Energetics of mixed phase cloud particle interactons

The ratio of the kinetic to surface energy of an ice crystal on impact gives a measure of the available energy for break-up. Such a break-up process may influence particle collision and also particle observations at high velocity (aircraft speed) and at low velocity (terminal velocity). The detailed physical processes of the impact determines the transformation of kinetic energy: 1) converted to thermal energy through viscous dissipation of deforming liquid or displacing air on impact, 2) to create new surfaces during break-up, 3) to form dislocations or melt part of the crystal and 4) retained by bouncing particles.

Ice crystals from convective and stratiform clouds were collected in formvar solution by continuous replicator mounted on the UND Citation aircraft with speed of 130 m/s. Crystal break-up was also video-recorded following impact on the optical flat of a Cloudscope mounted on the NCAR C130 and also at higher speed of 200 m/s mounted on the NASA DC8. Visible fractures account for about 0.6% of the kinetic energy loss; however, in other regions of the crystal severe break-up makes it impossible to measure the fracture length by optical procedures. In order to explain the kinetic energy loss three processes are considered: 1) fractures parallel to simple crystallographic axes, 2) fractures that extend not only along a single plane, but in an arbitrary angle as brittle fracture, and 3) fragments in size down to the ice crystal lattice dimension. Severe break-up (most bonds broken) of 2% of the crystal or melting (1/5 bonds broken) of 6% is enough to explain the crystal kinetic energy losses during the encounter at aircraft speed.

Knowledge regarding ice crystal interactions with the aircraft surface instruments and also with other cloud particles as ice crystals, droplets, and graupel particles characterize ice particle impact and break-up. Further, detail of these two processes may also be of importance in relation to electrical properties of the particle after impaction and break-up, having implications for both aircraft icing and particle measurement instrumentation.