Using ADEOS/POLDER data to reduce angular variability of NOAA/AVHRR reflectances

Time series of National Oceanic and Atmospheric Administration (NOAA)/Advanced Very High Resolution Radiometer (AVHRR) Global Area Coverage (GAC) data, collected daily over two 50-km targets during March-June 1997 in Hungary, were corrected for angular effects using coincident multiangle Polarization and Directionality of the Earth's Reflectance (POLDER) Level-2 Land Surface data products. The POLDER data used consisted of narrow-band 0.67- and 0.765-mum reflectances corrected for ozone and water vapor absorption and Rayleigh scattering effects. The AVHRR visible (0.55-0.75 mum) and near-IR (0.68-1.05 mum) data were converted to reflectances, screened for clouds, and corrected for the same atmospheric effects as the POLDER data. Neither POLDER nor AVHRR data were collected for aerosol effects. POLDER reflectances were used to derive bidirectional reflectance distribution function (BRDF) for each 6-km(2) POLDER grid box. The BRDFs were normalized to the near-nadir values at 45 degrees solar zenith angle, resulting in the anisotropic factors, which were derived for each month for each grid box. Thus, seasonal variability of local anisotropy for each POLDER, grid box in the target areas was established. The anisotropic factors were then applied to the AVHRR/GAC visible and near-IR reflectances mapped into the POLDER grid. The anisotropy-corrected AVHRR reflectances exhibit less fluctuation than the original uncorrected values, thus, facilitating the interpretation of short-term variability in surface conditions. Application of the POLDER BRDFs to AVHRR data is especially advantageous for processing AVHRR temporal composites because of the scarce angular statistics in the areas of frequent clouds, which hampers derivation of BRDFs from AVHRR data itself. Assuming that the local BRDF does not substantially vary from year to year, this approach could be extended to data from the growing seasons of other years. Ultimately, the multiyear time series could be corrected so that the variability, related to angular effects, inherent to AVHRR time series, is reduced. The current approach suggests a paradigm for a synergistic use of TERRA/EOS Multiangle Imaging SpectroRadiometer (MISR)/MODerate resolution Imaging Spectrometer (MODIS) data stream. (C) 2001 Elsevier Science Inc. All rights reserved.