Design of a Novel Spectral Albedometer for Validating the MODerate Resolution Imaging Spectroradiometer Spectral Albedo Product

Abstract

Land surface shortwave broadband albedo is a key parameter in general circulation models and surface energy budget models. Multispectral satellite data are typically used to generate broadband albedo products in a three-step process: atmospheric correction, for converting the top-of-atmosphere observations to surface directional reflectance; angular modeling, for converting the surface directional reflectance to spectral albedo of each individual band; and finally, narrowband-to-broadband conversion, for transforming the spectral albedos to broadband albedos. Spectroradiometers can be used for validating surface directional reflectance products and pyranometers or broadband albedometers, for validating broadband albedo products, but spectral albedo products are rarely validated using ground measurements. In this study, we designed a new type of albedometer that can measure spectral albedos. It consists of multiple interference filters and a silicon detector, for measuring irradiance from 400-1100 nm. The linearity of the sensors is 99%, and the designed albedometer exhibits consistency up to 0.993, with a widely-used commercial instrument. A field experiment for measuring spectral albedo of grassland using this new albedometer was conducted in Yudaokou, China and the measurements are used for validating the MODerate Resolution Imaging Spectroradiometer (MODIS) spectral albedos. The results show that the biases of the MODIS spectral albedos of the first four bands are −0.0094, 0.0065, 0.0159, and −0.0001, respectively. This new instrument provides an effective technique for validating spectral albedos of any satellite sensor in this spectral range, which is critical for improving satellite broadband albedo products.

Land Surface Albedo Estimation and Cross Validation Based on GF-1 WFV Data

“…where α(θ i , λ) is the blue-sky albedo of the band λ at a solar zenith angle of θ, α bs (θ i , λ) is the black-sky albedo, α ws (θ i , λ) is the white-sky albedo, and s(θ i τ(λ)) is the fraction of diffuse skylight when the solar zenith angle is θ, which is a function of aerosol optical depth and can be calculated using a predetermined lookup table (LUT) based on the 6S atmospheric radiative transfer code [40].…”

Re-understanding of land surface albedo and related terms in satellite-based retrievals

“…Typically, the pyranometers measure the full solar shortwave spectrum. However, the data may be filtered to eliminate shorter wavelengths to get the bi-hemispheric reflectance in the required spectral domain, such as visible (0.3-0.7 μm), nearinfrared (0.7-3.0 μm), shortwave bands (0.3-5.0 μm), and as that performed in the most recent developed spectral albedo-meter (Zhou, Wang, & Liang., 2018). These surface measurements are also referred to as "land surface albedo".…”

Time Series High-Resolution Land Surface Albedo Estimation Based on the Ensemble Kalman Filter Algorithm

“…where α(θ i , λ) is the blue-sky albedo of the band λ at a solar zenith angle of θ, α bs (θ i , λ) is the black-sky albedo, α ws (θ i , λ) is the white-sky albedo, and s(θ i , τ(λ)) is the fraction of diffuse skylight when the solar zenith angle is θ, which is a function of aerosol optical depth and can be calculated using a predetermined look-up table (LUT) based on the 6S atmospheric radiative transfer code [33]. There was some data missing in MCD43A3 due to retrieval failure, so we used a gap filling algorithm to construct a continuous albedo data set.…”