Daytime aerosol optical depth above low-level clouds is similar to that in adjacent clear skies at the same heights: airborne observation above the southeast Atlantic

Abstract

To help satellite retrieval of aerosols and studies of their radiative effects, we demonstrate that daytime aerosol optical depth over low-level clouds is similar to that in neighboring clear skies at the same heights. Based on recent airborne lidar and sun photometer observations above the southeast Atlantic, the mean aerosol optical depth (AOD) difference at 532 nm is between 0 and −0.01, when comparing the cloudy and clear sides, each up to 20 km wide, of cloud edges. The difference is not statistically significant according to a paired t test. Systematic differences in the wavelength dependence of AOD and in situ single scattering albedo are also minuscule. These results hold regardless of the vertical distance between cloud top and aerosol layer bottom. AOD aggregated over ∼2∘ grid boxes for each of September 2016, August 2017 and October 2018 also shows little correlation with the presence of low-level clouds. We posit that a satellite retrieval artifact is entirely responsible for a previous finding of generally smaller AOD over clouds (Chung et al., 2016), at least for the region and time of our study. Our results also suggest that the same values can be assumed for the intensive properties of free-tropospheric biomass-burning aerosol regardless of whether clouds are present below.