Aerosol breezes drive cloud and precipitation increases.

Aerosol-cloud interactions are a major source of uncertainty in weather and climate models. These interactions and associated precipitation feedbacks are modulated by spatial distributions of aerosols on global and regional scales. Aerosols also vary on mesoscales, including around wildfires, industrial regions, and cities, but the impacts of variability on these scales are understudied. Here, we first present observations of covarying mesoscale aerosol and cloud distributions on the mesoscale. Then, using a high-resolution process model, we show that horizontal aerosol gradients of order 100 km drive a thermally-direct circulation we call an "aerosol breeze". We find that aerosol breezes support initiation of clouds and precipitation over the low-aerosol portion of the gradient while suppressing their development on the high-aerosol end. Aerosol gradients also enhance domain-wide cloudiness and precipitation, compared with homogenous distributions of the same aerosol mass, leading to potential biases in models that do not adequately represent this mesoscale aerosol heterogeneity.