Characterization of MODIS land albedo (MOD43) accuracy with atmospheric conditions in Africa

The MODIS albedo product (MOD43) provides estimates of spectral and broadband albedo for most of earth's land masses. With 1 km spatial resolution and 16 day temporal resolution, this unique product can be used to set spatially and temporally dynamic boundary conditions in large scale models requiring surface energy fluxes. Nevertheless, the MODIS product does not provide estimates of actual "blue sky" albedo, which depends on irradiance conditions and is the quantity measured in the field. Instead, MOD43 provides theoretical white sky (bihemispherical) and black sky (directional- hemispherical) values. In practice, these values are linearly combined to estimate blue sky albedo using direct and diffuse irradiance estimates. In this study, we investigate the accuracy of MODIS Collection 003 albedo using tower-based albedo measurements collected over African savannas during SAFARI 2000. We calculate MODIS blue sky albedo using an online surface flux model [1] that interfaces with daily sunphotometer data available from the AERONET program. We characterize albedo uncertainty as a function of spectral band, solar zenith angle and season (wet, dry). Our results suggest that the accuracy of our blue sky albedo is less than 2% (absolute), though seasonal biases and significant variance exist. An improved cloud filter will likely reduce the variance.