Global climate models and reanalysis products have revealed large, persistent downwelling shortwave radiation biases over the Southern Ocean and coastal Antarctica. The biases are hypothesized to be caused by the incapability of models to accurately simulate the frequent occurrence of low-level mixed-phase clouds in these regions. In this study, we use the ground-based observations collected at Davis, Antarctica during the Precipitation over Land and The Southern Ocean (PLATO) field campaign in austral summer of 2019 to assess the capability of the high-resolution regional Unified Model (UM) to reproduce precipitating clouds off coastal Antarctica. We test the new UM double-moment Cloud AeroSol Interacting Microphysics (CASIM) scheme, running at the spatial resolution of 1.5-km and 100-m. We compare it to the previous single-moment cloud microphysics scheme at the same resolutions. The atmospheric configurations with double-moment cloud microphysics exhibit marginally degraded meteorological conditions relative to single-moment configurations compared with observations. For cloud properties, the UM regional models can generally simulate the phase, vertical structure and timing of events during the sublimation and precipitation periods. Nevertheless, overestimated ice water path and potentially underestimated liquid water path contribute to positive surface shortwave biases and negative longwave biases. The single moment microphysics simulates more liquid water path, though we suggest for the wrong reasons due to its ice nucleating parameterization. Our results suggest that the new double-moment cloud microphysics scheme, while having reduced performance in some respects, has large potential to better represent low-level mixed phase clouds for this region.

Antarctic landfast sea ice (fast ice) is stationary sea ice that is attached to the coast, grounded icebergs, ice shelves, or other protrusions on the continental shelf. Fast ice forms in narrow (generally up to 200 km wide) bands, and ranges in thickness from centimeters to tens of meters. In most regions, it forms in autumn, persists through the winter and melts in spring/summer, but can remain throughout the summer in particular locations. Despite its relatively limited horizontal extent (comprising between about 4 and 13 \% of overall sea ice), its presence, variability and seasonality are drivers of a wide range of physical, biological and biogeochemical processes, with both local and far-ranging ramifications for various Earth systems. Antarctic fast ice has, until quite recently, been overlooked in studies, likely due to insufficient knowledge of its distribution, leading to its reputation as a “missing piece of the Antarctic puzzle”. This review presents a synthesis of current knowledge of the physical, biogeochemical and biological aspects of fast ice, based on the sub-domains of: fast ice growth, properties and seasonality; remote-sensing and distribution; interactions with the atmosphere and the ocean; biogeochemical interactions; its role in primary production; and fast ice as a habitat for grazers. Finally, we consider the potential state of Antarctic fast ice at the end of the 21st Century, underpinned by Coupled Model Intercomparison Project model projections. This review also gives recommendations for targeted future work to increase our understanding of this critically-important element of the global cryosphere.