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.

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The properties of the ice phase in a number of cloud types are investigated to improve the ice phase parametrization in atmospheric global-climate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North Atlantic, and continental convective clouds over New Mexico and Montana in the USA are studied.Ice concentrations are seen to be several orders of magnitude higher than those which could be attributed to primary nucleation of ice nuclei at cloud-top temperatures. Thus secondary ice multiplication processes must be operating in each cloud type. Evidence suggests that the process of ice splinter production during riming, the Hallett-Mossop process which operates at temperatures around -6 "C, is the dominant mechanism operating.The data analysed are parametrized as phase ratios, the fraction of cloud condensed water found in the liquid phase, and the variation of this phase ratio with temperature is examined. The greatest differences are observed between frontal and convective clouds, although smaller differences between continental and maritime clouds of the same type are also seen. In general, frontal clouds possess very high fractions of ice across a wide range of temperature. In contrast, convective clouds exhibit a wide range of phase ratio across the whole temperature range observed. These differences are attributed to the greater vertical wind velocities present in convective clouds.These parametrizations have been used in the UK Meteorological Office Global Climate Model. They are valid for clouds which span the Hallett-Mossop splinter-production temperature range.

<p><strong>Abstract.</strong> In situ airborne observations of cloud microphysics, aerosol properties and thermodynamic structure over the transition from sea ice to ocean are presented from the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign. A case study from 23 March 2013 provides a unique view of the cloud microphysical changes over this transition under cold air outbreak conditions. Cloud base and depth both increased over this transition, and mean droplet number concentrations also increased from approximately 80 cm<sup>−3</sup> over the sea ice to 90 cm<sup>−3</sup> over the ocean. The ice properties of the cloud remained approximately constant. Observed ice crystal concentrations averaged approximately 0.5–1.5 L<sup>−1</sup>, suggesting only primary ice nucleation was active; however, there was evidence of crystal fragmentation at cloud base over the ocean. The liquid-water content increased almost four-fold over the transition and this, in conjunction with the deeper cloud layer, allowed rimed snowflakes to develop which precipitated out of cloud base. Little variation in aerosol particle number concentrations was observed between the different surface conditions; however, some variability with altitude was observed, with notably greater concentrations measured at higher altitudes (> 800 m) over the sea ice. Near-surface boundary layer temperatures increased by 13 °C from sea ice to ocean, with corresponding increases in surface heat fluxes and turbulent kinetic energy. These significant thermodynamic changes were concluded to be the primary driver of the microphysical evolution of the cloud. This study represents the first investigation, using in situ airborne observations, of cloud microphysical changes with changing sea ice cover and addresses the question of how the microphysics of Arctic stratiform clouds may change as the region warms and sea ice extent reduces.</p>

SUMMARYA model has been constructed of the orographic enhancement of snowfall by the seeder-feeder mechanism. It is found that the collection of water from the feeder cloud is much more efficient than with rainfall under similar conditions, producing significantly larger enhancements of up to three times the seeder cloud precipitation rate. Over long hills growth is mostly by vapour diffusion, with riming dominating over shorter hills. It is found that the peak snowfall enhancement is strongly dependent on hill height, wind speed and feeder cloud depth over a long hill. These sensitivities are much weaker or are absent over short hills due to the large effects of wind drift.