Aerosol indirect effect dictated by liquid clouds

Abstract

AbstractAnthropogenic aerosols have been shown to enhance the solar reflection from warm liquid clouds and mask part of the warming due to the buildup of greenhouse gases. However, very little is known about the effects of aerosol on mixed‐phase stratiform clouds as well as other cloud regimes including cumulus, altocumulus, nimbostratus, deep convection, and anvil cirrus. These additional cloud categories are ubiquitous and typically overlooked in satellite‐based assessments of the global aerosol indirect forcing. Here we provide their contribution to the aerosol indirect forcing estimate using satellite data collected from several colocated sensors in the A‐train for the period 2006–2010. Cloud type is determined according to the 2B‐CLDCLASS‐LIDAR CloudSat product, and the observations are matched to the radiative flux measurements from CERES (Clouds and the Earth's Radiant Energy System) and aerosol retrievals from MODIS (MODerate resolution Imaging Spectroradiometer). The oceanic mean aerosol indirect forcing is estimated to be −0.20 ± 0.31 W m−2 with warm low‐level cloud largely dictating the strength of the response (−0.36 ± 0.21 W m−2) due to their abundance and strong cloud albedo effect. Contributions from mixed‐phase low‐level cloud (0.01 ± 0.06 W m−2) and convective cloud (0.15 ± 0.23 W m−2) are positive and buffer the system due to strong aerosol‐cloud feedbacks that reduce the cloud albedo effect and/or lead to convective invigoration causing a countering positive longwave warming response. By combining all major cloud categories together, aerosol indirect forcing decreases and now contains positive values in the uncertainty estimate.