A closed-to-open cell mixed-phase cloud transition observed over the Nordic Seas under high aerosol loading and strong surface fluxes

Abstract. Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm^-3, decreasing to 90 cm^-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m^-2 to 270 g m^-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (N_i) are generally 2 L^-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m^-2 collocated with strong updrafts, adjacent to glaciated cloud. N_i surpasses 10 L^-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr^-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m^-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.