Articles | Volume 16, issue 2
Research article
27 Jan 2016
Research article |  | 27 Jan 2016

Observations of high droplet number concentrations in Southern Ocean boundary layer clouds

T. Chubb, Y. Huang, J. Jensen, T. Campos, S. Siems, and M. Manton

Abstract. Cloud physics data collected during the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) campaigns provide a snapshot of unusual wintertime 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 48° S to the south of Tasmania. Cloud droplet number concentrations were consistent with climatological values in the northernmost profiles but were exceptionally high for wintertime in the Southern Ocean at 100–200 cm−3 in the southernmost profiles. Sub-micron (0.06  < D <  1 µm) aerosol concentrations for the southern profiles were up to 400 cm−3.

Analysis of back trajectories and atmospheric chemistry observations revealed that while conditions in the troposphere were more typical of a clean remote ocean airmass, there was some evidence of continental or anthropogenic influence. However, the hypothesis of long-range transport of continental aerosol fails to explain the magnitude of the aerosol and cloud droplet concentration in the boundary layer. Instead, the gale force surface winds in this case (wind speed at 167 m above sea level was  > 25 m s−1) were most likely responsible for production of sea spray aerosol which 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 occur regularly, and are expected to increase in frequency, over windy parts of the Southern Ocean.

Short summary
The remote Southern Ocean is known to be one of the most pristine environments on the planet, but we found that cloud droplet and aerosol concentrations during one research flight in June 2009 were higher than expected. We were unable to attribute this to continental aerosol sources, and we hypothesize that strong winds resulted in local aerosol production in the form of sea salt. This has several consequences for climate modelling and cloud physics research.
Final-revised paper