Observations of Mixed‐Phase Stratiform and Convective Cloud Regimes Over the Southern Ocean: A Case Study Near the Antarctic Polar Front

Abstract Supercooled liquid water (SLW) in mixed‐phase clouds over the Southern Ocean is a key source of uncertainty in climate sensitivity. This case study examines contrasting cloud morphologies and phases in post‐frontal cold‐air masses observed during the Southern Ocean Cloud Radiation and Aerosol Transport Experimental Study. Widespread mixed‐phase stratiform clouds over the cold flank of the Antarctic Polar Front and shallow mixed‐phase convective clouds over the warm flank were observed by coordinated aircraft measurements and satellite retrievals. Correspondingly, the 1.2 km‐thick, well‐mixed marine atmospheric boundary layer (MABL) over the cold flank transformed into a 2 km‐thick, stratified structure over the warm flank. The melting layer was observed to be closer to a stratified layer that decoupled the above cloud layer from the underlying surface layer. This suggests that, in addition to the previously recognized deepening‐warming and drizzle‐induced decoupling mechanisms, melting‐induced cooling in mixed‐phase clouds may contribute to MABL decoupling. Convective cloud tops exhibited higher concentrations of ice particles, with ice particle concentrations exceeding 20 L −1 , at least twice as high as in stratiform clouds. Given previously estimated concentrations of ice‐nucleating particles below 10 −3 L −1 at temperatures between −4°C and −10°C in summertime marine boundary layers, secondary ice production appears to be more active in convective than in stratiform clouds. The cloud‐regime differences were associated with an approximately 130 W m −2 change in reflected solar radiation at the top of the atmosphere, assuming full cloud cover, which is substantially underestimated in state‐of‐the‐art reanalysis.