Aurelia Butler

Observations of Earth’s atmosphere show that thunderstorms are often stronger in the presence of high concentrations of aerosols — airborne particles too small to see with the naked eye.
For instance, lightning flashes are more frequent along shipping routes, where freighters emit particulates into the air, compared to the surrounding ocean. And the most intense thunderstorms in the tropics brew up over land, where aerosols are elevated by both natural sources and human activities.
While scientists have observed a link between aerosols and thunderstorms for decades, the reason for this association is not well-understood.
Now MIT scientists have discovered a new mechanism by which aerosols may intensify thunderstorms in tropical regions. Using idealized simulations of cloud dynamics, the researchers found that high concentrations of aerosols can enhance thunderstorm activity by increasing the humidity in the air surrounding clouds.
This new mechanism between aerosols and clouds, which the team has dubbed the “humidity-entrainment” mechanism, could be incorporated into weather and climate models to help predict how a region’s thunderstorm activity might vary with changing aerosol levels.
“It’s possible that, by cleaning up pollution, places might experience fewer storms,” says Tim Cronin, assistant professor of atmospheric science at MIT. “Overall, this provides a way that humans may have a footprint on the climate that we haven’t really appreciated much in the past.”
Cronin and his co-author Tristan Abbott, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences, have published their results today in the journal Science.
Clouds in a box
An aerosol is any collection of fine particles that is suspended in air. Aerosols are generated by anthropogenic processes, such as the burning of biomass, and combustion in ships, factories, and car tailpipes, as well as from natural phenomena such as volcanic eruptions, sea spray, and dust storms. In the atmosphere, aerosols can act as seeds for cloud formation. The suspended particles serve as airborne surfaces on which surrounding water vapor can condense to form individual droplets that hang together as a cloud. The droplets within the cloud can collide and merge to form bigger droplets that eventually fall out as rain.
But when aerosols are highly concentrated, the many tiny particles form equally tiny cloud droplets that don’t easily merge. Exactly how these aerosol-laden clouds generate thunderstorms is an open question, although scientists have proposed several possibilities, which Cronin and Abbott decided to test in high-resolution simulations of clouds.
For their simulations, they used an idealized model, which simulates the dynamics of clouds in a volume representing Earth’s atmosphere over a 128-kilometer-wide square of tropical ocean. The box is divided into a grid, and scientists can observe how parameters like relative humidity change in individual grid cells as they tune certain conditions in the model.
In their case, the team ran simulations of clouds and 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.
“That told us these two previously proposed ideas weren’t what were producing changes in convection in our simulations,” Abbott says.
In other words, some other mechanism must be at work.

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A simulation of one day of cloud formation in a region of low aerosol concentration. The colored surface represents the air temperature at the surface. Many of the clouds (in grey) are 10 to 15 kilometers tall, reaching at or above the cruising altitudes of most aircraft. These simulated clouds are similar in size to clouds that produce thunderstorms in the real-world tropics.

Driving storms
The team dug through the literature on cloud dynamics and found previous work that pointed to 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.
Cronin and Abbott 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 the 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 put their idea to the test using the same simulation of cloud dynamics, this time 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 humid layer of air that made it easier for air to rise quickly through the atmosphere as strong, storm-brewing updrafts.
“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,” Abbott says. “That bubble will have an easier time ascending to altitudes of 10 to15 kilometers, which is the depth clouds need to grow to 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,” Abbott says.
This research was supported, in part, by the National Science Foundation. More

