New evidence from satellites, models, and ground
observations reveal urban areas, with all their asphalt,
buildings, and aerosols, are impacting local and possibly
global climate processes. This is according to some of the
world’s top scientists convening in a special session at the
Fall Meeting of the American Geophysical Union in San

To study urban impact on local rainfall, Dr. J. Marshall
Shepherd of NASA’s Goddard Space Flight Center, Greenbelt,
Md., and Steve Burian of the University of Utah, Salt Lake
City, used the world’s first space-based rain radar, aboard
the Tropical Rainfall Measuring Mission (TRMM) satellite, and
dense rain gauge networks on land to determine there are
higher rainfall rates during the summer months downwind of
large cities like Houston and Atlanta. Burian and Shepherd
offer new evidence rainfall patterns and daily precipitation
trends have changed in regions downwind of Houston from a
period of pre-urban growth, 1940 to 1958, to a post-urban
growth period, 1984 to 1999.

Cities tend to be one to 10 degrees Fahrenheit (0.56 to 5.6
degrees Celsius) warmer than surrounding suburbs and rural
areas. Warming from urban heat islands, the varied heights of
urban structures that alter winds, and interactions with sea
breezes are believed to be the primary causes for the findings
in a coastal city like Houston.

In related work, Dr. Daniel Rosenfeld, an atmospheric
scientist at Hebrew University, Jerusalem, reveals the
increased amount of aerosols, tiny air particles, added by
human activity to those naturally occurring also alter local
rainfall rates around cities. Rosenfeld suggests the particles
provide many surfaces upon which water can collect, 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 ‘delayed’ rainfall
resulting from the presence of aerosols, creating bigger
clouds and heavier rain.

To help scientists like Shepherd and Rosenfeld improve
understanding of links between city landscapes and climate
processes like rainfall, NASA’s suite of Earth observing
satellites provides information about the land cover/land use
properties that initiate the urban effects. The satellites
also track the aerosols, clouds, water vapor, and temperature
that describe atmospheric conditions in urban environments.
Their measurements allow scientists to make end-to-end studies
of urban impacts on the climate system practically anywhere on

“The space-borne instruments on Terra, Aqua, TRMM, and Landsat
provide a wealth of new observations of aerosol particles near
and downwind of cities, the cloud optical properties, and
surface reflectance characteristics that can help us
understand the effects that urban environments have on our
atmosphere and precipitation patterns,” said Dr. Michael King,
NASA Earth Observing System Senior Project Scientist. “Aura,
to be launched in 2004, will add even more data,” he said.

With growing evidence of the effects of urbanization on
climate, climate modelers, like Georgia Institute of
Technology’s Dr. Robert Dickinson, hope to account for the
cumulative effects of urban areas on regional and global
climate models. For example, since asphalt has such a large
effect on local heat transfer, water run-off, and how winds
behave, characterizing asphalt cover is probably the biggest
urban effect to be factored into global models.

NASA’s Earth Science Enterprise is dedicated to understanding
the Earth as an integrated system and applying Earth System
Science to improve prediction of climate, weather and natural
hazards using the unique vantage point of space.

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Shepherd, King, Rosenfeld, and Dickinson will present their
findings during a press conference on Thursday, December 11,
2003, at 6 p.m., EST in Room 2012, Moscone West, at the 2003
Fall Meeting of the American Geophysical Union in San

They also will convene a special session, organized by
Shepherd and Dr. Menglin Jin of the University of Maryland,
detailing these results on Human-Induced Climate Variations
Linked to Urbanization: From Observations to Modeling,
sessions U51A and U51C, starting on Friday morning, December
12, at 11:00 a.m. EST at MCC 3001-3003. B-roll of video is
available on this topic, by calling Wade Sisler of NASA-TV at