An analysis of clear sky and contextual biases using an operational over ocean MODIS aerosol product

[1] Clear sky and other cloud-related contextual biases are critical yet unsolved mysteries for aerosol related climatological studies using satellite observations. For the first time, we simulated contextual biases over ocean using 2-years of Navy Aerosol Analysis and Prediction System (NAAPS) products that include the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) assimilation. We compared model-derived AOD in regions with and without observations, and found that sampling results in negligible seasonal globally averaged AOD bias (<5%). Biases are more pronounced in regions with frequent overcast skies and high aerosol loadings, such as Southeast Asia, and mid-latitude South America. This suggests that contextual biases may develop from transport covariance and other observing biases. Lastly, we found that over remote oceans, under cloud decks, a slight increase aerosol optical depth values could exist, comparing with cloud free regions. But this is still small relative to cloud artifacts in the retrieval.