Clouds over the Southern Ocean as Observed from the RV Investigator during CAPRICORN: The Properties of Shallow Convection and Extended Stratiform Clouds

The properties of clouds derived from measurements collected using a suite of remote sensors on board the Australian R/V Investigator during two voyages into the Southern Ocean during March and April, 2016 and January and February, 2018 are examined. We focus our attention on a subset of marine boundary layer (MBL) clouds that form a substantial portion of the cloud coverage fraction. We find that the MBL clouds that dominate the coverage fraction often occur in decoupled boundary layers near the base of marine inversions. The thermodynamic conditions under which these clouds are found are reminiscent of marine stratocumulus studied extensively in the subtropical eastern ocean basins except that here they are often supercooled with a rare presence of the ice phase, quite tenuous in terms of their physical properties, rarely drizzling, and tend to occur in migratory high pressure systems where there is a transition from cold to warm air advection. We develop a simple cloud property retrieval algorithm that uses as input the lidar attenuated backscatter, the w-band radar reflectivity, and the 31 GHz brightness temperature. We find that the stratocumulus clouds examined have water paths in the 15 to 25 g m^-2 range, effective radii near 8 um and number concentrations in the 20 cm^-3 range in the Southern Ocean with optical depths in the range of 3 to 4. We speculate that addressing the high bias in absorbed shortwave radiation in climate models will require understanding the processes that form and maintain these marine stratocumulus clouds in Southern middle and high latitudes.