Annual cycle of surface-coupling effects on Arctic mixed-phase clouds during MOSAiC

Abstract. Persistent mixed-phase clouds frequently occurred in the Arctic and have significant impacts on the Arctic climate. The surface mixed-layer (SML) coupling status of these clouds impacts their microphysical properties. During an Arctic summer cruise in 2017, surface-coupled clouds were observed to contain ice more often than decoupled clouds at low-supercooling temperatures. Here, an annual cycle of Arctic mixed-phase cloud ice-formation temperatures is presented for the Arctic ice-drift experiment Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) in 2019 and 2020. From October until March no clouds with cloud minimum temperatures above −10 °C were observed. From April to September an increased fraction of ice-containing clouds was observed for clouds with minimum temperatures between −7.5 °C and −5 °C (between 40% and 70%). Between April and July SML-coupled clouds with a minimum temperature above −7.5 °C showed an enhanced fraction of ice-containing clouds, compared to decoupled clouds (2–3 times higher). Also, SML-coupled clouds were 2–4 times more likely to be observed during this period. In August + September the ratio of coupled-to-decoupled ice-containing clouds reduced to 1.3, due to a higher frequency of occurrence of ice-containing decoupled clouds. Using surface-based ice-nucleating particle (INP) measurements the observed phenomena could likely be attributed to the presence of INPs active above −15 °C at the surface. Analysis of sea-ice concentration in the surrounding region, the distance to the ice edge, and the travel time along the back-trajectories to the marginal ice zone supports this finding.