A survey is presented of microphysical properties of boundary layer clouds sampled over the Southern Ocean during the First Aerosol Characterization Experiment (ACE 1, November-December 1995). Observations were confined between the latitudes of 40 degrees S and 55 degrees S and between the longitudes of 135 degrees E and 160 degrees E, which covers a region south of the island of Tasmania, Because of the remoteness of the region that was sampled, the microphysical cloud structure represents background conditions in which anthropogenic influences are minimal. Maximum cloud droplet concentrations for individual case studies ranged between 45 and 200 cm(-3). These numbers are consistent with those reported from the summer phase of the Southern Ocean Cloud Experiment (SOCEX II, February 1995) and confirm that the droplet concentrations in an unpolluted atmosphere are largely constrained to values below 200 cm(-3). The often observed weak decoupling of the cloud layer from the surface layer suggests that the entrainment of aerosol from the free atmosphere cannot be ruled out as a significant source of cloud condensation nuclei. The effective radius of the cloud droplets ranged from 4.5 to 16.6 mu m. On many occasions the condition for cloud top entrainment instability was satisfied. This implies che occurrence of dry conditions aloft, turbulence generation due to entrainment, and more generally, conditions of broken cloud cover. It is premature to suggest that these conditions are prevalent over the Southern Ocean as the experiment may have inadvertently targeted atmospheric conditions with regions of broken clouds.
