Measurement-Based Estimates of Direct Radiative Effects of Absorbing Aerosols above Low-level Clouds

The elevated layers of absorbing smoke aerosols from western African (e.g. Gabon, and Congo) biomass burning activities have been frequently observed above low level Stratocumulus clouds off the African coast, which presents an excellent natural laboratory for studying the effects of aerosols above clouds (AAC) on regional energy balance in tropical and sub-tropical environments. Using spatially and temporally collocated Moderate Resolution Imaging Spectroradiometer (MODIS), Ozone Monitoring Instrument (OMI), and Clouds and the Earth's Radiant Energy System (CERES) data sets, the top-of-atmosphere (TOA) shortwave Aerosol Direct Shortwave Radiative Effects (ARE) of absorbing aerosols above low-level water clouds in the Southeast Atlantic Ocean was examined in this study. The regional averaged instantaneous ARE has been estimated to be 36.7±20.5 Wm-2 (regional mean ± standard deviation) along with a mean positive OMI Aerosol Index (AI) at 1.3 in August 2006 based on multi-sensors measurements. The highest magnitude of instantaneous ARE can even reach 138.2 Wm-2. We assess that the 660nm Cloud Optical Depth (COD) values of 8-12 is the critical value above (below) which aerosol absorption(scattering) effect dominates and further produces positive (negative) ARE values. Sensitivity studies based on both observations and radiative transfer model calculations have been performed to study several uncertainties due to issues such as the positions, physical and optical properties of aerosols and clouds. For example, the results show that ARE values are more sensitive to aerosols above lower COD values than cases for higher COD values. This is among the first studies to provide quantitative estimates of shortwave ARE due to AAC events from an observational perspective. Keywords: Absorbing Aerosol above Clouds; Direct Radiative Effect; Remote Sensing