Yu, M; Lu, P; Cao, XW; Tang, MG; Wang, QK; Li, ZJ (2020). Field Observations of the Bidirectional Reflectance Characteristics of Lake Ice. SPECTROSCOPY AND SPECTRAL ANALYSIS, 40(8), 2453-2461.

Ice surface albedo is an essential parameter associating with the energy exchange between water and atmosphere in cold regions, which can be retrieved from satellite remote sensing data. However, reflectivity data is not equal to the ice surface albedo because satellites always have a limited field of view at specific wavelength bands and observational angles. Anisotropy corrections are also needed according to the conditions of underlying surfaces. Since ice has strong forward scattering and its optical properties are sensitive to its physics, there is large difference in reflectance among different ice types and at different observational angles, which results in uncertainties in ice albedo retrievals. The field observations on lake ice were conducted in Wuliangsuhai lake, Inner Mongoliain February 2019. Spectral measurements of surface albedo, bidirectional reflectance distribution function(BRDF) and anisotropy reflectance factor(ARF) were conducted for the five types of lake ice: (I) ice with non-uniform bubbles under overcast sky; (II) ice with sands on surface; (III) ice with big bubbles inside; (N) ice with dense small bubbles inside, and (V) melting ice. The differences among them are discussed. The results reveal that lake ice albedo increases with the solar zenith, except for the melting ice case, showing an opposite trend. The bidirectional reflectance characteristics of ice present an obvious anisotropy. Peak reflectivity takes place in the direction of the forward scattering, and the location is significantly affected by the ice surface condition. The reflected light in the other directions mainly come from volume scattering in ice, which is insensitive to the observed zenith and can be affected by ice's uniformity at shortwave band rather than at longwave band. Results on BRDF indicate that the spectral shape of volume scattering is similar to albedo, but the attenuation rate in the longwave band of BRDF is faster than that of albedo. That is, the energy of volume scatter is more concentrated in the shortwave band. However, in the direction near the reflectivity peak, the energy is more concentrated in the longwave band than in the shortwave band. ARF results reveal that the contribution of volume scattering to surface albedo decrease with wavelength, but the role of surface reflection is on the contrary. More importantly, the order of the ARF was not the same as that of the BRDF, which suggests that the retrieval parameters for the surface albedo of different ice types are not identical even under the same observational angle.