Environmental conditions favoring ice pellet aggregation

Winter precipitation is an important issue for many countries because of its common occurrence and its potential for destructive consequences. However, correct prediction of the precipitation type when temperatures are near 0 °C is often difficult because many different types can occur. These different precipitation types can occur alone, in close succession, or in combination. This study examines one aspect of this overall issue, the ability of ice pellets to form aggregates through collisions with supercooled drops and the consequent reduction of freezing raindrops at the surface. This issue was examined using a bin model (initiated with a Marshall–Palmer precipitation size distribution) to model the collisions between ice pellets and supercooled rain drops as they fall through a refreezing layer to the surface. Under certain conditions, collisions aloft resulting in ice pellet aggregates can significantly reduce the number of supercooled drops that reach the surface. For example, a reduction of supercooled drop precipitation rate of 56% is achieved in an initial overall precipitation rate of 25 mm/h and this in turn leads to 40% of the ice pellets at the surface being aggregated particles. Lower precipitation rates and a higher proportion of particles that contain ice at the top of the layer lead to a smaller reduction in precipitation rates but not necessarily a smaller fraction of aggregation. These findings imply that the likelihood of ice pellet aggregation must be accounted for in order to predict freezing rain intensity.

Research Highlights
► Collisions between ice pellets and supercooled drops can produce ice pellet aggregates.
► Aggregation can significantly reduce the amount of freezing rain reaching the surface.
► Ice pellet size, precipitation rate and depth of sub-freezing layer can affect aggregation.