Abstract
Abstract. During austral summer 2015, the Microphysics of Antarctic Clouds (MAC) field campaign collected unique and detailed airborne and ground-based in situ measurements of cloud and aerosol properties over coastal Antarctica and the Weddell Sea. This paper presents the first results from the experiment and discusses the key processes important in this region, which is critical to predicting future climate change. The sampling was predominantly of stratus clouds, at temperatures between −20 and 0 °C. These clouds were dominated by supercooled liquid water droplets, which had a median concentration of 113 cm−3 and an interquartile range of 86 cm−3. Both cloud liquid water content and effective radius increased closer to cloud top. The cloud droplet effective radius increased from 4 ± 2 µm near cloud base to 8 ± 3 µm near cloud top. Cloud ice particle concentrations were highly variable with the ice tending to occur in small, isolated patches. Below approximately 1000 m, glaciated cloud regions were more common at higher temperatures; however, the clouds were still predominantly liquid throughout. When ice was present at temperatures higher than −10 °C, secondary ice production most likely through the Hallett–Mossop mechanism led to ice concentrations 1 to 3 orders of magnitude higher than the number predicted by commonly used primary ice nucleation parameterisations. The drivers of the ice crystal variability are investigated. No clear dependence on the droplet size distribution was found. The source of first ice in the clouds remains uncertain but may include contributions from biogenic particles, blowing snow or other surface ice production mechanisms. The concentration of large aerosols (diameters 0.5 to 1.6 µm) decreased with altitude and were depleted in air masses that originated over the Antarctic continent compared to those more heavily influenced by the Southern Ocean and sea ice regions. The dominant aerosol in the region was hygroscopic in nature, with the hygroscopicity parameter κ having a median value for the campaign of 0.66 (interquartile range of 0.38). This is consistent with other remote marine locations that are dominated by sea salt/sulfate.
Highlights
Antarctic clouds have a central role in the weather and climate at high southern latitudes (Lubin et al, 1998; Lawson and Gettelman, 2014)
“in-cloud” measurements were determined as periods when the liquid water content (LWC) was greater than 0.01 g m−3 or when particles were detected by the 2DS
The ice mass is taken as the sum of the high irregular (HI) and medium irregular (MI) 2DS categories, while the liquid mass is taken as the sum of the CAS droplets (> 3 μm) and the 2DS low irregular (LI) category
Summary
Antarctic clouds have a central role in the weather and climate at high southern latitudes (Lubin et al, 1998; Lawson and Gettelman, 2014). Through snow precipitation and their radiative effects, they are key to the mass balance of the Antarctic ice sheet, which impacts global sea levels (van den Broeke et al, 2011) and Southern Ocean circulation (Bromwich et al, 2012). It has been suggested that changes in Antarctic clouds can influence weather patterns as far away as the tropics and even the extratropics of the Northern Hemisphere (Lubin et al, 1998). O’Shea et al.: In situ measurements of cloud microphysics and aerosol over coastal Antarctica
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