冰面反照率是用来研究寒区水域大气和水体之间能量交换的重要参数, 可以通过遥感数据反演得到。 但是卫星遥感观测的波长和角度都相对固定, 而且视场角有限, 因此获得的表面反射率并不等价于反照率, 还需要根据下垫面情况进行各向异性校正。 冰的前向散射很强, 而且光学性质对其物理参数非常敏感, 因此不同物理性质的冰在不同方向上的反射率也有很大的差异, 给冰面反照率的反演带来了不确定性。 为了解决这些问题, 2019年2月在内蒙古乌梁素海湖对五种性质各异的冰进行了光学性质的现场观测, 测量类型包括: (Ⅰ)阴天不均匀气泡冰; (Ⅱ)表层有大量泥沙的冰; (Ⅲ)稀疏大气泡冰; (Ⅳ)致密小气泡冰; (Ⅴ)融化冰, 并对彼此之间的反照率、 双向反射因子(BRDF)以及各向异性反射因子(ARF)光谱特征的差异进行对比分析。 结果表明: 随着太阳天顶角的增大, 除融化冰的反照率出现了下降以外, 其余冰面反照率都出现不同程度的增大。 冰的双向反射特征呈现出明显的各向异性, 在前向散射方向会形成一个反射的峰值点, 该点的位置会受冰表情况的影响, 而峰值点以外方向上的反射光主要来自冰内体散射, 这部分对观测的天顶角不敏感, 而对方位角的敏感性在短波段会受到冰内部组分均匀程度的影响, 在长波段几乎不受影响。 在对冰的BRDF谱线分析时发现, 冰的体散射谱线形状和反照率谱线很相似, 但是在长波段的减小速度更快, 即体散射在短波段能量更加集中, 而在前向散射方向接近反射峰值点位置上, 能量在长波段更加集中, 短波段较少。 分析ARF谱线可以发现, 冰的体散射对反照率的贡献随波长的增大而减小, 而冰表面反射情况相反。 更重要的是各测量点的ARF谱线的大小顺序和BRDF谱线的顺序并不相同, 说明即使在观测角度一致时, 不同冰的反照率反演参数并不相同。

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; (Ⅳ) 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.