The cloud properties and governing processes in Southern Ocean marine
boundary layer clouds have emerged as a central issue in understanding
the Earth's climate sensitivity. While the simulated cloud feedbacks in
Southern Ocean clouds have evolved in the most recent climate model
intercomparison, the background properties of simulated summertime
clouds in the Southern Ocean are not consistent with measurements due to
known biases in simulating cloud condensation nuclei concentrations.
This paper presents several case studies collected during the Capricorn
2 and Marcus campaigns held aboard Australian research vessels in the
Austral Summer of 2018. Combining the surface-observed cases with MODIS
data along forward and backward air mass trajectories, we demonstrate
the evolution of cloud properties with time. These cases are consistent
with multi-year statistics showing that long trajectories of air masses
over the Antarctic ice sheet are critical to creating high droplet
number clouds in the high latitude summer Southern Ocean. We speculate
that secondary aerosol production via the oxidation of biogenically
derived aerosol precursor gasses over the high actinic flux region of
the high latitude ice sheets is fundamental to maintaining relatively
high droplet numbers in Southern Ocean clouds during Summer.