| Summary: | The amount of supercooled liquid and ice within mixed-phase clouds (MPCs) influences precipitation formation, cloud lifetime and controls their radiative properties. The importance of better representing MPCs in Earth System Models for improving climate projections is undisputed, but how the balance between liquid and ice in high-latitude MPCs will change with warming is still unclear. Until recently, they were believed to transition towards a more liquid state. However, recent studies suggest that due to retreating sea ice, local marine sources of ice-nucleating particles (INPs) will become more abundant, leading to more cloud ice. In this regard, cold air outbreaks can provide a natural laboratory to study the influence of sea ice on the ice formation within Arctic MPCs. In order to disentangle the different processes of ice formation, in this study we track 5600 clouds in a Lagrangian way from the CloudSat and CALIPSO satellites as they transition from sea ice to open ocean. We use the Lagrangian statistics to contrast the transition temperature at which INPs are activated over both surfaces. We find that the distance from the sea ice edge serves as a good indicator for categorizing the phase composition of Arctic MPCs, and that the open ocean is an important INP source, fostering ice nucleation at warmer temperatures. By comparing with ice crystal number concentrations, we can identify temperature ranges where secondary ice processes become more relevant. The presented characterization of the phase transition of MPCs provides an important stepping stone for improving estimates of the MPC feedback.
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