The evolution of cloud microphysical properties during Cold Air Outbreaks – a composite approach to in situ measurements

Abstract. When cold, dry polar air is advected over a warmer ocean, a rapid development of clouds is observed. Several airborne field campaigns have been dedicated to these marine Cold Air Outbreaks (mCAOs). However, their properties, as well as their impact on the energy budget and water cycle, remains poorly understood. This study investigates the evolution of cloud microphysics during mCAOs through use of airborne in situ observations of ice crystals and water droplets from a recent Spring campaign in the Norwegian Sea (ISLAS2022). As individual flights only offer snapshots into certain parts of the mCAO evolution, a composite approach has been developed that integrates the in situ microphysical observations from multiple flights in order to capture the entire mCAO development. Thin, low ice clouds were observed over sea ice, reaching cloud top altitudes around 1200 m high. A rapidly developing "stratiform" region has been observed, where liquid-topped mixed-phase clouds increase in vertical extent, as the boundary layer deepens with increasing distance from the sea ice edge. At around 600 km fetch, rapid glaciation in a subsequent "convective" region leads to deep (up to 4500 m cloud top), almost completely glaciated, precipitating clouds that are reaching the end of their lifetime. While the observed microphysical properties are in agreement with earlier studies, the study highlights the potential of the composite approach in moving away from individual case studies to a more holistic microphysical picture of mCAOs, especially when statistics are improved by including additional campaign datasets in the future.