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International audience The correct representation of Antarctic clouds in atmospheric models is crucial for accurate projections of the future Antarctic climate. This is particularly true for summer clouds which play a critical role in the surface melting of the ice shelves in the vicinity of the Weddell Sea. The pristine atmosphere over the Antarctic coast is characterized by low concentrations of ice nucleating particles (INPs) which often result in the formation of supercooled liquid clouds. However, when ice formation occurs, the ice crystal number concentrations (ICNCs) are substantially higher than those predicted by existing primary ice nucleation parameterizations. The rime-splintering mechanism, thought to be the dominant secondary ice production (SIP) mechanism at temperatures between −8 and −3 ∘C, is also weak in the Weather and Research Forecasting model. Including a parameterization for SIP due to breakup (BR) from collisions between ice particles improves the ICNC representation in the modeled mixed-phase clouds, suggesting that BR could account for the enhanced ICNCs often found in Antarctic clouds. The model results indicate that a minimum concentration of about ∼ 0.1 L−1 of primary ice crystals is necessary and sufficient to initiate significant breakup to explain the observations, while our findings show little sensitivity to increasing INPs. The BR mechanism is currently not represented in most weather prediction and climate models; including this process can have a significant impact on the Antarctic radiation budget.

• The Arctic has warmed by around 2°C since 1850, approximately double the global average. Even if the Paris Agreement successfully limits global warming to a further 0.5°C, the Arctic is expected to warm by at least another 1°C. • The United Kingdom's (UK) weather is linked to conditions in the European Arctic. For example, high atmospheric pressure in the Nordic Seas divert damaging storms across the UK and mainland Europe, with the potential to cause societal disruption from flooding. • It is possible, although presently unconfirmed, that alterations in Arctic conditions provoked the 'Beast from the East' winter storm in 2018. • Scientists need to take observations and improve their understanding of climatic processes in the Nordic Seas and the Arctic Ocean to fill gaps in knowledge about the links between the Arctic climate and the UK's weather; a risk identified by the Intergovernmental Panel on Climate Change (IPCC). • The UK has significant research expertise and experience to understand how global warming will change the Arctic's environment and affect the UK. • This strength, allied with the capabilities of the UK's new polar research ship the RRS Sir David Attenborough, warrants an integrated programme of research, including advanced numerical modelling, to improve predictions of future extreme weather events. • Such a programme must acknowledge that the Arctic is politically an increasingly congested and contested space. It should be designed in collaboration with key Arctic and near-Arctic nations to increase the UK's influence and ability