In situ-based spectral characteristics of first-year landfast ice in eastern Liaodong Bay, Bohai sea

Understanding the optical properties of different sea ice types is crucial for improving knowledge of energy balance, heat and mass budgets, and for enhancing the accuracy of remote sensing retrievals. The in situ-based spectral characteristics of coastal first-year sea ice in eastern Liaodong Bay, Bohai Sea, were systematically analysed. A regional reflectance database was established in January 2025, collecting 49 ice samples with diverse types, thicknesses, densities, sampling locations, and observation angles. The results show that scale-like ice exhibited the highest visible-band reflectance (>0.7), followed by young ice with considerable variability, while frazil ice, nilas, and needle ice exhibited lower reflectance due to their thin structure, loose texture, and uneven surfaces, and sediment-laden ice had the lowest reflectance. Sea ice reflectance was strongly related to ice density, with lower-density ice showing higher values, and displayed a significant exponential relationship with ice thickness in both ultraviolet and visible bands (p < 0.01), with the ultraviolet band yielding higher coefficients of determination. The Hemispherical Directional Reflectance Factor (HDRF) increased with azimuth angle, reaching a minimum at nadir (0° zenith), and reflectance grew with zenith angle, with forward scattering being dominant, followed by side and back scattering. Additionally, comparisons between Geostationary Ocean Colour Imager II (GOCI-II) satellite data and in situ measurements showed similar spectral patterns, including primary peaks in the ultraviolet–blue bands and a secondary maximum near 600–700 nm, although GOCI-II reflectance was generally 10–20% lower. Moreover, preliminary results based on four machine learning algorithms indicate that GOCI-II satellite data have clear discriminatory potential for classifying sea ice by both region and thickness.