Characterization of aerosol above-cloud incidence and optical properties over the southeastern Atlantic

Description

A major source of uncertainty in aerosol climate effects is related to the vertical distribution of atmospheric aerosols with respect to clouds. Recent observations reveal that aerosols are not limited to the boundary layer but rather frequently observed above clouds, particularly in marine environments with low cloud cover. Compared to aerosols that occur beneath or in clouds, above-cloud aerosols (ACA), especially those that are light-absorbing, can result in an amplification of aerosol absorption and thus a smaller negative or even positive direct radiative effect. In this study, we evaluate aerosol vertical distribution simulated by a global climate model (DOE/E3SM), with a focus on elevated aerosols in one of the most active ACA regions identified by satellite observations: southeastern Atlantic. We retrieved the 18-month aerosol extinction profiles, using the 18-month AMF-1 MPL measurements on Ascension Island (ASI) from the DOE LASIC campaign (May 2016 to Oct 2017). This allows us to examine the monthly and diurnal aerosol variations combined with the LASIC meteorological measurements. Calibration of the retrieved aerosol extinction profiles with the AERONET column AOD shows encouraging agreement. We find that the observed seasonality in aerosol vertical distribution and ACA is modulated strongly by biomass burning activities as well as changes of boundary layer depth in this region. Summer peak biomass burning increases both column AOD and ACA, while winter biomass burning and dust aerosols have a larger effect on ACA than AOD due to lower boundary layer height. The global model predicts the summer peak AOD and places the elevated aerosol layers at the approximate altitudes on the monthly mean basis, but underestimates the ACA loadings throughout the year. Since the ACA aerosols are strongly absorbing in observations and modeling, this model bias indicates a possible underestimation of aerosol direct and semi-direct radiative effects over the subtropical marine stratocumulus region. The radiative impact is being investigated. Additionally, we find that the changes of the underlying cloud cover affect the seasonal and spatial variations in the estimated aerosol direct radiative effects. This study highlights the importance of characterizing the ACA distributions and properties.