Observations of Secondary Ice in Southern Ocean Stratiform Clouds

Clouds in the Southern Ocean (SO) have unique microphysical properties due to their existence in a pristine environment. The SO is dominated by clouds with supercooled liquid tops that contribute significantly to the scattering of the incoming solar radiation in the southern hemisphere. Despite their significance for the global climate, the processes maintaining these clouds are fairly unknown, which leads to significant biases in current climate and general circulation models.

One of the largest uncertainty in the SO clouds are their ice particle concentrations and processes related to ice formation. Whereas satellite retrievals tend to overestimate the fraction of liquid in clouds, in-situ observations in the SO region have been scarce and limited in detection of small (<100 μm) ice particles important for ice formation processes. In this contribution, we present a unique dataset of the ice microphysical properties in the SO stratiform clouds performed with the NSF NCAR G-V during the recent Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES).

During the SOCRATES campaign, in-situ measurements were performed with ice microphysical probes covering the size range from few tens of micrometers to several millimetres. The typical sampled clouds were multi-layer stratocumulus clouds at temperatures between 5°C and -20°C with ice particle concentrations from few to hundreds of particles per litre. Evidence of secondary ice formation was observed frequently, including observations of small pristine columns together with rimed needles. The observed ice particle concentrations will be discussed with relation to aerosol properties, droplet size distributions, cloud temperature and cloud coupling to the surface. Furthermore, the dominant secondary ice processes in the SO will be evaluated, including the role of supercooled drizzle droplets in the secondary ice formation.