A comparison of satellite-derived spectral albedos to ground-based broadband albedo measurements modeled to satellite spatial scale for a semidesert landscape

This study describes the use of ground-based albedometer measurements for validating albedo measurements acquired from space at the 1-km spatial resolution of Moderate Resolution Imaging Spectroradiometer (MODIS). This work forms an element of the Grassland Prototype Validation Exercise initiative, a prelaunch Earth Observing System field experiment conducted in May 1997 at Jornada, NM, USA. In lieu of MODIS data, we present albedo comparisons using 1.1-km Advanced Very High Resolution Radiometer and 6x7-km Polarisation and Directionality of the Earth's Radiation data. To derive broadband albedo from the satellite data, we calculate spectral albedo through the multiangular inversion of a semiempirical bidirectional reflectance distribution function model and perform narrowband-to-broadband albedo conversion using an observed spectrum as interpolator and extrapolator of spectral albedo. Comparisons of ground-based albedo values to the satellite-derived values meet the +/-0.05 suggested accuracy requirement for global climate modeling and are close to the +/-0.02 suggested standard. Spatial modeling based on assigning typical albedos to three land cover classes obtained through classification of 12-m resolution Thematic Mapper Simulater data over the 1.1X1.1-km area of an Advanced Very High Resolution Radiometer pixel shows that the instrument tower at the field site, in the vicinity of which the ground albedo measurements were made, was located in an area where the local spatial heterogeneity of albedo is similar to that at the coarser satellite resolution. This explains the agreement between the ground-based and the satellite-derived albedo values despite the difference in spatial scale. These results provide the first step in establishing appropriate strategies for albedo validation for MODIS. (C) Elsevier Science Inc., 2000.