Why Do Warm Clouds Rain: A Comparison of Southern Ocean and South-East Pacific Marine Stratocumulus Clouds

Aircraft observations from three NSF/NCAR deployments are used to determine the primary causes of warm rain from marine stratocumulus decks. The three data sets are from the 2008 VOCALS deployment in the southeast Pacific off the coast of northern Chile, the 2016 ORCAS deployment over the Southern Ocean off the southern tip of Chile, and the 2018 SOCRATES deployment over the Southern Ocean south of Tasmania, Australia. For each of these deployments, about 60 slant aircraft profiles (“vertical soundings”) are used to characterize the cloud microphysics.

Despite somewhat similar drop concentrations and mean radii for the subtropical Pacific and Southern Ocean marine stratus decks, this study shows that the warm rain process appears retarded over the Southern Ocean. Median drizzle drop concentrations are about a factor 2-5 lower over the Southern Ocean. Some studies have suggested that the persistence of marine stratus over the Southern Ocean is due to the relative lack of ice nuclei in these air masses. However, an important finding of the present study is that the warm rain process may also be less active, thus potentially leading to more persistent Southern Ocean clouds.

Given the recent completion of SOCRATES, analysis of the levels of giant sea-salt cloud condensation nuclei (GCCN), turbulence levels, and the degree of decoupling has only just started, but each of these may have considerable impact on the respective strengths of the warm rain processes. Currently a thorough classification of the different marine cloud types is underway; this is done to select only profiles with stratocumulus decks, with mixed cumulus and stratocumulus, and with only cumulus in order to select appropriate vertical profiles from the three regions for further analysis.

Drizzle drop concentrations from VOCALS are shown below. The figure shows the drizzle drop concentration (averaged over the cloud depth) as a function of near-cloud top cloud droplet concentration (N_ct) cloud droplet mean volume radius (r_cvt). Each symbol corresponds to single vertical profile through the cloud decks. Similar figures, but with important differences, are available for the ORCAS and SOCRATES deployments.