On the Relative Efficiency of Coalescence and Riming

Abstract

Riming growth rates for graupel and frozen raindrops are compared with those of unfrozen drops growing by coalescence. The results indicate that graupel can have a considerable advantage over unfrozen raindrops in regions where the cloud droplets are large (e.g., 20 μm diameter). In this case, the graupel growth rates win also be enhanced by reductions in the graupel bulk densities, at least down to values as low as 0.2 g cm⁻³ or less. When encountering smaller cloud droplets, however, the apparent advantage of the graupel particles fades, and at times may even be reversed.