The other Manhattan project: A thankless quest to understand New York’s atmosphere

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In 2003, a group of atmospheric scientists set out to illuminate the little-understood behavior of New York City’s urban atmosphere. And despite some significant advances, years later, they are left with the same problem as when they started: the movements of the atmosphere in a city as complicated as New York defy prediction.

“There’s still a lot of uncertainty,” said Steve Hanna, one meteorologist who participated in the project. “A model may be showing the wind at the bottom of the building having a certain direction, and if you go out and measure it, it may be 180 degrees off.”

If scientists do master the workings of urban atmospheres, city dwellers could see their lives improve in both every day and extraordinary circumstances: urban planners could ensure cleaner air on city streets, and first responders could respond more quickly and accurately to toxic releases. The intent of the Urban Atmosphere Observatory, as the 2003 project was initially called, was to provide the city with a new tool to respond to chemical and biological attacks, and to create a destination for research on the burgeoning field of urban meteorology. But the project ended with little fanfare in 2007. And though individual scientists have continued to study the city’s air patterns, the city has yet to see tangible results.

The atmosphere is the most underexplored physical component of New York City, and although scientists have known for more than 200 years that cities can impact climate, only since the 1960s have they begun to model these impacts. It is always difficult to understand precisely how air moves around and interacts with a city’s many surfaces, but New York City is a particular challenge. As a coastal city, it "has a pronounced sea-land breeze system that creates daily, sudden shifts in wind speed and direction,” as the first report on the Urban Atmosphere Observatory explained. It is also what oceanographer Julie Pullen explained was a “corrugated urban landscape”—it has more than one peak of very tall buildings, and even within areas like Midtown, the height of buildings varies wildly.

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“As a potential target, New York looms quite large in everyone's mind, and New York is probably the most complex of cities in terms of urban structures and how they interact with the meteorology,” Pullen says. “It makes New York as a city very complicated to do this type of work in.”

Understanding movement within the atmosphere is particularly complicated. “When the weather report says it's 39 degrees, that’s what you're going to see around the city,” says Stuart Gaffin, who studies climate at Columbia University’s Earth Institute. “If you move to rain, that's nowhere near as smooth. I have a lot of rain guages around the city. It's not uniform, by any means. Winds are an order of magnitude more variable.”

“At a fundamental level, rainfall and wind is turbulence, or that clichéd word, chaos,” he said. “Turbulence is one of the hardest thing to deal with in science.”

The only reason the city’s atmosphere has been studied as much as it has in recent years is because of how city air can spirit along with it dangers like pollutants or chemicals released by a terrorist’s bomb, and even then, opportunities to collect data on New York in particular have been scarce. Bob Bornstein, a blustery, Bronx Science-educated meteorologist, has perhaps spent more time than anyone else thinking about New York City’s atmosphere, and even he has based his work primarily on one set of data, collected in the 1960s.

Bornstein graduated in 1963 from City College of New York, with a degree in meteorology and a dream of working for NASA. But when he was interviewing for Ph.D. programs in meteorology, he quickly caught wind of another opportunity. In the 1960s, air pollution was where the money was. New York University had received millions of dollars from what would become the Environmental Protection Agency to study air pollution in the city, and Bornstein signed on.

During a series of field study periods, a team that included Bornstein collected data on things like wind speed and moisture measurement. A helicopter flew over the city, starting two hours before sunrise and ending two hours afterward, taking temperature measurements. Bornstein supervised teams of students who were taking wind measurements with balloons, in the lowest mile of the atmosphere.

That one round of data collection lasted Bornstein for decades. In 1969, he moved to San Jose State University, and arranged to bring the data with him. From California, he and his students began to map out the structure of the air over New York City.

As a rule, cities create urban heat islands—areas that retain heat longer than more rural or suburban landscapes. (Anyone who has spent a summer in New York understands this instinctually: it’s why at 11:30 p.m., long after a cool breeze should have relieved the heat of the day, going outside still feels likes entering a sauna.) The data from the NYU experiment showed Bornstein, first of all, the vertical extent of New York’s urban heat island effect—300 meters, on average, above the city. It also showed that, when wind hit New York, the city’s physical structures initially slowed it down, but eventually the city’s heat overcame that hump, and the wind sped up again.

Few people were looking at the behavior of urban atmospheres, and Bornstein’s discoveries brought him acclaim in his field. “Those are my first two coups,” he said.

He continued mining “this golden data set.” He worked on one of the first three-dimensional models for air pollution. He learned that, contrary to what one might expect, the city’s atmosphere retained moisture better than rural areas. Also that it could delay a cold front passing through. In some cases, the city acted in the same way as a mountain; it shunted fronts right over it. Bornstein’s work also brought him into contact with forecasters, who gave him two years’ worth of radar echoes that proved that thunderstorms that begin outside the city don’t go over it; they go around it. The city splits them right in two.

The majority of Bornstein’s work focused on the mesoscale, or the urban boundary layer: the mile of air above the city, which plays an important role in regulating pollution.

Beginning in the late 1990s, and in particular after 9/11, funding for this sort of work began coming from a new source: government agencies concerned with the airborne threats from chemical and biological weapons. Even before 9/11, the federal government had been interested in how a plume of toxic material would move through a city. It had funded studies in Salt Lake City and in Oklahoma City in which scientists had released tracer gas and tracked its movements. But in cities like those, with fewer tall buildings and less variation, those movements were fairly predictable. (“Oklahoma City was pretty mundane,” was how one scientist put it.)

