Regional aerosol warming enhanced by the diurnal cycle of low cloud

Abstract

Atmospheric aerosols are an important component of the Earth’s climate system and contribute substantial uncertainties to predictions of 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) induces warming, but the magnitude of this effect varies widely among climate models. Thus, it is essential to improve estimates based on observations to help constrain model uncertainties. However, the impact of the observed cloud diurnal cycle on DARE remains unclear. Here we quantify DARE using radiative transfer modelling based on hourly satellite observations of clouds focusing on the region 20° S–0° and 10° 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 a more than twofold increase (+1.7 ± 0.4 W m⁻²) in the regional mean aerosol radiative warming. The increase in DARE results from morning hours when cloud fractions and optical depths are higher. 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 observations-based estimates of aerosol climatic effects need to account for the cloud diurnal cycle.