A humidity-entrainment mechanism for microphysical invigoration of convection.

Cloud-aerosol interactions are thought to influence vertical velocities in deep convection, and past research has proposed several mechanisms which which high aerosol concentrations might invigorate convection. However, these mechanisms largely ignore coupling between clouds and their environment and therefore neglect possible invigoration mechanisms that involve that coupling. In this work, we propose a novel invigoration mechanism that arises through the influence of aerosols on environmental humidity. We find that increasing the liquid cloud droplet number concentration (Nc, a proxy for aerosol concentration) produces robust increases in peak vertical velocities in idealized convection-permitting simulations. We show that environmental humidity also increases robustly with Nc, and we use calculations with simple entraining plume models to link the humidity increase to warmer and faster updrafts. Additionally, we argue that humidity increases at high Nc because clouds form rain less readily and instead detrain more condensate into their environment. Finally, we show that links between aerosol concentration and peak vertical velocity are weakened in scenarios where strong large-scale forcing replaces cloud detrainment as the dominant control on atmospheric humidity. Our results emphasize the importance of interactions between clouds and their environment in determining the cloud-scale response to changes in cloud microphysics and suggest that a possible response to increased aerosol loading is invigoration by a "humidity-entrainment" mechanism.