Strong increases in cloud water path and cloud fraction downwind of anthropogenic aerosol hotspots

The impact of anthropogenic aerosols on clouds remains the most uncertain driver of climate change, largely owing to the noise induced by the meteorological covariability between aerosols and clouds. Natural experiments of aerosol-cloud interactions at anthropogenic aerosol hot spots have recently emerged as a great possibility to overcome the noise of meteorological covariability and to quantify causal impacts of aerosols on clouds. Here, we present observational evidence for increases in cloud water path and coverage downwind of anthropogenic aerosol hot spots. Analysis of the temporal evolution of properties of liquid-water clouds in MODIS satellite data reveals gradual increases in cloud water path and coverage in response to increased cloud droplet numbers in precipitating cloud decks. 

Importantly, we also identify a near-instantaneous decrease in cloud water path, which seems to be unphysical and is likely explained by satellite retrieval error. Such a satellite retrieval error has previously likely led to an underestimation of the average increase in cloud water path in response to aerosols and the associated climate cooling effect (e.g. in Toll et al 2019 Nature https://doi.org/10.1038/s41586-019-1423-9). Additionally, we find a stronger decrease in cloud water path downwind of industrial aerosol sources when liquid-water clouds are supercooled below -10 °C, suggesting a potential influence of ice-nucleating particles, consistent with recently discovered glaciation events at anthropogenic aerosol hot spots (Toll et al 2024 Science https://doi.org/10.1126/science.adl0303).