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.
Highlights
The land surface albedo is a key input parameter required in current general circulation models
We designed a measuring system that allowed us to accurately obtain spectral albedos corresponding to satellite sensors
Our design effectively avoids errors associated with traditional broadband surface albedo product verification due to wavelength conversion and band mismatch; it mitigates the defects in spectroradiometer measurements, which are labor- and time-consuming to gather and are difficult to use when conducting regional-scale time-series observations
Summary
The land surface albedo is a key input parameter required in current general circulation models. The basic validation of remote sensing albedo products is primarily centered on the broadband products, which are compared directly against field measurements, using a combination of pyranometers or albedometers [2,13,14]. There are many advantages to spectral albedo products, compared to broadband albedo products—they represent a more fine-tuned understanding of the land surface, for example, and they include more spectral information [18] When they are retrieved directly from spectral observations, using a Bidirectional Reflectance Distribution Function (BRDF) model, the retrieval model can be accurately assessed by evaluating the spectral albedo product accuracy. They may even more accurate than measuring with albedometers, but apply only to smaller areas To this effect, there is urgent demand for a new instrument, which can directly observe spectral albedo in supplying time series observations.
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