In 2020, with many aspects of our everyday lives turned upside-down, news and views from around the Institute continued to draw a great deal of media interest. Despite the challenges of this unusual and unprecedented year, the MIT community still found ways to grab headlines by breaking barriers, innovating, making discoveries, and taking a stand. Below are just some of the stories that captured the great work of MIT students, faculty, and staff in 2020.
Opinion: Has the coronavirus finally taught us how to listen to science?After MIT and other area institutions acted swiftly to rearrange how we live and work in response to the coronavirus pandemic, President L. Rafael Reif wrote about how we might confront another big challenge: climate change. “If we can take the right lessons from the crisis, we will find ourselves better prepared to tackle the health of our fevered planet.” Full story via The Boston Globe
MIT fast-tracking face shields to country’s busiest hospitals treating coronavirus
Professor Martin Culpepper spoke with Cynthia McFadden of NBC News about his team’s work designing a new face shield that can be rapidly manufactured. “It’s the kind of ingenuity that MIT is known for,” says McFadden, noting that MIT “has long been on the front lines of solving America’s problems.”Full story via NBC News
Meet MIT’s first Black female student body president
Danielle Geathers, president of the MIT Undergraduate Association, joined Kelly Clarkson to discuss the goals of her presidency. She highlighted the Talented Ten Mentorship program she founded, which aims to help increase matriculation of Black women by pairing Black girls and women in high school with Black women at MIT.Full story via The Kelly Clarkson Show 
Opinion: The 2020 election meltdown that didn’t happen
Professor Charles Stewart III published numerous opinion pieces examining the administration of the 2020 presidential election. In The Wall Street Journal, Stewart wrote that “the U.S. should be thankful for the heroic — and successful — efforts of election administrators around the country.”Full story via The Wall Street Journal 
Trump administration rescinds rules on foreign students studying online
In response to a lawsuit filed by MIT and Harvard University, the Department of Homeland Security rescinded a new policy that would have prevented thousands of foreign students from studying in the U.S. “This case made abundantly clear that real lives are at stake in these matters, with the potential for real harm,” said MIT’s president.Full story via The Wall Street Journal 
Related: “I’m the President of MIT. America needs foreign students” via The New York Times
Karilyn Crockett appointed head of city’s new equity and inclusion office
Karilyn Crockett, a lecturer in the Department of Urban Studies and Planning, was named chief of equity for the City of Boston. “Do we have the will and the courage to dream new dreams for populations long denied what we actually deserve?” Crockett asked. “I believe we do.”Full story via The Boston Globe
Lessons from a study of the digital economy
Three years after answering an “intellectual call to arms” to examine the impact of technology on jobs, the MIT Task Force on the Work of the Future published its final set of recommendations. “In an extraordinarily comprehensive effort, they included labor market analysis, field studies and policy suggestions for changes in skills-training programs, the tax code, labor laws and minimum-wage rates,” wrote reporter Steve Lohr.Full story via The New York Times
Astronomers find possible sign of life on Venus 
In one of the most talked about discoveries this year, scientists at MIT and elsewhere reported that they have found phosphine in the atmosphere of Venus.Full story via CBS This Morning 
Adapting to social media’s disruptions in “The Hype Machine”
Professor Sinan Aral explored the benefits and downfalls posed by social media. “I’ve been researching social media for 20 years. I’ve seen its evolution and also the techno utopianism and dystopianism,” said Aral. “I thought it was appropriate to have a book that asks, ‘what can we do to really fix the social media morass we find ourselves in?’”Full story via NPR
Compact nuclear fusion reactor is “very likely to work,” studies suggest
In a series of peer reviewed papers, MIT researchers provided evidence that plans to develop a next-generation compact nuclear fusion reactor, known as SPARC, should be feasible.Full story via The New York Times 
Community ingenuity in the face of Covid-19
The coronavirus affected many aspects of Institute life, both directly for our community members, and indirectly, as a new challenge needing to be addressed worldwide. MIT students, staff, researchers, and other community members deftly answered the challenge.
MIT out-MITs itself; builds full scale campus replica on Minecraft
MIT community members recreated the MIT campus in Minecraft, providing an opportunity for students to enjoy MIT’s “intensely collaborate culture” from afar. “Being able to meet in a virtual space and have some kind of social interaction, even while being socially distant — it’s just really important to a lot of students,” explained first-year student Shayna Ahteck.Full story via Boston Magazine
New York needed ventilators. So they developed one in a month.
A team in New York, inspired by the open-source ventilator design from the MIT E-Vent group, developed a lower cost ventilator now in production. The “hurry-up engineering feat” relied on a network of MIT professors, students, and alumni.Full story via The New York Times
A few MIT students produced one of the best hackathons on Covid-19
A team of MIT students hosted the Africa Takes on Covid-19 virtual hackathon, which brought together participants from around the world to “create tech-driven solutions to address the most critical unmet needs caused by the Covid-19 outbreak across the continent.”Full story via True Africa 
How MIT, Harvard are managing to keep COVID-19 numbers low
Ian Waitz, vice chancellor for undergraduate and graduate education, and Suzanne Blake, director of MIT Emergency Management, discussed MIT’s work to mitigate Covid-19 transmission on campus this fall.Full story via Cambridge Chronicle
A pandemic upended their communities, so these teen inventors built apps to make life easier
When the MIT App Inventor team moved its hackathon online due to the coronavirus pandemic, it gave aspiring coders from all over the world an opportunity to enter the competition. “There was a sense of helplessness that was settling down. And a big theme in our workplace is empowerment,” said curriculum developer Selim Tezel. “We wanted to give them a context in which they could be creative and sort of get rid of that feeling of helplessness.”Full story via CNN
In effort to fight Covid-19, MIT robot gets to work disinfecting The Greater Boston Food Bank
A robotic system developed by CSAIL researchers in collaboration with Ava Robotics uses UV-C light to kill viruses and bacteria on surfaces and aerosols.Full story via TechCrunch
Media moments for math
The people of MIT were frequently recognized and profiled in the media, but one department in particular saw a number of stories that inspire: mathematics.
From NFL to MIT: John Urschel looking to increase diversity in mathematicsGraduate student John Urschel, a trustee of the National Museum of Mathematics (MoMath), spoke with ESPN about his efforts aimed at empowering and encouraging more Black students to pursue careers in STEM fields.Full story via ESPN
A math problem stumped experts for 50 years. This grad student solved it in days.
Assistant Professor Lisa Piccirillo, who solved the Conway knot problem as a graduate student, reflected on what drew her to math.Full story via The Boston Globe Magazine
Undergraduate math student pushes the frontier of graph theoryIn a profile of graduate student Ashwin Sah, Quanta Magazine reported that he “produced a body of work that senior mathematicians say is nearly unprecedented for a college student.”Full story via Quanta Magazine
More of the latest MIT In the Media summaries, with links to the original reporting, are available at news.mit.edu/in-the-media. More

