Summer storms are generally more frequent, intense and concentrated over cities than over rural areas, according to new, detailed observations of eight cities and their surroundings. The results could change how city planners prepare for floods in their cities, especially as urban areas expand and as climate change alters global weather patterns.

The new study finds that more storms form over urban areas and their boundaries than in surrounding areas, and that larger cities intensify rainfall more than smaller cities. The research was published in Earth’s Future.

“Cities are expected to become more populated and increase in size in the coming decades,” said Herminia Torelló-Sentelles, an atmospheric scientist at the University of Lausanne and the study’s lead author. “Being able to quantify urban flood risk is important for urban planning and when designing urban drainage systems.”

The rain effect has been reported in studies of single cities, but the new research looked for trends and differences across multiple cities. Differences in urban rainfall patterns highlight the need to keep studying storm activity in cities across the globe, Torelló-Sentelles said.

Taking cities by storm

Some storms have rain that falls evenly, like a sprinkler, while others drop rain in concentrated bursts, like a fire hose. The new study finds that cities can turn storms into fire hoses, dropping bursts of rainfall over small urban areas instead of spreading out the rain over a larger area. Those concentrated bursts of rainfall can exacerbate flood risks if city infrastructure cannot handle the deluge.

Most cities are producing more fire hose-like storms than rural areas. Cities are also spawning more storms than their surroundings, and bigger cities are generating stronger storms than smaller cities.

“It’s not only intensity of rainfall that matters when you look at flood risk. It’s also how it’s distributed over space,” Torelló-Sentelles said. “If you have a very large amount of rainfall falling over a very small area, that can collapse the drainage system in an urban area.”

Several factors could be causing urban storm creation and intensification, Torelló-Sentelles said. Cities are generally warmer than their cool, moist and vegetation-dense surroundings, which could cause air to be drawn toward the cities and uplifted. That warm, uplifted air then condenses into rain clouds over urban centers.

Storms are also often formed as air is uplifted over mountain ranges, producing rain clouds at the mountains’ peaks. Like mini mountain ranges, city skylines can create favorable environments for the uplift of air masses and the creation of storms.

“You can think of a city like an obstacle,” Torelló-Sentelles said. “When a storm is moving toward it, the air can be lifted over and around it.”

Aerosol pollution suspended in the atmosphere over cities may also either enhance or suppress rainfall.

The researchers used seven years of high-resolution weather data from eight cities in Europe and the United States (Milan, Italy; Berlin, Germany; London and Birmingham, United Kingdom; Phoenix, Arizona; Charlotte, North Carolina; Atlanta, Georgia; and Indianapolis, Indiana) to track summer storm formation and intensity in cities and their surroundings. The cities varied in size, climate and urban shape, but all are in relatively flat regions and far from large bodies of water—factors which could influence local rainfall patterns.

The researchers tracked storm formation and evolution outside of and over cities and their boundaries, identifying the average direction, average intensity, maximum intensity and area of each storm.

They found that more storms overall formed over cities and their boundaries compared to nearby rural areas. Storms typically were most intense over city centers, or over the city edges as in Berlin and Birmingham. Larger cities had greater rainfall intensification than smaller cities: in smaller cities, rainfall intensified by 0.9% to 3.4%, while it increased from 5.2% to 11% in the largest cities compared to outlying areas. Some cities also had much higher rainfall intensification during specific times of the day.

Rainfall also became more spatially concentrated over urban areas by up to 15%. Concentrated bursts of rainfall can tax urban water management systems more than rainfall that is evenly distributed.

Strong storms increase urban flood risk

Increasingly large urban areas could generate and amplify more storms than their surroundings, even as climate change continues to intensify storms worldwide. The combined impact of urban growth and climate change could stress urban stormwater systems and lead to more frequent and severe floods.

While the researchers found some consistent trends across all cities, every city changed rainfall patterns in unique ways. For instance, while most of the cities had storms with stronger bursts of rainfall than their surroundings, Berlin and Charlotte had more dispersed rainfall. In Atlanta, storms intensified the most in the daytime hours, while storms in Birmingham only intensified overnight. And unlike the other six cities studied, Berlin and Phoenix did not have more storms initiate over the city than in surrounding areas.

These results highlight the need for individual city planning strategies and studies including more cities, Torelló-Sentelles said. As the climate changes and the world urbanizes, individual cities will need to develop their own adaptation and mitigation strategies.

“We need to study a wider variety of cities so that we can generalize findings and determine which city characteristics have the largest effects on cities’ rainfall-modifying potential,” she said. “The mechanisms driving urban rainfall are quite complex, and we still need to research these processes more.”

Scientists working on the Short-Baseline Near Detector (SBND) at Fermi National Accelerator Laboratory have identified the detector’s first neutrino interactions.

The SBND collaboration has been planning, prototyping and constructing the detector for nearly a decade. After a few-months-long process of carefully turning on each of the detector subsystems, the moment they’d all been waiting for finally arrived.

