Publications

Chang, YX; Jiao, ZT; Zhang, XN; Mei, LL; Dong, YD; Yin, SY; Cui, L; Ding, AX; Guo, J; Xie, R; Zhu, ZD; Li, SJ (2021). Assessment of Improved RossLi BRDF Models Emphasizing Albedo Estimates at Large Solar Angles Using POLDER Data. IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, 59(12), 9968-9986.

Abstract
Surface albedo is closely related to the Earth's energy budget and is usually estimated by integrating remotely sensed bidirectional reflectance distribution function (BRDF) data based on the widely used Ross-Li kernel-driven models. However, for large solar zenith angles (i.e., SZAs > 70 degrees), albedo estimation using the operational algorithm of the Moderate Resolution Imaging Spectroradiometer (MODIS), i.e., RossThick-LiSparseReciprocal (RTLSR), is not recommended because it is reported to somewhat underestimate the black-sky albedo (BSA) at large SZAs based on ground albedo measurements. Recently, various combinations of the Ross-Li BRDF models with improved capabilities have been developed, and the assessments of these models based on worldwide satellite BRDF data with good spatial sampling, particularly at the large view and solar angles, will be important to improve an understanding of their performance in estimating intrinsic albedos. Following previous studies, the objective of this study is to further assess a series of hotspot-corrected Ross-Li models by demonstrating their ability to fit the POLarization and Directionality of the Earth's Reflectances (POLDER) data sets and estimate albedo, especially at large SZAs, based on selected concurrent POLDER and MODIS data. The hotspot-corrected RTLSR model obtained by combining the RossThickChen and LiSparseReciprocalChen kernels (RTLSR_C) shows the best fitting ability, with a high cumulative frequency of small root-mean-square errors (RMSEs), thus confirming previous conclusions. Model differences mainly appear in albedo estimates, especially BSA estimates at large SZAs. The BSAs estimated by other models are significantly different from the RTLSR_C estimates in the near-infrared (NIR) and red bands as the SZA increases to approximately 60 degrees and 70 degrees, respectively. In this case, RossThinChen-LiSparseReciprocalChen (RTNLSR_C) yields higher BSA estimates than those of RTLSR_C. Comparisons of the MODIS and POLDER albedos estimated with Ross-Li models show that models with the RossThinChen kernel yield higher BSA estimates than those of the RTLSR_C model as the SZA increases. The results indicate that the retrieved albedo is likely to be more accurate with appropriately selected kernels for BRDF models at large SZAs, providing guidance for selecting suitable combinations of multiple kernels.

DOI:
10.1109/TGRS.2020.3030948

ISSN:
0196-2892