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Eddies are often seen as the weather of the ocean. Like large-scale circulations in the atmosphere, eddies swirl through the ocean as slow-moving sea cyclones, sweeping up nutrients and heat, and transporting them around the world.
In most oceans, eddies are observed at every depth and are stronger at the surface. But since the 1970s, researchers have observed a peculiar pattern in the Arctic: In the summer, Arctic eddies resemble their counterparts in other oceans, popping up throughout the water column. However, with the return of winter ice, Arctic waters go quiet, and eddies are nowhere to be found in the first 50 meters beneath the ice. Meanwhile, deeper layers continue to stir up eddies, unaffected by the abrupt change in shallower waters.
This seasonal turn in Arctic eddy activity has puzzled scientists for decades. Now an MIT team has an explanation. In a paper published today in the Journal of Physical Oceanography, the researchers show that the main ingredients for driving eddy behavior in the Arctic are ice friction and ocean stratification.
By modeling the physics of the ocean, they found that wintertime ice acts as a frictional brake, slowing surface waters and preventing them from speeding into turbulent eddies. This effect only goes so deep; between 50 and 300 meters deep, the researchers found, the ocean’s salty, denser layers act to insulate water from frictional effects, allowing eddies to swirl year-round.
The results highlight a new connection between eddy activity, Arctic ice, and ocean stratification, that can now be factored into climate models to produce more accurate predictions of Arctic evolution with climate change.
“As the Arctic warms up, this dissipation mechanism for eddies, i.e. the presence of ice, will go away, because the ice won’t be there in summer and will be more mobile in the winter,” says John Marshall, professor of oceanography at MIT. “So what we expect to see moving into the future is an Arctic that is much more vigorously unstable, and that has implications for the large-scale dynamics of the Arctic system.”
Marshall’s co-authors on the paper include lead author Gianluca Meneghello, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences, along with Camille Lique, Pal Erik Isachsen, Edward Doddridge, Jean-Michel Campin, Healther Regan, and Claude Talandier.
Beneath the surface
For their study, the researchers assembled data on Arctic ocean activity that were made available by the Woods Hole Oceanographic Institution. The data were collected between 2003 and 2018, from sensors measuring the velocity of the water at different depths throughout the water column.
The team averaged the data to produce a time series to produce a typical year of the Arctic Ocean’s velocities with depth. From these observations, a clear seasonal trend emerged: During the summer months with very little ice cover, they saw high velocities and more eddy activity at all depths of the ocean. In the winter, as ice grew and increased in thickness, shallow waters ground to a halt, and eddies disappeared, whereas deeper waters continued to show high-velocity activity.
“In most of the ocean, these eddies extend all the way to the surface,” Marshall says. “But in the Arctic winter, we find that eddies are kind of living beneath the surface, like submarines hanging out at depth, and they don’t get all the way up to the surface.”
To see what might be causing this curious seasonal change in eddy activity, the researchers carried out a “baroclinic instability analysis.” This model uses a set of equations describing the physics of the ocean, and determines how instabilities, such as weather systems in the atmosphere and eddies in the ocean, evolve under given conditions.

