Observations of high droplet number concentrations in Southern Ocean boundary layer clouds
Thomas Chubb, Yi Huang, Jorgen Jensen, Teresa Campos, Steven Siems and Michael Manton.
Monash University
Data from the standard cloud physics payload during the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) campaigns provide a unique snapshot of the microphysical conditions in the boundary layer over the Southern Ocean. On 29 June 2011, the HIAPER sampled the boundary layer in a region of pre-frontal warm air advection between 58 and 48S to the south of Tasmania. Cloud droplet number concentrations were unusually high for wintertime in the Southern Ocean at 150-250/cc in the southernmost profiles, with mean particle diameters of about 10 μm. Sub-micron (0.06 < D < 1 μm) aerosol concentrations were up to 400/cc.
For these profiles, aerosol concentrations in the free troposphere (about 20/cc) were more typical of a clean remote ocean airmass. Analyses of back trajectories and atmospheric chemistry observations indicate that the aerosol loading of the boundary layer was not due to long range transport from the Australian continent. Instead, the gale force surface winds in this case (160 m wind speed was 20–25 m/s ) were most likely responsible for production of sea spray aerosol which has influenced the microphysical properties of the boundary layer clouds. The smaller size and higher number concentration of cloud droplets is inferred to increase the albedo of these clouds, and these conditions may occur regularly over the Southern Ocean.
