Dr.Vaughan Phillips, Senior University Lecturer
Aggregation of ice crystals (< 1 mm) in collisions to form snowflakes (> 1 mm) is one of the most important processes in the growth of precipitation particles in natural clouds. Snow from quiescent layer-clouds comprises much of the precipitation during the year at high latitudes, in colder seasons and anywhere that freezing level (0 degC) is typically close to the ground. Equally, rebounding ice-ice collisions are essential for the charge separation causing lightning in vigorous convective clouds. Numerical models of clouds and their microphysical processes are generally applied to forecast snowfall and severe weather. The most uncertain aspect of simulating aggregation in ice-ice collisions is currently the 'sticking efficiency', namely the probability that two particles remain stuck together given that they have collided. A new scheme for modeling the sticking efficiency and a new theory are described in the presentation. Past laboratory observations of ice-ice collisions from the 1960s until recently are pooled to create a unified framework for sticking efficiency. Fundamental dependencies on collision kinetic energy and on surface area of colliding particles are inferred from past observations. Implications of the new scheme for forecasting weather are discussed.