Estimations of Global Shortwave Direct Aerosol Radiative Effects Above Opaque Water Clouds Using a Combination of A-Train Satellite Sensors

Abstract. All-sky Direct Aerosol Radiative Effects (DARE) play a significant yet still uncertain role in climate. This is partly due to poorly quantified radiative properties of Aerosol Above Clouds (AAC). We compute global estimates of short-wave top-of-atmosphere DARE over Opaque Water Clouds (OWC), DARE OWC , using observation-based aerosol and cloud radiative properties from a combination of A-Train satellite sensors and a radiative transfer model. There are three major differences between our DARE OWC calculations and previous studies: (1) we use the Depolarization Ratio method (DR) on CALIOP (Cloud Aerosol LIdar with Orthogonal Polarization) Level 1 measurements to compute the AAC frequencies of occurrence and the AAC Aerosol Optical Depths (AOD), thus introducing fewer uncertainties compared to using the CALIOP standard product; (2) we apply our calculations globally, instead of focusing exclusively on regional AAC hotspots such as the southeast Atlantic; and (3) instead of the traditional look-up table approach, we use a combination of satellite-based sensors to obtain AAC intensive radiative properties. Our results agree with previous findings on the dominant locations of AAC (South and North East Pacific, Tropical and South East Atlantic, northern Indian Ocean and North West Pacific), the season of maximum occurrence, aerosol optical depths (a majority in the 0.01–0.02 range and that can exceed 0.2 at 532 nm) and aerosol extinction-to-backscatter ratios (a majority in the 40–50 sr range at 532 nm which is typical of dust aerosols) over the globe. We find positive averages of global seasonal DARE OWC between 0.13 and 0.26 W · m −2 (i.e., a warming effect on climate). Regional seasonal DARE OWC values range from −0.06 W · m −2 in the Indian Ocean, offshore from western Australia (in March–April–May) to 2.87 W · m −2 in the South East Atlantic (in September–October–November). High positive values are usually paired with high aerosol optical depths (> 0.1) and low single scattering albedos ( 0.94), representative of, e.g., biomass burning aerosols. Because we use different spatial domains, temporal periods, satellite sensors, detection methods, and/or associated uncertainties, the DARE OWC estimates in this study are not directly comparable to previous peer-reviewed results. Despite these differences, we emphasize that the DARE OWC estimates derived in this study are generally higher than previously reported. The primary reasons for our higher estimates are (i) the possible underestimate of the number of dust-dominated AAC cases in our study; (ii) our use of Level 1 CALIOP products (instead of CALIOP Level 2 products in previous studies) for the detection and quantification of AAC aerosol optical depths, which leads to larger estimates of AOD above OWC; and (iii) our use of gridded 4° × 5° seasonal means of aerosol and cloud properties in our DARE OWC calculations instead of simultaneously derived aerosol and cloud properties from a combination of A-Train satellite sensors. Each of these areas is explored in depth with detailed discussions that explain both rationale for our specific approach and the subsequent ramifications for our DARE calculations.