Contrasting Arctic Cirrus Clouds: Properties and Processes from Observations and Simulations

Abstract. Cirrus clouds in the Arctic play a central role in the regional radiation budget, yet their formation pathways and properties remain poorly constrained, especially during Warm Air Intrusions (WAI). In this study, we analyse and contrast cirrus observed during WAI events (WAI cirrus) with those formed under undisturbed Arctic Conditions (AC cirrus) using remote sensing measurements from the HALO–(AC)³ campaign, combined with backward trajectory modelling with CLaMS-Ice and LAGRANTO. This allows us to connect the optical, macro- and microphysical properties measured via lidar and radar to the cloud origin and dynamics during their evolution. We find that WAI cirrus encountered stronger vertical transport along their trajectories compared to AC cirrus. At the time of detection, WAI cirrus were geometrically and optically thicker, exhibited higher depolarization ratios and ice supersaturation, and contained more numerous but smaller ice crystals, consistent with homogeneous ice nucleation at colder temperatures. AC cirrus, in contrast, showed larger ice crystals and a comparably greater contribution from heterogeneous ice nucleation. As WAI events are projected to become more frequent and long-lasting, a resulting increase in the formation of WAI cirrus may enhance their influence on the Arctic radiation budget leading to cooling. Our results provide observational constraints on cirrus variability in the Arctic and emphasize the need for their improved representation in weather and climate models.