Measurement Report: Influence of particle density on secondary ice production by graupel and ice pellet collisions

Abstract. We present a laboratory study dedicated to fragmentation due to graupel-graupel and ice pellet-ice pellet collisions and their role in augmenting ice particle concentration in clouds. For this, graupels of different sizes and densities were created utilizing dry growth condition in a cold chamber at -7 °C and -15 °C using a setup that simulates the natural rotation and tumbling motion of freely falling graupels. Ice pellets were generated by freezing water in 3D-printed spherical molds. We conducted collision experiments inside the cold chamber utilizing a fall tube that ensures central and repeatable collision of ice particles at different collision kinetic energies. The number of fragments generated in the collisions were analyzed following a theoretical framework as a function of the collision kinetic energy. Our results revealed a strong dependency of the fragment number on the density of the colliding ice particles, which can be attributed to the particles’ structure. The number of fragments varies between 1 and 20 and, thus, comparable or larger than those resulted in drop freezing experiments. The size of the fragments was in the sub-mm range for graupels, and up to 3 mm for ice pellets. Another set of experiments, focusing on multiple collision of graupel revealed that the number of fragments generated becomes zero when the particle suffers more than three collisions in a row.