Quasi-Lagrangian observations of cloud transitions during the initial phase of marine cold air outbreaks in the Arctic – Part 1: Temporal and spatial evolution

This work aims to quantify the macrophysical and microphysical properties of Arctic mixed-phase clouds and their temporal and spatial evolution during marine cold air outbreaks in the Arctic. In particular, cloud thermodynamic phase partitioning and phase transitions are discussed. To this end, high-resolution observations from the airborne hyperspectral and polarized imaging system specMACS during the HALO–(AC)³ campaign are analyzed within a quasi-Lagrangian framework based on backward airmass trajectories. Six flights targeting marine cold air outbreaks of different intensity are compared to investigate the variability of cloud evolution. With increasing time the airmass spent above open, sea ice-free ocean, rising cloud top heights, increasing horizontal cloud extents, and growing effective radii of liquid cloud droplets are observed for all cases. In addition, a phase transition from the liquid water to the mixed-phase cloud regime is detected and the ice fraction increases with time. The variability between the observed cloud properties during the cold air outbreaks is large. Larger and faster increasing cloud top heights and effective radii of liquid cloud droplets are observed during stronger events. In addition, the phase transition from the liquid water to the mixed phase occurs earlier and larger ice fractions are reached during the more intense events. The presented data and analyses provide unique observational data, which can be used to improve the representation of low-level Arctic mixed-phase clouds and their evolution during marine cold air outbreaks in models in the future.