Reflectance Anisotropy from MODIS for Albedo Retrieval from a Single Directional Reflectance
Reflectance Anisotropy from MODIS for Albedo Retrieval from a Single Directional Reflectance
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DOI：
10.3390/rs14153627
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发表时间：
2022-07
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影响因子：
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通讯作者：
Hu Zhang;Mengzhu Zhao;Z. Jiao;Y. Lian;Lei Chen;Lei Cui;Xiaoning Zhang;Yan Liu;Yadong Dong-Yadong-D
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文献类型：
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作者：
Hu Zhang;Mengzhu Zhao;Z. Jiao;Y. Lian;Lei Chen;Lei Cui;Xiaoning Zhang;Yan Liu;Yadong Dong-Yadong-D
地表反射率各向异性和多角度观测不足是卫星反照率估算面临的主要挑战。通过将从粗分辨率双向反射分布函数（BRDF）数据中提取的各向异性信息相关联，从单一方向反射中提取反照率，已经开展了许多研究。在这些方法中，土地覆盖类型（LCT）和归一化植被指数（NDVI）在区分反射率各向异性方面的贡献存在争议。本文首先提出了一种考虑模型参数分布特征，从MODIS BRDF/反照率乘积中提取先验BRDF (F)的方法。基于LCT和ndvi的F也从相应的子集中提取。然后，利用模拟或卫星的方向反射率和F的各向异性信息得到F-based反照率，最后将方向反射率和F-based反照率与MODIS反照率或地面测量结果进行比较，以显示F在反照率反演过程中补偿反射各向异性影响的能力。全球和时间序列MODIS BRDF数据充分验证了该方法的有效性。结果表明，反射各向异性具有聚集分布模式，F可以代表一个贴图内大部分像素的反射各向异性。LCT和NDVI的改善只发生在含有大面积植被和贫瘠地面的情况下。基于f的短波反照率与MODIS反照率产品具有较高的一致性，但在前半球存在热点和较大的观测天顶角区域。与地面测量和MODIS反照率的比较表明，基于f的单方向反射反照率基本达到绝对精度要求，均方根误差（RMSE）分别为0.027和0.036。
Surface reflectance anisotropy and insufficient multi-angular observations are the main challenges in albedo estimation from satellite observations. Numerous studies have been developed for albedo retrieval from a single directional reflectance by associating the anisotropy information extracted from coarse-resolution bidirectional-reflectance distribution function (BRDF) data. The contribution of land-cover type (LCT) and the Normalized Difference Vegetation Index (NDVI) in distinguishing reflectance anisotropy in these methods remains controversial. This study first proposed an approach to extracting a priori BRDF (F) from the MODIS BRDF/albedo product by considering the distribution characteristics of the model parameters. LCT- and NDVI-based F were also extracted from the corresponding subset. Then, the F-based albedo was derived from simulated or satellite directional reflectance and the anisotropic information of F. Finally, the directional reflectance and F-based albedo were compared with the MODIS albedo or ground measurement, in order to show the ability of F to compensate for the effect of reflectance anisotropy in the albedo retrieval process. The method was fully validated by the global and time-series MODIS BRDF data. The results showed that reflectance anisotropy has an aggregated distribution pattern, and F can represent the reflectance anisotropy of most pixels within a tile. The improvement of LCT and NDVI only occurs when the tile contains a large area of vegetated and barren ground. With the exception of the hotspot and large viewing-zenith-angle area in the forward hemisphere, the F-based shortwave albedo has high consistency with the MODIS albedo product. A comparison with the ground measurements and MODIS albedo showed that the F-based albedo from a single directional reflectance generally achieves an absolute accuracy requirement, with a root-mean-square error (RMSE) of 0.027 and 0.036.