First Assessment of SWOT KaRIn Measurements Over Lake Ice and Overlying Snow

Abstract. The Surface Water and Ocean Topography (SWOT) mission, launched on December 16, 2022, carries a Ka-band Radar Interferometer (KaRIn) and provides high-resolution, wide-swath measurements of Earth's surface waters. During the Cal/Val phase (March 29 to July 09, 2023; local date), near-daily observations were acquired over Teshekpuk Lake, Alaska, providing an opportunity to examine the response of KaRIn measurements to lake ice and overlying snow conditions. Using optical and SAR imagery, reanalysis products, and field observations during the same period, we found that SWOT heights provide reasonable elevations over the fully ice-covered (before melt onset) and open water surfaces. Meanwhile, Ka-band backscatter effectively tracks the evolution of snow conditions, freeze/melt events, surface melt onset, and ice break-up, as well as features such as pressure ridges, the inlet stream, cracks, and fragmented ice. Limitations were also found in the SWOT Level 2 KaRIn High-Rate Pixel Cloud Version D data used, particularly in the quality of height retrievals over melting ice, in areas contaminated by specular ringing, and on surfaces with little to no wind. Complementary simulations with the Snow Microwave Radiative Transfer (SMRT) model confirmed the sensitivity of Ka-band backscatter to evolving snow properties (snow density and grain size). These results highlight SWOT's ability to advance the monitoring of ice-covered lakes and their seasonal dynamics, and to support the development of retrieval algorithms of ice and overlying snow properties in preparation for the launch of future satellite missions providing Ka-band frequency measurements, such as the Copernicus polaR Ice and Snow Topography ALtimeter (CRISTAL) and Sentinel-3 Next Generation Topography (S3NG-TOPO) missions.