The effects of cloud morphology and sun-satellite angle on MODIS studies of cloud/aerosol interactions over Amazonia during the burning season

The MODIS instrument and operationally generated products have become widely used tools for measurements of cloud-aerosol interactions. But cumulus clouds are three dimensional, while the calculations used for operational satellite retrievals are universally one-dimensional. Recent papers have demonstrated systematic biases in the cloud retrievals due to cloud morphology and sun-satellite geometry. The current study of cloud-aerosol interactions in Amazonia demonstrates that the same types of cloud morphology that produce biases in the MODIS cloud retrieval products may also be responsible for cloud contamination of the MODIS land aerosol product. The result is a false correlation between aerosol loading and cloud effective radius that is strong enough to bias or even reverse the measured aerosol-Reff relationships depending on the type of screening used for cloud and experiment geometry. Surface contamination tends to produce the same type of biases, making it difficult to determine the magnitude of the pure 3D bias. When standard screening techniques are applied it is found that 3D effects are reduced but not completely eliminated. Guidelines for sun-satellite geometry and 1 km resolution cloud morphology are provided that should limit the influence of false correlations.