Ice nucleation through immersion freezing in mixed-phase stratiform clouds: Theory and numerical simulations

Mixed-phase stratiform cloud layers have commonly been observed at several Arctic locations. However, mechanisms governing production of ice in these cloud layers remain poorly understood. Because of this, models have difficulties correctly representing a steady-state mixed-phase stratus layer. Based on recent lidar and radar measurements of these clouds, along with measurements of Arctic aerosols, the idea of an active immersion freezing regime is presented. This nucleation mechanism provides a pathway for limited production of ice within a super cooled liquid layer, and avoids rapid glaciation through the Bergeron-Findeissen process. The nucleation theory and supporting observations and simulations are presented, along with a discussion of implications of results on future simulation and understanding of mixed-phase stratus. (c) 2009 Elsevier B.V. All rights reserved.