Exploring Aerosols Near Clouds With High‐Spatial‐ Resolution Aircraft Remote Sensing During SEAC4RS

Since aerosols are important to our climate system, we seek to observe the variability of aerosol properties within cloud systems. When applied to the satellite‐borne Moderate‐resolution Imaging Spectroradiometer (MODIS), the Dark Target retrieval algorithm provides global aerosol optical depth (AOD; at 0.55 μm) in cloud‐free scenes. Since MODIS' resolution (500‐m pixels, 3‐ or 10‐km product) is too coarse for studying near‐cloud aerosol, we ported the Dark Target algorithm to the high‐resolution (~50‐m pixels) enhanced‐MODIS Airborne Simulator (eMAS), which flew on the high‐altitude ER‐2 during the Studies of Emissions, Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys Airborne Science Campaign over the United States in 2013. We find that even with aggressive cloud screening, the ~0.5‐km eMAS retrievals show enhanced AOD, especially within 6 km of a detected cloud. To determine the cause of the enhanced AOD, we analyze additional eMAS products (cloud retrievals and degraded‐resolution AOD), coregistered Cloud Physics Lidar profiles, MODIS aerosol retrievals, and ground‐based Aerosol Robotic Network observations. We also define spatial metrics to indicate local cloud distributions near each retrieval and then separate into near‐cloud and far‐from‐cloud environments. The comparisons show that low cloud masking is robust, and unscreened thin cirrus would have only a small impact on retrieved AOD. Some of the enhancement is consistent with clear‐cloud transition zone microphysics such as aerosol swelling. However, 3‐D radiation interaction between clouds and the surrounding clear air appears to be the primary cause of the high AOD near clouds. Plain Language Summary Since aerosols are important components of our climate system, we seek to use observations to quantify aerosol properties and their interactions with clouds. To study aerosols close to clouds, we adapt a well‐known aerosol retrieval algorithm used for satellite remote sensing (lower spatial resolution) and port to airborne remote sensing (higher spatial resolution). We apply the retrieval to observations collected over the southeastern United States during late summer 2013. The resulting product suggests that total aerosol optical depth (AOD) can be greatly enhanced near clouds. To validate, we compare this retrieved AOD to other data sets and find that the enhanced AOD near clouds is only partially observed by other sensors. This suggests that the enhanced AOD is primarily a cloud adjacency or 3‐dimensional radiative effect. High‐resolution, passive remote sensing measurements can characterize aerosol/cloud environments, thus helping to interpret global AOD data sets from more comprehensive but coarser‐resolution satellite retrievals.