In situ measurements of cloud microphysics and aerosol over coastal Antarctica during the MAC campaign

Sebastian O'Shea, Thomas Choularton, Michael Flynn, Keith Bower, Martin Gallagher, Jonathan Crosier, Paul Williams, Ian Crawford, Z L Fleming, Constantino Listowski, A. Kirchgaessner, R. S. Ladkin, Thomas Lachlan-Cope

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    During austral summer 2015 the Microphysics of Antarctic Clouds (MAC) field campaign collected 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.

    The sampling was predominantly of stratus cloud, 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 inter-quartile range of 86 cm−3. The concentration of large aerosols (0.5 to 1.6 μm) decreased with altitude and were depleted in airmasses 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 0.64 (interquartile range = 0.34). This is consistent with other remote marine locations that are dominated by sea salt/sulphate.

    Cloud ice particle concentrations were highly variable with the ice tending to occur in small isolated patches. Below ca 2000 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 Hallet-Mossop mechanism lead 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. However, higher ice concentrations were found in updrafts and downdrafts compared to quiescent regions. 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.
    Original languageEnglish
    JournalAtmospheric Chemistry and Physics
    Early online date21 Apr 2017
    Publication statusPublished - 2017


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