Life cycle studies and liquid-phase characterization of Arctic mixed-phase clouds: MOSAiC 2019–2020 results

Abstract. Height-resolved monitoring of life cycles of mixed-phase clouds (MPCs) was performed in the free troposphere over the central Arctic during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition from October 2019 to September 2020. The research icebreaker Polarstern served as a platform for state-of-the-art remote sensing of aerosols and clouds. The use of the recently introduced dual field-of-view polarization lidar technique in combination with the well-established lidar-radar retrieval technique provided, for the first time, a robust instrumental basis to monitor the evolution of the liquid and the ice phase of MPCs and the interplay between the two phases. We discuss two long-lasting Arctic MPC cases observed close to the North Pole. During the late summer MPC event, most likely three gravity waves strongly disturbed the cloud evolution. We documented this perturbation in detail in terms of liquid and ice-phase properties and the recovery of the strongly disturbed liquid phase within a few hours. For the first time, cloud statistics, covering all seasons of a year, are presented for liquid-bearing stratiform clouds in the central Arctic. The focus is on the optical and microphysical properties of the liquid phase which is of key importance for a long MPC lifetime. The observations confirmed that ice formation occurs predominantly via immersion freezing. We also found that activation of aerosol particles to form droplets is of great importance for the longevity of MPCs and that the free tropospheric reservoirs of cloud-condensation nuclei and ice-nucleating particles seem to be usually well-filled.