Spatiotemporal Heterogeneity of Aerosol and Cloud Properties Over the Southeast Atlantic: An Observational Analysis

The southeast Atlantic has expansive aerosol plumes overlying clouds for a third of each year. Aerosol optical depths (AODs) were measured from the airborne Sun photometer and lidar during the 2016 NASA ObseRvations of Aerosols above CLouds and their intEractionS field campaign. We compare these measurements with one another and with collocated Moderate Resolution Imaging Spectroradiometer (MODIS) observations at native spatial resolutions using <15-min and 3-h temporal collocation criteria. We find better statistical relationships for the <15-min temporal resolution, indicating that AODs in the southeast Atlantic commonly vary below three-hourly temporal scales over MODIS spatial resolutions. We also use the airborne Solar Spectral Flux Radiometer (SSFR) to conduct the first comprehensive evaluation of attenuation-corrected below-aerosol cloud optical depths (CODs) from MODIS and the Spinning Enhanced Visible and Infrared Imager (SEVIRI). MODIS COD retrievals improve their agreement with the SSFR when accounting for overlying aerosol attenuation whereas SEVIRI CODs are mostly underestimated. Plain Language Summary Aerosols are suspended particles in the atmosphere that affect the Earth's regional and global climate both by absorbing and scattering solar light and through their interaction with clouds. Hotspots of aerosols above clouds are present in some parts of the world, yet aerosol products from satellites are largely unavailable in cloudy conditions in part because of large uncertainties at estimating aerosol properties above bright backgrounds such as clouds. In 2016, two research aircraft from the National Aeronautics and Space Administration flew over the southeast Atlantic Ocean, a region with significant biomass burning aerosol (from agricultural fires) and a large oceanic cloud deck, to measure the properties of each. We use the aircraft data to evaluate satellite estimates of liquid cloud thicknesses and aerosol amounts above clouds. We then confirm that changes in aerosols and clouds over time affect the accuracy of satellite measurements. This information is crucial for refining satellite measurements and improving our understanding of how aerosols and clouds affect Earth's climate.

Automated summary

In the southeast Atlantic, aerosols above clouds are present for a third of the year. Analysis shows better statistical relationships for aerosol optical depths (AODs) when measured at <15-min temporal resolution compared to 3-h temporal resolution. Additionally, SEVIRI cloud optical depths (CODs) are mostly underestimated compared to MODIS COD retrievals.