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

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.

Radiative Effects of Secondary Ice Enhancement in Coastal Antarctic Clouds

“…N ice occurs in patches during both flights, in agreement with the average findings of the campaign (O'Shea et al, 2017). Modeled clouds were predominantly liquid, with high N isg>80 occurring in isolated patches ( Figure S8), in agreement with our observations.…”

“…However, this parametrization lags behind current theories of H-M SIP, as observational evidence suggests that this process is also dependent on the cloud droplet distribution and cloud type (Hobbs & Rangno, 1998;Mossop, 1985b;. Where ice was observed at temperatures spanning the H-M range, the number concentrations were enhanced by up to three orders of magnitude with respect to predicted INP concentrations (O'Shea et al, 2017). To better understand the role of SIP in coastal Antarctic clouds, the Microphysics of Antarctic Clouds (MAC) aircraft campaign was conducted in the austral summer of 2015, from 21 November to 14 December, sampling aerosol particle, cloud, and boundary layer (BL) properties over the Weddell Sea.…”

“…While these clouds are dominated by liquid droplets, isolated patches of high N ice occur (Grosvenor et al, 2012;O'Shea et al, 2017): concentrations which have been attributed to secondary ice production (SIP). SIP is poorly understood and can occur via numerous mechanisms.…”

“…H-M rime splintering is often parametrized using the temperature-dependent triangle function detailed by Reisner et al (1998). Data have been summarized previously by O'Shea et al (2017). SIP has the potential to affect cloud microphysical structure and radiative properties from the high N ice produced and subsequent interactions with the cloud liquid phase through the Wegener-Bergeron-Findeisen (WBF) mechanism.…”

“…The Meteorological Airborne Science INstrumentation used aboard the British Antarctic Survey's Twin Otter research aircraft during the MAC campaign has been detailed previously (O'Shea et al, 2017). Ice particle measurements from the 2-Dimensional Stereo (2-DS) particle imaging probe (SPEC Inc., Lawson et al, 2006) are used for comparison with the PolarWRF model.…”