Low-level cloud diurnal cycle drives regional direct aerosol radiative warming

Atmospheric aerosols are an important component of the Earth’s climate system, contributing substantial uncertainties in predicting future climate change. In the southeast Atlantic, where expansive light-absorbing smoke aerosol plumes overlie semi-permanent stratocumulus clouds, the direct aerosol radiative effect (DARE) introduces warming in the region but DARE estimates vary widely among climate models. Thus, an improvement in observational constraints of DARE with accurate representations of aerosol and cloud properties is needed to reduce model uncertainties. Since observational-based estimates of DARE are mostly based on polar-orbiting satellite sensors, the impact of the cloud diurnal cycle on DARE uncertainties remains unclear. This study quantifies the first DARE using hourly cloud observations from Spinning Enhanced Visible and Infrared Imager on board the Meteosat Second Generation geostationary platform and aerosol properties from the Modern-Era Retrospective-analysis for Research and Applications- Version 2 over the domain 20°S – 0°N/S and 15°W – 15°E. Accounting for the observed cloud diurnal cycle over the southeast Atlantic, rather than assuming a constant early-afternoon cloud field throughout the entire day, results in about a twofold (~ 1.4 Wm^-2) increase in the regional DARE at the top of the atmosphere. This increase primarily results from the morning hours when cloud fractions and cloud optical depths are high. The neglect of the cloud diurnal cycle at TOA can be equivalent to overestimating the SSA by 4%. Future estimates of observationally-based aerosol radiative effects need to account for the cloud diurnal cycle.