Wintertime/summertime contrasts of cloud condensation nuclei and cloud microphysics over the Southern Ocean

Wintertime/summertime contrasts of cloud condensation nuclei (CCN) and cloud microphysics observed in clean maritime environments off the west coast of Tasmania, Australia in the Southern Ocean Cloud Experiment, are presented. The average wintertime CCN concentration (N_CCN) was 32 cm^-3 at 1% supersaturation (S), but it was 19 cm^-3 when only baseline (maritime airflow with minimal anthropogenic influences) flights were considered. In contrast, the average summertime N_CCN were more than a factor of 5 higher than wintertime for all S ranges. The seasonal contrast was larger when only baseline flights were considered, especially at lower S: summertime more than an order of magnitude higher than wintertime at S below 0.1%. Corresponding cloud droplet concentrations (N_c) showed similar contrasts but to a smaller extent. Summertime average cloud droplet concentrations [N_c(ave)] were only 2.5 times higher than wintertime concentrations (70 cm^-3 versus 28 cm^-3). This difference was nearly a factor of 3 when only baseline flights were considered (57 cm^-3 versus 20 cm^-3). Flight-average N_CCN and various representations of adiabatic cloud droplet concentrations (N_a generally showed good correlations, indicating that the original effects of CCN are somehow retained in the N_c(ave). The average mean diameter (MD) of the cloud droplets was 13.9 and 17.1 μm for the summer and winter clouds, respectively. For baseline only, average MDs were 15.4 and 18.2 μm, respectively. Average MD was thus above or close to the 15-μm threshold for drizzle production, except for the summertime nonbaseline clouds, which had an average MD smaller than 10 μm. Because of the larger droplet sizes, conversion to drizzle was more efficient in the winter clouds, where the average drizzle liquid water content (L_d) of 0.12 g m^-3 was twice that of the summer L_d. The L_d for the summer nonbaseline clouds was negligible. Average L_d was also highly dependent on cloud depth. Winter baseline clouds were sometimes too thin to produce significant drizzle even though they contained very low N_c and very large MDs. The thickest clouds contained the highest L_d although their MD was not always the largest.