“It isn’t every day that a detector sees its first neutrinos,” said David Schmitz, co-spokesperson for the SBND collaboration and associate professor of physics at the University of Chicago. “We’ve all spent years working toward this moment and this first data is a very promising start to our search for new physics.”

SBND is the final element that completes Fermilab’s Short-Baseline Neutrino (SBN) Program and will play a critical role in solving a decades-old mystery in particle physics. Getting SBND to this point has been an international effort. The detector was built by an international collaboration of 250 physicists and engineers from Brazil, Spain, Switzerland, the United Kingdom and the United States.

The Standard Model is the best theory for how the universe works at its most fundamental level. It is the gold standard that particle physicists use to calculate everything from high-intensity particle collisions in particle accelerators to very rare decays. But despite being a well-tested theory, the Standard Model is incomplete. And over the past 30 years, multiple experiments have observed anomalies that may hint at the existence of a new type of neutrino.

Neutrinos are the second most abundant particle in the universe. Despite being so abundant, they’re incredibly difficult to study because they only interact through gravity and the weak nuclear force, meaning they hardly ever show up in a detector.

Neutrinos come in three types, or flavors: muon, electron and tau. Perhaps the strangest thing about these particles is that they change among these flavors, oscillating from muon to electron to tau.

Scientists have a pretty good idea of how many of each type of neutrino should be present at different distances from a neutrino source. Yet observations from a few previous neutrino experiments disagreed with those predictions.

“That could mean that there are more than the three known neutrino flavors,” explained Fermilab scientist Anne Schukraft. “Unlike the three known kinds of neutrinos, this new type of neutrino wouldn’t interact through the weak force. The only way we would see them is if the measurement of the number of muon, electron and tau neutrinos is not adding up like it should.”

The Short Baseline Neutrino Program at Fermilab will perform searches for neutrino oscillation and look for evidence that could point to this fourth neutrino. SBND is the near detector for the Short Baseline Neutrino Program while ICARUS, which started collecting data in 2021, is the far detector. A third detector called MicroBooNE finished recording particle collisions with the same neutrino beamline that same year.

The Short Baseline Neutrino Program at Fermilab differs from previous short-baseline measurements with accelerator-made neutrinos because it features both a near detector and far detector. SBND will measure the neutrinos as they were produced in the Fermilab beam and ICARUS will measure the neutrinos after they’ve potentially oscillated. So, where previous experiments had to make assumptions about the original composition of the neutrino beam, the SBN Program will definitively know.

“Understanding the anomalies seen by previous experiments has been a major goal in the field for the last 25 years,” said Schmitz. “Together SBND and ICARUS will have outstanding ability to test the existence of these new neutrinos.”

Beyond the hunt for new neutrinos

In addition to searching for a fourth neutrino alongside ICARUS, SBND has an exciting physics program on its own.

Because it is located so close to the neutrino beam, SBND will see 7,000 interactions per day, more neutrinos than any other detector of its kind. The large data sample will allow researchers to study neutrino interactions with unprecedented precision. The physics of these interactions is an important element of future experiments that will use liquid argon to detect neutrinos, such as the long-baseline Deep Underground Neutrino Experiment, known as DUNE.

Whenever a neutrino collides with the nucleus of an atom, the interaction sends a spray of particles careening through the detector. Physicists need to account for all the particles produced during that interaction, both those visible and invisible, to infer the properties of the ghostly neutrinos.

It’s relatively easy to model what happens with simple nuclei, like helium and hydrogen, but SBND, like many modern neutrino experiments, uses argon to trap neutrinos. The nucleus of an argon atom consists of 40 nucleons, making interactions with argon more complex and more difficult to understand.

“We will collect 10 times more data on how neutrinos interact with argon than all previous experiments combined,” said Ornella Palamara, Fermilab scientist and co-spokesperson for SBND. “So, the analyses that we do will also be very important for DUNE.”

But neutrinos won’t be the only particles SBND scientists will keep an eye out for. With the detector located so close to the particle beam, it’s possible that the collaboration could see other surprises.

“There could be things, outside of the Standard Model, that have nothing to do with neutrinos but are produced as a byproduct of the beam that the detector would be able to see,” said Schukraft.

One of the biggest questions the Standard Model doesn’t have an answer for is dark matter. Although SBND would only be sensitive to lightweight particles, those theoretical particles could provide a first glimpse at a “dark sector.”

“So far ‘direct’ dark matter searches for massive particles haven’t turned anything up,” said Andrzej Szelc, SBND physics co-coordinator and professor at the University of Edinburgh. “Theorists have devised a whole plethora of dark sector models of lightweight dark particles that could be produced in a neutrino beam and SBND will be able to test whether these models are true.”

These neutrino signatures are only the beginning for SBND. The collaboration will continue operating the detector and analyzing the many millions of neutrino interactions collected for the next several years.

“Seeing these first neutrinos is the start of a long process that we have been working towards for years,” said Palamara. “This moment is the beginning of a new era for the collaboration.”