Impact of 3-D clouds on clear-sky reflectance and aerosol retrieval in a biomass burning region of Brazil

Three-dimensional (3-D) cloud radiative effects on clear-sky reflectances and associated aerosol optical depth retrievals are quantified for a cumulus cloud field in a biomass burning region in Brazil through a Monte Carlo simulation. In this study the 1-km Moderate Resolution Imaging Spectroradiometer cloud optical depth and surface reflectance datasets are used to compute the 3-D radiation fields with ambient aerosol optical thickness of 0.1 at a wavelength of 0.66 μm. The 3-D radiative effects range from -0.015 to 0.018 with an average of 0.004 and standard deviation of 0.006. The 3-D effects are most pronounced and variable for cloud neighboring pixels, where both large negative effects over shadows and positive effects near sunlit cloud edges are found. The clear next-to-cloud pixels, that contain /spl sim/83% of the clear pixel population, are affected in the most complex way and not reliable for aerosol retrieval. In the area 2 km away from clouds, the 3-D effects enhance the reflectance in clear patches. The average and variability of enhancements gradually decrease as a function of the cloud-free distance, resulting in a systematically higher aerosol optical depth estimates for pixels closer to clouds in one-dimensional (1-D) retrieval. At a distance of 3 km away from clouds, the 3-D effect is still appreciable with the average enhancement slightly less than 0.004. This enhancement will lead to an over estimate of aerosol optical thickness of /spl sim/0.04 in 1-D retrieval, which is significant for an ambient atmosphere with aerosol optical thickness of 0.1.