3:09 pm Jan. 19, 2011
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.
“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.
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