The Generation of Secondary Ice Particles in Clouds by Crystal–Crystal Collision

The number of fragments generated by crystal collisions in a cloud is a product of the number of fragments produced per collision and the collision frequency. The first term, called the fragment generation function, was obtained experimentally by taking high-speed photographs of collisions of natural ice crystals with a fixed plate. The number of fragments in a collision was found as a function of the change in momentum on impact with a fixed plate and as a function of crystal type and degree of rime. The difference in the change in momentum for collisions in a cloud compared to the fixed plate is treated theoretically and developed into a mathematical model. The collision frequency is incorporated into the model and rates of fragment generation studied for different crystal combinations, sizes and concentrations. The generation of secondary particles by mechanical fracturing does not explain the presence of large concentrations of ice crystals in relatively warm clouds. The additional crystals generated in smooth stratiform clouds may reach a maximum of 10 times the expected natural crystal concentration at or near cloud base. Isolated convective clouds do not appear to contain the proper conditions to produce significant additional crystals by mechanical fracturing. Stratiform clouds with embedded convection appear to provide the greatest opportunity for second" particle generation. Here the secondary crystal concentrations could reach 100 to 1000 times the expected natural concentrations.