Investigating Urban Atmospheres:
  From Downtown to the Whole Planet

Beijing at sunset. (Photo by Richard Anthes, UCAR.)

Spend a little time in the heart of a major city and you'll notice it often feels warmer there than in nearby suburban or rural areas. Scientists are taking renewed interest in this urban heat island effect as they refine methods for representing cities in complex computer models of weather and climate.

Cities have subtle influences on the atmosphere for a number of reasons:

• paved surfaces tend to radiate heat from the Sun
• tall buildings disrupt wind patterns
• heavy traffic sends pollutants into the air
• concrete canyons absorb sunlight

As more of the world's population moves into already crowded cities, researchers are beginning to investigate how densely populated areas affect day-to-day weather, as well as long-term climate. Their work can help policymakers anticipate changing rainfall and temperature patterns. The research is also useful as emergency managers and security officials prepare to respond in case toxins are accidentally or deliberately released into the atmosphere.

Here's a look at some of NCAR's research into the atmospheric impacts of urban areas.

Protecting New York and Other Cities

Since New York City is considered a top terrorist target, local and national security agencies are determined to map its weather patterns. If terrorists were to release radiation or another type of toxin into the air, emergency response officials would want to determine the direction of the plume so they could evacuate people in harm's way.

NCAR's Daran Rife is heading up an effort to produce fine-scale weather forecasts of the New York metropolitan area. He and colleagues are building on previous national security projects, including the use of NCAR computer models to forecast atmospheric conditions around Salt Lake City during the 2002 Winter Olympics. NCAR also is involved in a project using cutting-edge sensors and software to scan for potential airborne toxins near the Pentagon and predict their motion and impact on the building.

To get a clear picture of New York's landscape, the new team will take advantage of the vast amount of information streaming from NASA's fleet of Earth-observing satellites. The resulting Earth science data sets will reflect the latest housing boom in the New York metropolitan area which, starting in the 1990s, swallowed up farms and undeveloped land on the city's fringes.

NCAR scientists ran detailed simulations like this one during the 2002 Winter Olympics so officials could map out evacuation routes in the event that terrorists released a toxin into the atmosphere. This simulation started with a scenario specifying temperature, winds, and other key factors to produce a dispersal pattern for a generic gas over Salt Lake City. The highest gas concentration is in red. To create the simulations, NCAR researchers and colleagues used the Weather Research and Forecasting model (WRF) to drive a fine-scale model of fluid behavior called the Computational Fluid Dynamics model (CFD). Click here or on the image to enlarge. (Illustration courtesy William Coirier, CFD Research Corporation.)

The new data sets, along with NASA satellite weather observations, will be fed into state-of-the-art NCAR weather models. The team will fine-tune these high-resolution computer models to capture such atmospheric details as the movement of winds through river valleys and the prevailing flows of sea breezes, as well as finer-scale temperature and wind patterns in downtown New York.

For the fine-scale prediction that's the goal of this work, it's essential to capture an enormous amount of information about the area's topography and land use. As Rife warns, "if you don't get the details right, a plume might go in the exact opposite direction [of what] your model indicates."

Cities Are Shaping Our Future

Even if you live in a rural area, a city that's an hour or two down the highway may affect your weather.

For example, an analysis by NCAR scientist Fei Chen revealed that Houston's relative warmth altered the directions of winds at least 100 miles away. That's because, as urban air heats up, it rises and allows cool air from surrounding areas to move in, thereby affecting local air currents.

Chen has also worked with colleagues at NCAR and the Hong Kong University of Science and Technology to understand changes in Hong Kong's air quality. They found that the harbor city's relatively high nighttime temperatures were reducing the strength of nocturnal land breezes that used to send city air toward the South China Sea. As a result, pollution tends to hover over the city instead of dispersing as it once did.

Chen and colleagues are now looking at land use changes in the Dallas area. Their goal is to determine how new development along a city's edges can affect weather in the area.

Looking at Local Climate Change

Researchers use powerful tools like the Community Climate System Model (CCSM), a multiagency collaboration led by NCAR, to simulate changes in climate at the global level. Like other global climate models, the CCSM has "coarse resolution," meaning it starts with data from widely spaced points on an imaginary grid stretched over the globe to portray the behavior of the atmosphere as it interacts with the land surface, the ocean, and sea ice.

Now a team of scientists is starting to use this global model to zoom in on urban areas and determine how they may be affected by climate change, including global warming.

In a collaboration between NCAR and the University of Kansas, researchers are developing global data sets of urban characteristics that can be fed into the land component of the CCSM. The team is using observations from field campaigns in Vancouver, Mexico City, and other urban areas to ensure that the model's output reflects real conditions, a process called validation.

Getting fine-scale detail into the CCSM is particularly daunting. Perhaps even more than lakes, vegetation, and other rural features, cityscapes have subtle and hugely complex impacts on the local atmosphere. The size and density of an urban area, the darkness of its streets and sidewalks, and the prevalence of various construction materials can affect local climate. Dense concrete can store more heat than surfaces made of stone, which means for example that the historic core of Mexico City, with many stone structures, affects local temperatures differently than the glass and steel of Vancouver.

One of the team's ultimate goals is to help city planners prepare for an altered climate. Incorporating cities into the CCSM will yield projections that can guide policymakers contemplating the creation of more parks or rooftop gardens as a way of offsetting the higher temperatures that come with global warming.

Measuring Megacity Impacts

To find out more about what happens to the air streaming out of a city, NCAR and a large team of collaborators in March 2006 mounted a large-scale field campaign outside Mexico City. Their goal: to investigate the chemical and physical transformation of air pollution as it flows downwind from the city, assessing pollution's impact on regional and global air quality, climate, and ecosystems.

The results may be applicable not just to Mexico City, but also to other megacities (cities with 10 million or more inhabitants) around the world. Demographers project that there will be at least 26 megacities around the globe by 2015.

Researchers on the project, called MIRAGE (Megacity Impacts on Regional and Global Environments), used specially equipped aircraft, ground stations, and satellite observations to gather data on how Mexico City's air pollution ages as it disperses in the first hours and days after emission. They hope to shed light on four broad questions:

• How far downwind does Mexico City's pollution plume extend?
• How are the pollutants transformed by chemical reactions occurring downwind of the city?
• How do the pollutants affect visibility, as well as regional and global climate?
• How do the urban pollutants interact with pollutants from other sources, such as agricultural burning and wildfires?

MIRAGE is part of a larger set of coordinated field campaigns given the umbrella name of MILAGRO. By coordinating people, equipment, and data, MILAGRO participants were able to investigate the complex air chemistry flowing out of Mexico City on a variety of scales.

Smog in Mexico City. (© Julio Etchart)

More Air Quality Links Sharing our World's Air Our Research: Pollution and air chemistry NCAR Science Update: Air Quality Research For Learners Air Pollution's Effect On Us Air Pollution and Climate Read the Research—NCAR Annual Report 2005/06 MIRAGE Plans and Accomplishments New Approaches to Multiscale Modeling and Analysis