Southern Ocean Mixed Phase Clouds: New Insights from CALIPSO-CloudSat and Ship-Based Hydrometeor Phase Analyses

Abstract:

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 govern the occurrence frequency of mixed-phase clouds are not understood. We have reimagined the CALIPSO lidar phase algorithm to specifically identify mixed phase clouds thereby effectively increasing the known diagnosed presence of ice in shallow boundary-layer based supercooled clouds over the SO by ~70%. In combination with phase diagnostics from several thousand hours of ship-based depolarization lidar, we demonstrate, for the first time, latitudinal and seasonal structures in mixed phase cloud occurrence over the Southern Ocean. Combining the CALIPSO data with CloudSat data, clouds diagnosed as mixed phase from lidar are shown to always co-occur with radar reflectivities indicative of precipitation. As illustrated in the attached figure the seasonally varying latitudinal gradient in mixed phase cloud occurrence that is geographically correlated across the hemisphere with the oceanographic Polar front where ice phase occurrence increases significantly just equatorward of that frontal boundary. We argue that the latitudinal gradient is indicative of enhanced convective motions that result from transitory cold air masses that pass north of the marine thermal boundary. Taken together, we argue that sparse ice nuclei concentrations over the SO enhance the importance of secondary ice production processes that in turn rely on convective motions that can loft precipitation sized hydrometeors above the freezing level. The attached figure shows the seasonal frequency of mixed phase occurrence for boundary layer-based clouds with thickness between 1 and 3 km and with layer top temperatures between freezing and 253 K. The dashed line that meanders around the hemisphere is the seasonal location of the Polar oceanographic front.