Wet deposition in shallow convection over the Southern Ocean

Abstract

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 (N CCN). N CCN 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, N CCN and precipitation observations from Kennaook/Cape Grim Observatory (CGO), we find a statistically significant difference in N CCN between when the upwind meteorology is dominated by open mesoscale cellular convection (MCC) and closed MCC. When open MCC is dominant, a lower median N CCN (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 N CCN and minimum precipitation observed during Austral summer (DJF). Furthermore, the observed MCC morphology strongly depends on meteorological conditions. The relationship between N CCN and precipitation can be further examined across a diurnal cycle during the summer season. Although there was again a negative relationship between precipitation and N CCN, the precipitation cycle was out of phase with the N CCN cycle, leading it by ~3 hours, suggesting other factors, specifically the meteorology play a primary role in influencing precipitation.