New York, on the other hand, posed a challenge. Most cities have only one cluster of tall buildings, toward the center; Manhattan alone has two distinct peaks, one in midtown, one in lower Manhattan, and lots of height variety within each. Pullen, the oceanographer, worked on a paper that gives one example of this disparity: in one area, four kilometers square, centered near Rockefeller Center, the average building height is 53 meters, and the standard deviation from that figure is 47 meters, denoting a jumble of buildings that soar to the sky and squat close to the ground.

There are other complicating factors. New York is on the water, which means sea breezes buffet the city. In Manhattan, in the middle of the corrugated interplay of buildings, Central Park is a relatively flat expanse composed of organic material that deals with heat differently than the concrete surrounding it. Manhattan and Brooklyn are divided by the East River, which acts as a corridor for the air flow. The atmosphere also extends underground, through the subway system, a particular concern for homeland security.

In 2003, when the group of scientists and policy makers met in New York to begin work on addressing that threat to the city, the New York Police Department sent a representative; so did the newly formed Department of Homeland Security.

“Urban meteorology is still in its infancy, especially for densely built-up downtown areas,” wrote Mike Reynolds of Brookhaven National Laboratory and Sam Lee of the Environmental Measurements Laboratory, both government-affiliated organizations, in their summary of the meeting. New York City, they asserted, would be the perfect place to start building new knowledge. They envisioned a permanent network of observational tools that would provide real-time data for both scientists and emergency managers.

Ideally, if a noxious, air-borne substance were to be released in New York, emergency managers would be able to predict its progress, understand where it would go, who to warn, where to focus their efforts. But on the city level—what scientists call the microscale, or the urban canopy layer—the tools they had to model the city’s air flow were not sensitive enough. Most models simply didn’t drill down to the level of detail that included buildings, even: a model with a resolution of 5 km, for example, might have one grid point in Manhattan and the next one in the Bronx.

To be useful to first responders, models would have to take into account individual buildings, at the very least. In this scale, the smallest details of the city start to make a difference. In a city like New York, the long corridors of tall buildings create wind canyons, which determine how air moves. But two buildings of the same height create different turbulence patterns than two of staggered heights. Air also moves differently around tall buildings than it does around shorter ones, which it can also go over. It can move more quickly up along the sides of sunny buildings, where heat helps it rise. The vast, underground subway network and its ventilation system complicates things, but so do fountains, which can affect the air’s buoyancy. The two-foot cornice of a building can change the pattern of the wind. So can trees, roads, courtyards, gardens; the impact can extend to within a meter, or within a hundred meters.

To make a model able to reckon with that level of detail requires extraordinary computer power, which has only recently been brought to bear on the urban weather studies. (For many years, the best models that scientists studying air and wind movement had were built in wind tunnels: plexiglass areas, as small as an East Village apartment or as large as a high school gym, where researchers would build a physical model to scale and, essentially, turn on a fan and make measurement. )

“Because we have the computer power, we can do the same kind of things, but we can run them over and over and over again,” said Teddy Holt, a Naval Research Lab* scientist who has worked on modeling. “It's the computer power that has allowed us to such high resolution that we can start predicting these kinds of things.”

By 2005, that initial meeting of the Urban Atmosphere Observatory had led to two experiments, one in the Madison Square Garden area, another in Midtown, in which scores of scientists and their assistants released and then monitored the path of an odorless, colorless gas. The media showed up but quickly lost interest, said Steve Hanna, one scientist involved in the experiment.

“They apparently were expecting visible smoke and spinning instruments,” he wrote in an email. “But the tracer gas was invisible and was released in tiny quantities from a small tank and hose which just sat there and did nothing. Similarly the winds were observed by sonic anemometers which also do not spin around and just sit there, not moving. The gas samplers were small stationary hoses and boxes which we carried around and set up on light posts and rooftops in what looked like beer-coolers. It was not like a Weather Channel special showing meteorologists chasing tornados and houses being blown apart.”

Two years later, when the project finished up, its leaders made no effort to trumpet its accomplishments. The City College of New York inherited the network of equipment that had been procured for the project. The government decided to keep portions of the data collected close to its chest, particularly those related to the subway. One of the main results of the tests was to show how limited the current models for predicting air flow were. As Teddy Holt, who was involved with the project put it: “Oftentimes it went places they thought it would go, but it also went places they thought it wouldn't go.”

The federal government is still working on the product it promised to the city’s emergency management office. The work begins with the models atmospheric scientists have created of the city’s air system. They are like maps of the city’s atmosphere: if you plug in certain starting conditions—location plus wind direction—they’ll show you where a released gas will go next. These models will eventually feed an application that emergency responders could load on a handheld device and, after plugging in those few pieces of data, get a read on where a toxic release is bound for. But creating the applications is arduous: the most detailed models run slowly, over a few hours on supercomputers, so to create a quick-response program, scientists have to run, in advance, a large range of scenarios, creating a library of look-up tables that will be more quickly available. And since New York is so complicated, the government scientists working on the project have essentially put the project here on hold, by focusing on creating the application for D.C.-based responders first.

Even these most sophisticated models do not account for things like the subtle effects of the heat coming from the sunny side of a building, for instance. But atmospheric scientists around the world are still very much interested in understanding and modeling what happens in the street canyons of cities like New York. They want to make pedestrians more comfortable, by creating systems that help whisk away heat and pollution from the city streets. They want to make cities more sustainable by understanding how they interact with sunlight, and trap heat, in order to fix those tendencies and abate climate change.

The funding for scientists pursuing this sort of work has shifted away from homeland security and back to environmental concerns. Steve Hanna has found that at conferences, everyone seems to be “becoming an urban climatologist.” Stuart Gaffin, the Earth Institute scientist, is studying white roofs.

“The time is right to revisit Lower Manhattan as another experiment site, because it's so different from Midtown,” Pullen says. “These types of things tend to go in cycles.”

*corrected from the original

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