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Abstract

In cold air outbreaks (CAOs), cold and dry air flows from the central Arctic southward often leading to extreme weather at mid-latitudes. Roll convection in the boundary layer is triggered within the marginal sea ice zone. During the air-mass transformation, this roll convection leads to cloud formation within several 100 km away from the sea ice edge. Mixed-phase processes and extreme surface heat fluxes play an important role in the transformation. Understanding air-mass transformation is very important for weather and climate modelling. However, only few observations of cloud macro- and microphysical characteristics during CAOs along their trajectory exist yet.Our study investigates cloud rolls over the open ocean and sea ice west of Spitsbergen using airborne remote sensing measurements obtained by the Microwave Radar/radiometerfor Arctic Clouds (MiRAC) and Airborne Mobile Aerosol Lidar for Arctic research (AMALi). We focus on two CAO events that took place during the HALO–(AC)3 campaign conducted in March/April 2021. The POLAR 5 aircraft flew several legs along the same track perpendicular to the cloud streets crossing the sea ice edge. This allows an investigation of the air masses with time and distance to sea ice edge. Hence, our analysis resolves the development of the roll convection. Dropsondes serve information of the thermodynamic state of the atmosphere, whereas remote sensing observations allow for a geometrical and microphysical analysis of clouds. In-situ measurements from the POLAR 6 aircraft, which flew collocated with P5 but inside the clouds, resolve aerosol number concentrations and droplet size distributions.