Researchers recognize a process by which more regular thunderstorms can be created by small particles in the atmosphere.
Observations of Earth’s atmosphere indicate that thunderstorms are much stronger when large amounts of aerosols are present – airborne particles too small to see with the naked eye.
For instance, lightning is more common than in ocean environments along shipping routes, where container ships emit particles into the air.
And the most severe thunderstorms in the tropics brew over land, where both natural causes and human activity lift aerosols.
While a connection between aerosols and thunderstorms has been observed by scientists for decades, the explanation for this connection is not well understood.
A new mechanism by which aerosols can exacerbate thunderstorms in tropical regions has now been discovered by MIT scientists. Using idealized cloud dynamics simulations, the researchers found that by increasing the humidity around the clouds, high concentrations of aerosols would intensify thunderstorm activity.
In order to predict how thunderstorm activity will vary in a region with changing aerosol concentrations, this new mechanism between aerosols and clouds, which the team calls the ‘moisture extraction’ mechanism, could be integrated into weather and climate models.
“It’s possible that by removing pollution, places will experience fewer storms,” says Tim Cronin, assistant professor of atmospheric sciences at MIT. “Overall, this provides a way that humans may have a footprint on climate that we haven’t really appreciated in the past.”
In the journal Science, Cronin and his co-author Tristan Abbott, a doctoral student in the Department of Earth, Atmospheric and Planetary Sciences of MIT, published their results on Jan. 1, 2021.
In a box, clouds
A series of small particles suspended in the air is an aerosol.
Anthropogenic processes, such as the burning and combustion of biomass in ships, factories and vehicle tailpipes, as well as natural phenomena such as volcanic eruptions, sea spray and dust storms, create aerosols.
Aerosols can function in the atmosphere as nuclei for the formation of clouds.
The suspended particles serve as an air surface on which individual droplets that hang together as a cloud will condense the surrounding water vapor to form.
The droplets will collide and combine within the cloud to form larger droplets that ultimately precipitate as rain.
When aerosols are highly concentrated, however, the many tiny particles form similarly tiny droplets of cloud that don’t coalesce easily.
It is an open question exactly how these aerosol-laden clouds cause thunderstorms, while scientists have suggested some theories that Cronin and Abbott wanted to test in high-resolution cloud simulations.
They used an idealized model for their simulations that simulates the dynamics of clouds in a volume reflecting the atmosphere of Earth over a 128-kilometer-wide square of the tropical ocean.
The field is divided into a grid, and when they set such conditions in the model, scientists will observe how parameters such as relative humidity shift in individual grid cells.
In their case, by increasing the concentration of water droplets in clouds, the team performed cloud simulations and described the effects of increased aerosol concentrations.
They then suppressed the processes thought to drive two mechanisms previously proposed to see whether when they increased aerosol concentrations, thunderstorms also increased.
The simulation also produced more violent thunderstorms with greater aerosol concentrations when those processes were switched off.
That told us that in our simulations, these two previously suggested ideas were not the cause of the convection changes,”That told us that these two previously proposed ideas were not the cause of the changes in convection in our simulations,”
In other words, at work, there has to be some other mechanism.
A simulation of a cloud formation day in a low aerosol concentration area.
The colored area reflects the surface air temperature. Many of the clouds (in gray) are 10 to 15 kilometers high, approaching or exceeding the cruising altitude of most aircraft.
These simulated clouds are comparable in size to the clouds in the actual tropics that create thunderstorms.
The team scoured the cloud dynamics literature and found previous work that suggested a relationship between cloud temperature and the ambient air humidity.
Such studies have shown that as clouds rise,