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 v alidating albedo measurements acquired from space at the 1-km spatial resol ution of Moderate Resolution Imaging Spectroradiometer (MODIS). This work f orms an element of the Grassland Prototype Validation Exercise initiative, a prelaunch Earth Observing System field experiment conducted in May 1997 a t Jornada, NM, USA. In lieu of MODIS data, we present albedo comparisons us ing 1.1-km Advanced Very High Resolution Radiometer and 6x7-km Polarisation and Directionality of the Earth's Radiation data. To derive broadband albe do from the satellite data, we calculate spectral albedo through the multia ngular 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. C omparisons of ground-based albedo values to the satellite-derived values me et the +/-0.05 suggested accuracy requirement for global climate modeling a nd are close to the +/-0.02 suggested standard. Spatial modeling based on a ssigning typical albedos to three land cover classes obtained through class ification 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 t he 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 spati al heterogeneity of albedo is similar to that at the coarser satellite reso lution. This explains the agreement between the ground-based and the satell ite-derived albedo values despite the difference in spatial scale. These re sults provide the first step in establishing appropriate strategies for alb edo validation for MODIS. (C) Elsevier Science Inc., 2000.