Clouds and precipitation in the initial phase of marine cold air outbreaks as observed by airborne remote sensing

Abstract. Marine cold air outbreaks (MCAOs) strongly affect the Arctic water cycle and, thus, climate through large-scale air mass transformations. The description of air mass transformations is still challenging partly because previous observations do not resolve fine scales, particularly for the initial development of a MCAO, and lack information about cloud microphysical properties. Therefore, we focus on the crucial initial development within the first 200 km over open water of two MCAO events with different strengths observed during the HALO-(AC)3 campaign. Based on unique sampling of high-resolution airborne remote sensing and in-situ measurements, the development of the boundary layer, formation of clouds, onset of precipitation, and riming are studied. For this purpose, we establish a novel approach, solely based on radar reflectivity measurements, to detect roll circulation that forms cloud streets. The two MCAO events observed in April 2022 just three days apart occurred under relatively similar thermodynamic conditions. However, for the first event, colder airmasses from the central Arctic led to a marine cold air outbreak index twice that high as for the second event. Thus, the two cases exhibit different properties of clouds, riming, and roll circulations though the width of the roll circulation is similar. For the stronger MCAO, cloud tops are higher, more liquid-topped clouds exist, the liquid layer at cloud top is wider, and the liquid water path, mean radar reflectivity, precipitation rate, and occurrence are increased. These parameters evolve with distance over open water, as seen by, e.g., boundary layer deepening and cloud top height rising. Generally, cloud streets form after traveling 15 km over open water. After 20 km, this formation enhances cloud cover to just below 100 %. After around 30 km, precipitation forms, though for the weaker event, the development of precipitation is shifted to larger distances. For the stronger event, we detect riming for cloud temperatures below -20 °C. The variability of rime mass has the same horizontal scales as the roll circulation implying the importance of roll circulation on precipitation. The detailed observations of the two MCAO events could serve as a valuable reference for future model intercomparison studies.