Wet deposition in shallow convection over the Southern Ocean

Abstract

Southern Ocean (SO) air is amongst the most pristine on Earth, particularly during winter. Historically, there has been a focus on biogenic sources as an explanation for the seasonal cycle in cloud condensation nuclei concentrations (NCCN). NCCN is also sensitive to the strength of sink terms, although the magnitude of this term varies considerably. Wet deposition, a process encompassing coalescence scavenging (drizzle formation), is one such process that may be especially relevant over the SO. Using a boundary layer cloud climatology, NCCN and precipitation observations from Kennaook/Cape Grim Observatory (CGO), we find a statistically significant difference in NCCN between when the upwind meteorology is dominated by open mesoscale cellular convection (MCC) and closed MCC. When open MCC is dominant, a lower median NCCN (69 cm−3) is found compared to when closed MCC (89 cm−3) is dominant. Open MCC is found to precipitate more heavily (1.72 mm day−1) and more frequently (16.7% of the time) than closed MCC (0.29 mm day−1, 4.5%). These relationships are observed to hold across the seasonal cycle with maximum NCCN and minimum precipitation observed during Austral summer (DJF). Furthermore, the observed MCC morphology strongly depends on meteorological conditions. The relationship between NCCN and precipitation can be further examined across a diurnal cycle during the summer season. Although there was again a negative relationship between precipitation and NCCN, the precipitation cycle was out of phase with the NCCN cycle, leading it by ~3 hours, suggesting other factors, specifically the meteorology play a primary role in influencing precipitation.