COASTAL CITIES TURN UP THE HEAT ON RAINFALL
The old song, asking rain to "go away" and "come again another day,"
may get even older for people who live in large coastal cities,
according to new NASA-funded research.
According to the study, urban heat islands, created from pavement and
buildings in big coastal cities like Houston, cause warm air to rise
and interact with sea breezes to create heavier and more frequent
rainfall in and downwind of the cities. Analysis of Houston-area
rain-gauge data, both prior to and since urbanization, also suggests
there have been observed increases in rainfall as more heat islands
The Houston-area study used data from the world's only space-based
rain radar on NASA's Tropical Rainfall Measuring Mission (TRMM)
satellite, and dense clusters of rain gauges.
Authors, J. Marshall Shepherd of NASA's Goddard Space Flight Center,
Greenbelt, Md., and Steve Burian, a University of Arkansas,
Fayetteville, Ark. researcher, believe the impact large coastal cities
have on weather, and possibly climate, will become increasingly
important as more people move into urban areas, with even greater
concentrations in coastal zones. The paper is in the current American
Meteorological Society and American Geophysical Union's journal, Earth
A recent United Nations report estimates 60 percent of Earth's
population will live in cities by 2025. Previous related studies have
shown urban heat islands create heavier rainfall in and downwind of
cities like Atlanta, St. Louis and Chicago. However, this is one of
the first studies to provide evidence of such an effect around a U.S.
coastal city. It is also the first to incorporate specific
satellite-derived rainfall data for a coastal urban area.
Urban areas with high concentrations of buildings, roads and other
artificial surfaces retain heat, which leads to warmer surrounding
temperatures and creates heat islands. Rising warm air, promoted by
the increased heat, may help produce clouds that result in more
rainfall around cities. Buildings of different heights cause winds to
converge, driving them upward, helping form clouds. The study shows
the urban heat island/rain effect may be even more pronounced near
coasts. In coastal cities like Houston, sea breezes also create rising
air and clouds. The combination of urban converging winds and coastal
sea breezes may enhance thunderstorm development.
"Recent publications have shown evidence of increased lightning
activity over and downwind of Houston," Shepherd said. "Since
lightning and rainfall are so closely related, we decided to use
TRMM's Precipitation Radar, and a network of rain gauges, to see if
urban-induced abnormal rainfall existed," he said.
Using data from 1998 to 2002, the researchers found mean rainfall
rates, during the warm season, were 44 percent greater downwind of
Houston than upwind, even though the regions share the same climate.
They also found rainfall rates were 29 percent greater over the city
than upwind. Rainfall rates indicate how hard it rains and can be an
indicator of enhanced thunderstorm activity.
To rule out any effects from the coastline curvature near Houston on
thunderstorm development, the researchers divided the entire Texas
coast into seven zones extending 100 kilometers (62 miles) inland and
including four or five major inlets or bays. Analysis of rainfall data
in these zones showed abnormal rainfall only occurred over and
downwind of Houston, which suggested effects from the urban landscape
were significant. At the coastlines, TRMM satellite data were
important, because they allowed researchers to assess rainfall data in
areas where there were no gauges and records, like over the ocean.
A companion paper by the researchers, presented in March at a
Geological Society of America meeting in Kansas City, Mo., stated
urban areas also affect the timing of rainfall. Compared to upwind
areas, there were nearly two times as many occurrences of rainfall
from noon to midnight in the urban area. This finding has significant
implications for flood control in Houston, Burian said.
NASA's Earth Science Enterprise, which supported this study, 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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