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An icy rub
The researchers plugged various conditions into the model, and for each condition they introduced small perturbations similar to ripples from surface winds or a passing boat, at various ocean depths. They then ran the model forward to see whether the perturbations would evolve into larger, faster eddies.
The researchers found that when they plugged in both the frictional effect of sea ice and the effect of stratification, as in the varying density layers of the Arctic waters, the model produced water velocities that matched what the researchers initially saw in actual observations. That is, they saw that without friction from ice, eddies formed freely at all ocean depths. With increasing friction and ice thickness, waters slowed and eddies disappeared in the ocean’s first 50 meters. Below this boundary, where the water’s density, i.e. its stratification, changes dramatically, eddies continued to swirl.
When they plugged in other initial conditions, such as a stratification that was less representative of the real Arctic ocean, the model’s results were a weaker match with observations.
“We’re the first to put forward a simple explanation for what we’re seeing, which is that subsurface eddies remain vigorous all year round, and surface eddies, as soon as ice is around, get rubbed out because of frictional effects,” Marshall explains.
Now that they have confirmed that ice friction and stratification have an effect on Arctic eddies, the researchers speculate that this relationship will have a large impact on shaping the Arctic in the next few decades. There have been other studies showing that summertime Arctic ice, already receding faster year by year, will completely disappear by the year 2050. With less ice, waters will be free to swirl up into eddies, at the surface and at depth. Increased eddy activity in the summer could bring in heat from other parts of the world, further warming the Arctic.
At the same time, the wintertime Arctic will be ice covered for the foreseeable future, notes Meneghello. Whether a warming Arctic will result in more ocean turbulence throughout the year or in a stronger variability over the seasons will depend on sea ice’s strength.
Regardless, “if we move into a world where there is no ice at all in the summer and weaker ice during winter, the eddy activity will increase,” Meneghello says. “That has important implications for things moving around in the water, like tracers and nutrients and heat, and feedback on the ice itself.”
This research is supported, in part, by the National Science Foundation. More

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