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

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.