Low cloud diurnal cycle drives regional 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 varies widely among climate models. Black carbon, a major component of light-absorbing aerosol, contributes to the largest climate warming after carbon dioxide. Moreover, the impact of the cloud diurnal cycle on DARE uncertainties remains unclear. Here, we quantify the first direct aerosol radiative effects using hourly satellite observations of clouds focusing on 20°S – 0°N/S and 15°W – 15°E. We find that 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 nearly twofold (about 1.4 Wm-2) increase in the regional mean aerosol radiative warming. This regional DARE increase mainly results from morning hours when cloud fractions and optical depths are higher. The neglect of the cloud diurnal cycle adds to the underestimated radiative warming in the southeast Atlantic associated with underestimated aerosol absorption among climate models. Future observationally-based estimates of aerosol climatic effects need to account for the cloud diurnal cycle.