High in the clouds, atmospheric aerosols, including anthropogenic air pollutants, increase updraft speeds in storm clouds by making the surrounding air more humid, a new study finds. The results offer a new mechanism explaining the widely observed - but poorly understood - atmospheric phenomenon and provide a physical basis for predicting increasing thunderstorm intensity, particularly in the high-aerosol regions of the tropics. Observations worldwide have highlighted aerosols' impact on weather, including their ability to strengthen convection in deep convective clouds, like those that form during thunderstorms, resulting in larger and more severe storms. Previous studies have suggested two mechanisms by which aerosol concentrations could affect the intensity of convection - both involving the release of latent heat into the atmosphere as moisture within clouds condenses (the "warm-phase") or freezes ("cold-phase") to airborne particles. However, the link between aerosols and increased convection remains unclear and represents a major obstacle to understanding current and future severe weather risks - a particularly salient topic as human activities have become a significant source of atmospheric aerosols. To address this, Tristan Abbot and Timothy Cronin use the System for Atmospheric Modeling (SAM), an atmospheric model that can simulate detailed cloud processes, to study cloud-aerosol interactions. While the results show that the high-resolution simulations could reproduce the observed link between aerosols and convection, Abbott and Cronin found that neither of the previously proposed mechanisms can fully explain this invigoration. The authors offer a third possibility: high aerosol concentrations increase environmental humidity by producing more clouds, which can mix more condensed water into the surrounding air. Because humid air favors stronger updrafts, atmospheric convection can intensify, producing invigorated thunderstorms.
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