Formation of Graupel

Abstract

Measurements were made of the sizes and concentrations of graupel and snow crystals occurring in seeded and untreated winter storms of the Sierra Nevada. The amounts of rime on individual snow crystals were also determined. The observations show that crystal riming, and formation and precipitation of graupel, are common to all stages of Sierran snowstorms. Graupel occurs simultaneously with all types of snow crystals, but individual crystal types do not consistently occur with or serve as kernels for particles of graupel.Graupel particles do not develop predominantly on kernel snow crystals that grow to relatively large sizes over relatively long periods by deposition and accretion. Graupel often forms instead on a select few small crystals, when it forms utilizing regular crystals as kernels. Graupel frequently develops without kernels, by alternate riming processes. One possible alternative is a mechanism that produces graupel from rime accumulated at localized points on parent snow crystals.