Observing Fast Glaciation of Steam Fog and its Impacts on Cold Air Outbreak Clouds

During Cold Air Outbreaks (CAOs), mixed-phase clouds form when cold air moves over warm water. Therefore, when, where, and how ice particles are generated impact mixed-phase cloud evolution, their radiative forcing, and feedback processes. The NSF CAESAR (Cold Air Outbreak Experiment in the Sub-Arctic Region) Campaign in 2024 produced a unique dataset for exploring diverse ice generation mechanisms in CAO clouds. On 16 March 2024, the NCAR C-130 flew from Greenland's marginal ice zone towards the Norwegian Sea to observe a complete evolution of CAO clouds with observations above, below, and within clouds. Steam fog was visible over the leads and detected by the nadir-pointing MARLi (Multi-function Airborne Raman Lidar) on a long flux leg at ~150 m above the surface. MARLi depolarization measurements indicate quick glaciation of steam fog with in situ cloud probes showing pristine ice crystals with number concentrations exceeding 10/L. The air temperature at the flight level is about -20 ℃, and wind speed ~13 m/s. This near-surface ice is transported upward, seeding supercooled liquid that forms later near the top of the convective boundary layer. When compared with cases first forming a supercooled liquid layer and then followed by subsequent ice generation, this fast ice production near the surface changes the vertical transport of mass and energy and the initial liquid-ice mass partition of mixed-phase clouds, and, therefore, the downstream cloud evolution. MARLi and other remote sensor measurements will be used to characterize cloud vertical and horizontal developments in detail, together with in situ measurements. Results from this case will be compared with CAO clouds where no steam fog was observed.