How well do the regional atmospheric and oceanic models describe the Antarctic sea ice albedo?

Abstract. A realistic representation of the Antarctic sea ice surface albedo, especially during the spring and summer periods, is essential to obtain reliable atmospheric and oceanic model predictions. We used regional climate (HCLIM, MAR, RACMO), regional oceanic (MetROMS-UHel, NEMO) models and ERA5 reanalysis to investigate how well these models describe the basic sea ice characteristics: sea ice albedo, snow and ice thickness. We analyse models against a range of observations, including in-situ measurements from the ISPOL (Weddell Sea, Dec. 2004) and Marsden (McMurdo Sound, Nov. 2022) field campaigns, as well as drone and satellite data. Models perform well in reproducing the sea ice in certain conditions: during the ISPOL campaign, characterised by thicker snow cover and mild weather that resulted in daytime melt-driven albedo changes and nighttime refreezing in the snow-covered sea ice most models did well; MetROMS-UHel, NEMO, HCLIM and MAR reproduce mean values found in observations, and MetROMS-UHel captures even the observed diurnal albedo variability. However, all models had difficulty reproducing the sea ice conditions in the McMurdo Sound. The observed mean surface albedo was largely influenced by variations in drifting snow accumulation patterns over very thin (few to few tens of cm) snow cover and most models clearly overestimated the albedo. Over the colder and drier sea-ice regions with thin or patchy snow cover, the key issues affecting the accuracy of albedo models are the treatment of fractional snow cover and the snow albedo dependence on snow depth. Over the broader Weddell and Ross seas, sea ice albedo is primarily determined by sea ice concentration fields. HCLIM, MAR, and RACMO rely on ERA5 input for sea ice concentration fields, whereas MetROMS-UHel and NEMO calculate them internally, resulting in differences in both sea ice concentration and albedo patterns. Albedo parameterisations are still relevant: RACMO and ERA5 predict significantly darker sea ice over the Weddell Sea during the ISPOL campaign, while their predictions align better with observations over the Ross Sea during the Marsden campaign. Sea ice albedo is typically parameterised in models as a function of one or more variables, including air temperature, surface temperature, snow/ice type, snow grain size, snow depth, density, sea ice thickness, cloud cover fraction and solar zenith angle. The simplest approaches, like those in ERA5 and RACMO, rely on prescribed sea ice albedo values based on Ebert and Curry (1993). In HCLIM, the intermediate-complexity snow model determines snow reflectivity based on snow grain size distribution, which is only a function of snow density, and the bare ice albedo follows a simple temperature-based relationship. When more sophisticated radiative transfer schemes are applied, albedo is calculated based on the inherent optical properties of the surface, such as in MetROMS-UHel. Integrating advanced radiative transfer models to the regional climate or ocean models, represents a significant advancement in simulating surface processes.