Bright boundary layer clouds over the Southern Ocean observed during the HIPPO campaigns

Clouds over the Southern Ocean (SO) have been receiving attention due to excessive absorbed solar radiation causing large biases in climate models (Trenberth and Fasullo, 2010). These clouds exist in a pristine environment that results in unique microphysical properties, and satellite climatologies (Bennartz, 2007) confirm that droplet concentrations are generally lower in the SO than in the remote oceanic environments in the Northern Hemisphere. However, in-situ observations are lacking and there remains much to be learnt about the interaction between cloud properties and meteorological conditions in the SO.

Data from the standard cloud physics payload during the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations global transects provide a unique snapshot the nature of low-level clouds in the Southern Ocean. On June 29, 2011, the HIAPER sampled boundary layer cloud in a region of pre-frontal warm air advection between 57 and 47°S to the south of Tasmania. Cloud droplet number concentrations were unusually high at 150-250/cc in the southernmost profiles, with mean particle diameters of about 10 μm. Sub-micron aerosol concentrations were around 400/cc, with peak values in the 100-200nm diameter range.

Aerosol concentrations in the free troposphere (about 20/cc) were more typical of a clean remote ocean airmass, so it is unlikely that the aerosol loading of the boundary layer was due to long range transport from the Australian continent. Instead, the high surface winds in this case (500 ft winds were 20-25m/s; gale force) are most likely responsible for production of sea-salt aerosol (see Woodcock (1953)) which has influenced the microphysical properties of the boundary layer clouds. The smaller size and higher number concentration of cloud droplets increases the albedo of these clouds and their impact on the SO radiation budget should be considered.