Abstract
Recently, much attention has been given to using geostationary Earth orbit (GEO) meteorological satellite data for retrieving land surface parameters due to their high observation frequencies. However, their bidirectional reflectance distribution function (BRDF) information content with a single viewing angle has not been sufficiently investigated, which lays a foundation for subsequent quantitative estimation. In this study, we aim to comprehensively evaluate BRDF information from time-series observations from the Advanced Himawari Imager (AHI) onboard the GEO satellite Himawari-8. First, ~6.2 km monthly multiangle surface reflectances from POLDER onboard a low-Earth-orbiting (LEO) satellite with good angle distributions over various land types during 2008 were used as reference data, and corresponding 0.05° high-quality MODIS (i.e., onboard LEO satellites) and AHI datasets during four months in 2020 were obtained using cloud and aerosol property products. Then, indicators of angle distribution, BRDF change, and albedos were retrieved by the kernel-driven Ross-Li BRDF model from the three datasets, which were used for comparisons over different time spans. Generally, the quality of sun-viewing geometries varies dramatically for accumulated AHI observations according to the weight-of-determination, and wide-ranging anisotropic flat indices are obtained. The root-mean-square-errors of white sky albedos between AHI and MODIS half-month data are 0.018 and 0.033 in the red and near-infrared bands, respectively, achieving smaller values of 0.004 and 0.007 between the half-month and daily AHI data, respectively, due to small variances in sun-viewing geometries. The generally wide AHI BRDF variances and good consistency in albedo with MODIS show their potential for retrieving anisotropy information and albedo, while angle accumulation quality of AHI time-series observations must be considered.
Highlights
Advanced Himawari Imager (AHI) was taken as an example, for which a series of bidirectional reflectance distribution function (BRDF) indicators and albedo retrievals were derived to evaluate the underlying BRDF information through the hotspot-revised kernel-driven RTLSR_C model
The geostationary Earth orbit (GEO)-LEO inter-comparisons were performed at a monthly scale between the AHI, MODIS, and polarization and directionality of the Earth’s reflectances (POLDER)-accumulated multiangle observations at a coarse spatial resolution of 0.05◦ over various land types and seasons, and concurrent AHI and MODIS data were compared at a half-month scale
The results show that the AHI can attain much more clear-sky multiangle data than those of MODIS
Summary
The intrinsic reflectance anisotropy has been of great concern for decades and is usually described as a bidirectional reflectance distribution function (BRDF), which plays a key role in quantitative remote sensing inversion [1,2,3]. Considering that only limited angles are available in practice, the pattern of sun-viewing geometry has a critical influence on BRDF reconstruction across the whole illumination and viewing of hemispherical space. A noise amplification factor was designed to quantitatively evaluate the quality of the sun-viewing angle distribution, which was called the weight-of-determination (WoD) [7,8,9]. A median sampling density of view zenith angle (VZA) every.
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