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, the radiative properties of the clouds, and the derivation of cloud properties from remote sensing measurements. High ice particle concentrations occur often in slightly to moderately supercooled clouds in the Arctic. This paper combines data collected in a common type of ice‐producing arctic cloud (stratocumulus) with calculations based on laboratory experiments to elucidate mechanisms that might be responsible for the ice. Ice splinters produced during riming could account for the relatively high concentrations of ice particles in clouds that encompass temperatures between −2.5°C and −8°C. However, it has generally been assumed that ice splinters grow into pristine ice crystal habits, whereas detailed measurements in an arctic stratocumulus cloud showed that only 32% of the ice particles were pristine crystals (needles, sheaths, short columns, and plates) and 10% were broken pieces of needles or sheaths. Thirty‐seven percent of the ice particles were not identifiable crystal types, 20% were frozen drops, and 1% were aggregates and graupel. The large numbers of unidentifiable ice particles could have originated from the fragmentation of delicate ice crystals and the shattering of some drops during freezing in free fall. These two mechanisms may also play a role in the production of relatively high ice particle concentrations in moderately supercooled arctic clouds that lie outside of the temperature zone where ice splinter production by riming occurs.