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Are Cities Changing Local And Global Climates?

From Mayans to Romans to people of today, some of the greatest achievements of Homo sapiens have been their ability to build fantastic cities. But newer, equally spectacular innovations, like the satellites and computer models created at NASA, are starting to show that humankind's ambitions regarding urban landscapes may be altering local and possibly global climates. That's because asphalt and cement absorb heat, buildings alter winds, and tiny airborne particles from pollution called aerosols change the way clouds form, all of which exerts unique forces on local climates.

More Summer Rain Near Cities?

While cities might be cool and interesting places in a cultural sense, scientists are now learning that these Mecca's tend to be one to 10 degrees Fahrenheit [0.56 to 5.6' Celsius] warmer than surrounding suburbs and rural areas. Hard and dark city surfaces absorb heat and create urban heat islands. That heating creates rising warm air, which combined with the varied heights of buildings and other urban structures that alter winds, appear to alter rainfall in and around cities.

Animation of city in the summer Left: Animation of How Pollution Increases Summer Precipitation: In summer, weaker winds move the clouds more slowly. Heat absorbed by the city and pollution's interference with raindrop formation interact to cause the clouds to intensify before producing precipitation. The onset of rainfall from a cloud leads eventually to its demise by cooling off the air near the ground. The air pollution delays the onset of precipitation, so that the intense storm clouds can build higher and larger before they start precipitating and subsequently dissipating. Therefore, these larger and more intense thunderstorm clouds produce eventually heavier rainfall on the city and the downwind areas. First is the unpolluted, then the polluted case. Credit: NASA

coastal city Right: Animation of Urban Rainfall Effect in Coastal Cities: During the warmer months, the added heat creates wind circulations and rising air that produces new clouds or enhances existing ones. Under the right conditions, these clouds evolve into rain-producers or storms. Scientists suspect that converging air due to city surfaces of varying heights, like buildings, also promotes rising air needed to produce clouds and rainfall. Credit: NASACredit: NASA

Animation of city in the winter Left: Animation of How Pollution Reduces Winter Precipitation: In winter, moist air flows off the ocean and rises over the hills downwind of a coastal city, dropping its rain and snow mainly as it ascends the hills. As pollution from the city is pushed into the clouds by the hills downwind of the city, it interferes with droplet formation in the clouds and makes them smaller, as observed by NASA's satellites. The smaller cloud droplets convert more slowly into precipitation. Instead of precipitating, much of the water in the clouds evaporates, reducing the net rainfall downwind of the urban area by up to 15% to 25% on a seasonal basis. First is the unpolluted, then the polluted case. Credit: NASA

Dr. J. Marshall Shepherd of NASA Goddard Space Flight Center and Steve Burian of the University of Utah used satellites and ground measurements to show higher rainfall rates during the summer months downwind of large cities like Houston and Atlanta. Burian and Shepherd also offered new evidence that rainfall patterns and daily precipitation trends have changed in regions downwind of Houston from a period of pre-urban growth in 1940-1958 to a post-urban growth period in 1984-1999. To obtain these findings the researchers used the world's first space-based rain radar aboard the Tropical Rainfall Measuring Mission (TRMM) satellite and dense rain gauge networks on the ground. The effects of urban heat islands, wind-altering buildings, and interactions with sea breezes are believed to be primary causes for the findings in a coastal city like Houston.

In related work, Dr. Daniel Rosenfeld, an atmospheric scientist at Hebrew University, Jerusalem, Israel, reveals that tiny air particles from cars and industry called aerosols also change local rainfall rates around cities. Rosenfeld suggests that the particles provide many surfaces for water to collect on, preventing droplets from condensing into larger drops, and slowing conversion of cloud water into precipitation. In summer, rain and thunder increases downwind of big cities, as rising air from urban heat islands combines with dispersed water in aerosol laced clouds, creating bigger clouds and heavier rain. In contrast, Rosenfeld finds that in clouds that form in winter over hills downwind of coastal cities as in Israel and California, the pollution-induced slowing of precipitation results in loss of rainfall.

NASA Keeps Watch

While scientists like Shepherd and Rosenfeld work to increase knowledge of links between city landscapes and precipitation, NASA's Earth Observing System (EOS) showcases another astounding human innovation in the form of state-of-the-art satellite technology. NASA's EOS has been designed, among other things, to gather information that allows people to detect changes in and connections between urbanization and global climate. For instance, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard both the Terra and Aqua satellites records measurements of clouds, aerosols, water vapor, and land and ocean surface properties. In addition, the Aura satellite, to be launched in 2004, will track sources of lower atmospheric gases that reduce air quality, and lower and upper atmospheric ozone.

satellite view of Houston

Satellite Image of Houston Metro Area: These images show the Houston metropolitan area, where buildings, roads and other built surfaces create urban heat islands that can affect local rain patterns. The images were taken by ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), an imaging instrument that is flying on Terra, a satellite launched in December 1999 as part of NASA's Earth Observing System (EOS). Click image to see larger picture. Credit: NASA/JPL

The Future

With growing evidence of the effects of city landscapes on climate, climate modelers, like Georgia Institute of Technology's Dr. Robert Dickinson, hope to account for the combined effects of urban areas in regional and global climate models. It is important that climate models that project future climate changes accurately account for urban processes. For example, while the web of roads across any city provides a smooth surface for us to get from one place to another, asphalt has large effects on local heat transfer, water run-off, and how winds behave. Being able to characterize these processes that result from asphalt cover is probably the biggest urban effect to be added into global models.

The mission of NASA's Earth Science Enterprise (ESE), which includes the EOS satellites, is to develop a scientific understanding of the Earth system and its response to natural or human-induced changes to enable improved prediction capability for climate, weather, and natural hazards.

Scientists presented their findings on these topics at a special session at the 2003 Fall Meeting of the American Geophysical Union.

For more information and high resolution images on the Internet, visit: NASA Goddard Space Flight Center

Krishna Ramanujan
NASA Goddard Space Flight Center