Natural and anthropogenic aerosols play a significant role in developing convective clouds, and more specifically, thunderstorms. Numerous studies also suggest that thunderstorms are often stronger in the presence of high concentrations of aerosols.
Well, the association between aerosols and thunderstorms are being studied for decades, but the mechanism behind this is not well-understood.
MIT scientists seem to figure out the mechanism through which aerosols shoot up thunderstorms in tropical regions.
Scientists used idealized simulations of cloud dynamics. They discovered that aerosols’ high concentrations increase the humidity in the air surrounding clouds, hence enhancing thunderstorm activity.
This new mechanism is being called as ‘humidity-entrainment’ mechanism. It could be used in weather and climate models to predict how a region’s thunderstorm activity might vary with changing aerosol levels.
Tim Cronin, assistant professor of atmospheric science at MIT, said, “It’s possible that, by cleaning up pollution, places might experience fewer storms. Overall, this provides a way that humans may have a footprint on the climate that we haven’t appreciated much in the past.”
The idealized model used in the study simulates the dynamics of clouds in a volume representing Earth’s atmosphere over a 128-kilometer-wide square of the tropical ocean. The box is divided into the grid so that scientists can observe how parameters like relative humidity change in individual grid cells as they tune certain conditions in the model.
When scientists ran simulations, it represented the effects of increased aerosol concentrations by increasing the concentration of water droplets in clouds. They then suppressed the processes thought to drive two previously proposed mechanisms to see if thunderstorms still increased when they turned up aerosol concentrations.
When these processes were shut off, the simulation still generated more intense thunderstorms with higher aerosol concentrations.
Abbott said, “That told us these two previously proposed ideas weren’t what were producing changes in convection in our simulations.”
By digging through the literature on cloud dynamics, scientists found previous work highlighting a relationship between cloud temperature and the humidity of the surrounding air. These studies showed that as clouds rise, they mix with the clear air around them, evaporating some of their moisture and, as a result cooling the clouds themselves.
If the surrounding air is dry, it can soak up more of a cloud’s moisture and bring down its internal temperature, such that the cloud, laden with cold air, is slower to rise through the atmosphere. On the other hand, if the surrounding air is relatively humid, the cloud will be warmer as it evaporates and will rise more quickly, generating an updraft that could spin up into a thunderstorm.
Scientists wondered whether this mechanism might be at play in aerosols’ effect on thunderstorms. If a cloud contains many aerosol particles that suppress rain, it might be able to evaporate more water to its surroundings. In turn, this could increase the humidity of the surrounding air, providing a more favorable environment for the formation of thunderstorms. This chain of events, therefore, could explain aerosols’ link to thunderstorm activity.
They tested this idea using the same simulation, noting the temperature and relative humidity of each grid cell in and around clouds as they increased the aerosol concentration in the simulation. The concentrations they set ranged from low-aerosol conditions similar to remote regions over the ocean to high-aerosol environments similar to relatively polluted air near urban areas.
They found that low-lying clouds with high aerosol concentrations were less likely to rain out. Instead, these clouds evaporated water to their surroundings, creating a sticky layer of air that made it easier for air to rise quickly through the atmosphere as strong, storm-brewing updrafts.
Abbott said, “After you’ve established this humid layer relatively low in the atmosphere, you have a bubble of warm and moist air that can act as a seed for a thunderstorm. That bubble will have an easier time ascending to altitudes of 10 to15 kilometers, which is the depth clouds need to grow to act as thunderstorms.”
“This ‘humidity-entrainment’ mechanism, in which aerosol-laden clouds mix with and change the humidity of the surrounding air, seems to be at least one explanation for how aerosols drive thunderstorm formation, particularly in tropical regions where the air, in general, is relatively humid.”
“We’ve provided a new mechanism that should give you a reason to predict stronger thunderstorms in parts of the world with lots of aerosols.”
The results of this study are published in the journal Science.