Simulating effects of aerosols on rainfall in southern Africa

Abstract

Climate-aerosol model ECHAM5-HAM is employed to study effects of aerosol air pollution on rainfall in southern Africa. Aerosols effect the climate through light scattering and absorption, modification of cloud properties, and other indirect effects. The simulation model simulates the global climate on a grid and aerosol emissions from all major economic sectors as provided by the GAINS emission model. Using different model setups, we can separate the effect of aerosol light absorption due to black carbon and other aerosols, that of aerosols interacting with clouds through acting as cloud condensation nuclei and that through the ocean response. This is the first study of its kind to employ aerosol station measurements for model validation. Comparisons are done with previous plentiful studies for south Asia with many similarities in aerosol and rainfall climatology. We conclude that aerosols likely have a weak, negative effect on rainfall with internal variability dominating the simulation results, consistently with observed historical records. Aerosol light absorption does not seem to have a strong impact on rainfall. Meanwhile, the complexity of the problem also leaves uncertainty to the results. Regionally, the results show an opposite pattern of greenhouse gas projections that suggest a wet-get-wetter and dry-get-drier development due to global warming.