Airborne observations of cloud properties during their evolution from organized streets to isotropic cloud structures along an Arctic cold air outbreak

Abstract. This case study explores the evolution of clouds during an Arctic cold air outbreak in the Fram Strait region observed during the HALO-(𝒜𝒞)³ aircraft campaign. Our research provides information about the formation, structure, micro- and macrophysical properties, radiative effects and investigates the role of vertical wind shear and buoyancy forces in the transition from regular cloud streets to rather isotropic cloud patterns. Our findings show that lower horizontal boundary layer wind speeds (< 12 m s^-1) disrupt the formation of cloud streets, leading to more isotropic cloud patterns, characterized by increasing cloud fraction (from 0.73 to 0.84), cloud top height (from 330 m to 390 m), and quantify the increase of liquid water path as well. In addition, we observe an increase of the number concentration of ice crystals in a size range between 100 µm and 1000 µm and notable riming processes within organized cloud streets. Concurrent radiation measurements in our case study reveal that isotropic cloud patterns can exhibit either low or high albedo as well as low or high F_net,TIR, suggesting that these patterns represent different developing stages.