An Analysis of Potential Cloud Artifacts in MODIS Over Ocean Aerosol Optical Thickness Products
Using spatially and temporally collocated Moderate Resolution Imaging Spectroradiometer (MODIS)products and data from seven Aerosol Robotic Network (AERONET) sun photometer sites, we explored the relationship between MODIS aerosol optical depth and cloud fraction over remote oceans that have been recently reported in the literature. We show that artifacts such as cloud contaminaton or adjacency effect contribute to the majority of the relationship in clean marine conditions. This cloud fraction effect could result in a 10-20% overestimation in monthly mean aerosol optical depth or aerosol direct forcing values that are derived using MODIS aerosol products over cloud-free oceans. It may also explain some of the high optical depth values derived in the mid-latitude southern oceans. We also suggest that covariances of meteorological phenomenon such as wind or humidity in cloudy regions, while logical, might only account for a minor portion of the ensemble relationship (Zhang et al., 2005).
MODIS Aerosol Product Analysis for Data Assimilation: Assessment of Over-Ocean Level 2 Aerosol Optical Thickness Retrievals
Currently, the Moderate Resolution Imaging Spectroradiometers (MODIS) level 2 aerosol product (MOD04/MYD04) is the best aerosol optical depth product suitable for near-real-time aerosol data assimilation. However, a careful analysis of biases and error variances in MOD04/MYD04 aerosol optical depth product is necessary before implementing the MODIS aerosol product in aerosol forecasting applications. Using 1 year's worth of sun photometer and MOD04/MYD04 aerosol optical depth (τ) data over global oceans, we studied the major biases in MODIS aerosol over-ocean product due to wind speed, cloud contamination, and aerosol microphysical properties. For τ less than 0.6, we found similar uncertainties in the mean MOD04/MYD04 τ as suggested by the MODIS aerosol group, while biases are nonlinear for τ larger than 0.6. We showed that uncertainties in MOD04/MYD04 data can be reduced and the correlation between MODIS and sun photometer τ can be improved by reducing the systematic biases in MOD04/MYD04 data through empirical corrections and quality assurance procedures. By removing noise and outliers and ensuring that only the highest-quality data was included, we created a modified aerosol optical depth product that removes most massive outliers and ultimately reduced the absolute error (MODIS-sun photometer) in MODIS τ at 0.55 µm (τ0.55) by 10-20%. Averaged over 1 year's worth of Terra MODIS aerosol product over global oceans, we found a 12% reduction in MODIS τ0.55 with extremes of 30% over the southern midlatitudes and the North Pacific due to a reduction in cloud contamination. This modified aerosol optical depth product will be used operationally.