Ice particles in stratiform clouds in the Arctic and possible mechanisms for the production of high ice concentrations

The presence of ice particles in clouds affects precipitation processes, th e radiative properties of the clouds, and the derivation of cloud propertie s from remote sensing measurements. High ice particle concentrations occur often in slightly to moderately supercooled clouds in the Arctic. This pape r combines data collected in a common type of ice-producing arctic cloud (s tratocumulus) with calculations based on laboratory experiments to elucidat e mechanisms that might be responsible for the ice. Ice splinters produced during riming could account for the relatively high concentrations of ice p articles in clouds that encompass temperatures between -2.5 degreesC and -8 degreesC. However, it has generally been assumed that ice splinters grow i nto pristine ice crystal habits, whereas detailed measurements in an arctic stratocumulus cloud showed that only 32% of the ice particles were pristin e crystals (needles, sheaths, short columns, and plates) and 10% were broke n pieces of needles or sheaths. Thirty-seven percent of the ice particles w ere not identifiable crystal types, 20% were frozen drops, and 1% were aggr egates and graupel. The large numbers of unidentifiable ice particles could have originated from the fragmentation of delicate ice crystals and the sh attering of some drops during freezing in free fall. These two mechanisms m ay also play a role in the production of relatively high ice particle conce ntrations in moderately supercooled arctic clouds that lie outside of the t emperature zone where ice splinter production by riming occurs.