The presence of aerosols – including human-created pollutants – in the atmosphere can increase the humidity of clouds, boosting updraft speeds and leading to more intense thunderstorms, argue researchers from the US. Understanding this “humidity entrainment” mechanism could help scientists understand and predict storm development, especially in high aerosol regions in the tropics.
The release of microscopic particles into the atmosphere – whether from human activity, or via natural sources like volcanoes or dust storms – is known to impact local weather conditions. Among other effects, such releases strengthen updrafts in deep convective clouds, generating larger and more severe storm systems. This helps to explain why, in the tropics, the most intense thunderstorms form over land, where atmospheric aerosol levels are higher. In contrast, lightning flashes that occur over the ocean are more commonly observed near shipping lanes, where vessels leave tracks of exhaust aerosols in their wake.
Cold phase and warm phase
The exact reason for the association between aerosols and storm activity has, however, long been unclear. Two mechanisms have been put forward to explain it, both of which involve the release of latent heat into the atmosphere. In the first mechanism, the presence of aerosols in shallow clouds increases the number of particles onto which liquid droplets can condense, thereby suppressing rain. This so-called “cold phase” mechanism allows shallow clouds to loft more condensed water through the level at which the atmospheric temperature drops to 0°C, resulting in the release of more latent heat and thereby increasing buoyant updraft.
The alternative “warm-phase” mechanism, meanwhile, proposes that higher concentrations of aerosol particles serve instead to reduce supersaturation in liquid clouds. This effect also increases the release of latent heat, by allowing greater volumes of water vapour to condense.
In their new study, atmospheric scientists Tristan Abbott and Timothy Cronin of the Massachusetts Institute of Technology put these theories to the test using the so-called System for Atmospheric Modeling (SAM), which produces high-resolution simulations of cloud processes in a 128 km2 patch of atmosphere over the tropical ocean. To emulate the impact of rising aerosol concentrations in their model, the researchers increased the concentration of water droplets in the clouds from the low-aerosol conditions typical of the atmosphere above remote ocean regions to the high-aerosol environment found around polluted urban areas.
Although the scientists found that their simulation could reproduce the observed association between higher cloud aerosol levels and increased convection rates, the degree of this “invigoration” – as they term it – could not be fully explained by either the cold- or warm-phase theories. Indeed, when the researchers supressed the mechanisms underpinning both, they found that the model still produced more intense thunderstorms when the concertation of aerosol particles was increased.
Seeding a thunderstorm
In place of the cold- and warm-phase mechanisms, Abbott and Cronin propose that clouds rich in aerosols – which, as established, serve to supress rainfall – can evaporate more water to their surroundings, increasing local humidity. Such clouds, they explain, make it easier for the air to rise quickly, in contrast to a drier setting in which the surrounding air would cool the cloud by evaporation and slow its ascent. This bubble of warm air can then act as a seed for a thunderstorm. When they ran their cloud dynamics model again – this time with a focus on relative humidities and temperatures – the results validated their theory as a mechanism by which increasing aerosol concentrations could invigorate atmospheric convection.
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Jiwen Fan, a senior atmospheric scientist at the Pacific Northwest National Laboratory in the US who was not involved in the study, notes that while the paper proposes an additional mechanism of aerosol-induced invigoration of convective storms, its findings are subject to large uncertainties. “The model used in the study has a simple representation of cloud microphysics, which is fundamental to modelling the interactions between aerosols and clouds,” she explains. “The simplifications, such as fixing the cloud droplet number concentrations and simplifying cloud condensation/evaporation calculation with saturation adjustment, may lead to excessive evaporation, as shown by past studies, and exaggerate the humidification effect.”
Fan also questioned whether the proposed humidity entrainment mechanism is significant in the presence of the warm-phase invigoration mechanism. While previous studies have shown that warm-phase invigoration is important in the invigoration of tropical convection, she notes that this study precluded it (that is, it did not allow the mechanism to work). More measurements and modelling studies are, she suggests, needed to evaluate the significance of the various mechanisms.
The research is described in Science.
- This article was amended on 14 January 2021 to clarify the role of the warm-phase invigoration mechanism in the study.