Arctic Freeboard and Snow Depth From Near-Coincident CryoSat-2 and ICESat-2 (CRYO2ICE) Observations: A First Examination of Winter Sea Ice During 2020-2022

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 first-year ice, but CRYO2ICE has limited seasonal accumulation over multi-year ice which is linked to a slow increase in ICESat-2, and to some extent CryoSat-2, freeboards. We present a first estimation of spaceborne along-track snow depth estimates with average uncertainty of 10-11 +/- 2-3 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. Estimates of snow depth on sea ice are currently either outdated or limited in resolution, thus a need to derive high-resolution snow depth on sea ice is crucial. Former studies have computed snow depth on sea ice using the difference in penetration between radar (Ku-band) and laser (or Ka-band radar) altimeters from different satellite missions, assuming the Ku-band radar and laser/Ka-band are reflected at the bottom and top of the snow pack, respectively. Those studies have resulted in monthly composites of snow depth due to a limited overlap of individual tracks between the satellite missions. Since the CRYO2ICE (CryoSat-2/ICESat-2 Resonance) campaign was initiated in July 2020, we have for the first time the possibility of investigating near-coincident observations of spaceborne radar and laser altimeters. These results are important to initiate relevant discussions on snow depth retrieval in preparation for the future dual-frequency altimeter mission, CRISTAL (Copernicus Polar Ice and Snow Topography Altimeter), expected to launch in 2028. CRYO2ICE (C2I) orbits are investigated over sea ice for the first time and snow depth is computed along the orbits C2I snow depth has an uncertainty of 10-11 cm along 7 km segments with 5-95% confidence interval of 7-16 cm for both winter seasons Along-track snow depth is compared with modeled and passive-microwave estimates and C2I is thinner for the Pacific and Atlantic Arctic