EFFECTS OF LAND-COVER TYPE AND GREENNESS ON ADVANCED VERY HIGH-RESOLUTION RADIOMETER BIDIRECTIONAL REFLECTANCES - ANALYSIS AND REMOVAL

The objectives of the study are (1) to examine the effects of land cov er type, green biomass, and solar zenith angle (SZA) on the bidirectio nal reflectance distributions (BRDs) of the visible and near-infrared advanced very high resolution radiometer (AVHRR) data from NOAA 11 ove r terrestrial surfaces; and (2) to correct for these effects by develo ping BRD functions (BRDF). Four land cover types are examined: barren, grassland, forest, and cropland. The data used consist of I-km daily AVHRR measurements for three growing seasons collected over 19 homogen eous land sites (20 x 20 km(2) for each site) in the conterminous Unit ed States and parts of Canada, BRD is found to be strongly land cover dependent, For the same cover type, BRD is altered significantly by th e green biomass present (represented by the normalized difference vege tation index (NDVI)). The effects of SZA on BRDs are also observed ove r all surface types under investigation, Semiempirical BRDFs were deve loped to account for these effects that are the functions of the SZA, satellite viewing zenith angle, relative azimuth angle, and NDVI. Good agreements were found between the observed and the modeled bidirectio nal dependencies for wide ranges of NDVI and SZA, A single BRDF appear s to be sufficient for bidirectional correction of the clear-sky AVHRR measurements made over a specific land cover type throughout a season . Finally, the developed BRDFs were used to normalize AVHRR reflectanc e data to a common geometry and to infer the hemispherical albedos for monitoring the seasonal variations of land surfaces.