hail stone formation

The effect of vapor fluctuations on ice initiation and on the growth of frozen drops was investigated using a detailed microphysical parcel model. The simulation of freezing was improved by distinguishing between water drops that have captured ice forming nuclei and those that remain `clearʹ. The size distribution of the nuclei was monodisperse. The numerical experiments were made with three different nucleus radii (1.0 μm, 0.1 μm and 0.01 μm). Calculations show that the effect of vapor fluctuations and ice initiation depends on the size of nuclei. In fluctuating case, the number of 0.01-μm nuclei collected by water droplets is less and that of 1.0-μm nuclei is higher than in non-fluctuating case. The number of 0.1-μm nuclei captured by drops practically does not change. Calculations also show that while the submicron size nuclei initiate ice formation by immersion nucleation, the micron size nuclei mostly act as contact ones. Although the immersional freezing probability does not depend on the drop volume in the model, calculated fractions of frozen drops—similar to the observations—show strong dependence on the drop size.