On the Frequency of Occurrence of the Ice Phase in Supercooled Southern Ocean Low Clouds Derived From CALIPSO and CloudSat

Supercooled liquid clouds are an important component of the albedo of the Southern Ocean (SO). While ice phase occurrence in liquid-dominant clouds (hereafter mixed phase) at temperatures warmer than the homogeneous freezing point is rare in the SO, the processes that create mixed-phase clouds are not understood. Using data from the CALIPSO lidar, we reconsider the thresholds of layer-integrated depolarization ratio and layer-integrated attenuated backscatter that are used to diagnose the phases of fully attenuating cloud layers. We argue that liquid-only clouds have understood physical bounds to these thresholds allowing for unique identification of layers that are not consistent with the presence of single-phase liquid tops. Compared to the original phase algorithm the application of these physically constrained thresholds results in a similar to 70% increase in mixed phase during the annual cycle considered. Combining the CALIPSO data with CloudSat data, mixed-phase clouds seem to typically cooccur with precipitation implying secondary ice forming processes.