Sea-ice ridges constitute a large fraction of the total Arctic sea-ice volume (up to 40%); nevertheless, they are the least studied part of the Arctic ice pack. Here we investigate sea-ice melt rates using rare underwater multibeam data that cover a period of one month during the advanced melt stage in the Arctic summer. We show that the degree of bottom melt increases with ice draft for first-year and second-year level ice, and a first-year ice ridge keel, with an average of 0.45 m, 0.55 m, and 0.95 m of total snow and ice melt in the observation period, respectively. While bottom melt rates of ridge keels are 3-4 times higher than first-year level ice, surface melt rates are almost identical. Our estimate attributes 57% of the ridge keel melt variability to keel draft (36%), slope (32%), and width (27%).

In the summer of 2020, ESA changed the orbit of CryoSat-2 to align periodically with NASA’s ICESat-2 mission, a campaign known as CRYO2ICE, which allows for near-coincident CryoSat-2 and ICESat-2 observations in space and time over the Arctic until summer 2022, where the CRYO2ICE Antarctic campaign was initiated. This study investigates the Arctic CRYO2ICE radar and laser freeboards acquired by CryoSat-2 and ICESat-2, respectively, during the winter seasons of 2020–2022, and derives snow depths from their differences along the orbits. Along-track snow depth observations can provide high-resolution snow depth distributions which are vital for air-ice-ocean heat and momentum transfer, understanding light transmission, and snow-ice-interactions. Generally, ICESat-2 is backscattered at a surface above the elevation of the CryoSat-2 signal. CRYO2ICE snow depths are thinner than the daily model- or passive-microwave-based snow depth composites used for comparison, with differences being most pronounced in the Atlantic and Pacific Arctic. Satellite-derived and model-based snow estimates show similar seasonal accumulation over FYI, but CRYO2ICE has limited seasonal accumulation over MYI which is linked to a slow increase in ICESat-2, and to some extent CryoSat-2, freeboards. We present a first estimation of along-track snow depth estimates with average uncertainty of 9 +/- 3 cm for 7-km segments, with random and systematic contributions of 7 and 4 cm. These observations show the potential for along-track dual-frequency observations of snow depth from the future Copernicus mission CRISTAL; but they also highlight uncertainties in radar penetration and the correlation length scales of snow topography that still require further research. 

There are a limited number of studies covering the temporal evolution and spatial distribution of under-ice meltwater and false bottoms for Arctic sea ice. At the same time, they both have a significant effect on the desalinization of sea ice and the ice bottom melt rates. Additionally, these observations are an important part of the meltwater budget. The MOSAiC drifting expedition was aimed to collect field data of coupled processes between ice, ocean, and atmosphere. During the melt season ice cores were collected every week from the unponded first- (FYI) and second-year level ice (SYI) of the investigated ice floe. In addition, ice mass balance buoys were installed in the vicinity of two coring sites, but in ponded areas. This allowed for the comparison of snow, ice, melt pond, under-ice meltwater layer, and false bottom thickness evolution, as well as ice and water physical parameters. Despite the 130 m distance between unponded and ponded FYI sites, the thickness of both under-ice meltwater layer and false bottoms was almost identical. For the SYI, the thicker unponded area had a draft below the meltwater layer and experienced only an ice bottom temperature rise to -1.2°C, while for thinner ponded SYI under-ice meltwater layer was observed. The depth of the seawater and under-ice meltwater layer interface was similar for FYI and SYI. The temperature of under-ice meltwater was close to 0°C, above its freezing point with pronounced diurnal cycles. The under-ice meltwater layer formed three weeks earlier below SYI than below FYI. Due to presence of under-ice meltwater, the FYI bulk salinity decreased from both top and bottom to bulk values below typical for multiyear ice due to only top surface flushing. The thickness of under-ice meltwater layer was stable, around 47 cm for FYI and 26 cm for SYI, in contrast to gradually increasing water equivalent of melted snow and ice. This imbalance indicates a significant horizontal transfer of meltwater.