URBANIZATION INCREASES RAINFALL IN COASTAL AREAS

FAYETTEVILLE, Ark. — Phenomena that influence weather include the jet stream and El Niño, but Steve Burian wants to add Houston to that list. The University of Arkansas researcher found that large urban areas like Houston can influence their own weather and areas downwind.

Burian and J. Marshall Shepherd of NASA Goddard Space Flight Center conducted a unique study to quantify the impact of urbanization on rainfall. Their conjunctive analysis, which used space-based and land-based rainfall data, shows elevated rainfall amounts within and directly downwind of Houston. Burian will present their results today at the annual meeting of the North-Central Section of the Geological Society of America in Kansas City, Mo.

"There is increasing evidence that large coastal cities like Houston can influence weather through complex urban land-use-weather-climate feedbacks," explained Burian, assistant professor of civil engineering. "During urbanization, natural land covers are removed and replaced by artificial structures and surfaces like buildings, parking lots and sidewalks. Ornamental landscapes replace natural trees and vegetation and the soil structure is modified."

These changes impact the exchange of water and energy between the land and the atmosphere. As a result, temperature, wind and precipitation patterns change.

Understanding these phenomena becomes more important as populations become more urbanized. At the beginning of the 19^th century in the United States, less than 5 percent of the population lived in cities. By the beginning of the 20^th century, it had increased to 40 percent, and in 2001 more than 80 percent lived in urban areas. The United Nations Population Fund estimates that by 2025, 60 percent of the world’s population will live in cities.

Burian and Shepherd used data from the world’s first satellite-based precipitation radar aboard NASA’s Tropical Rainfall Measuring Mission and 13 years of rainfall data from a dense rain gauge network in Houston. Data were divided into four time periods — 6 a.m. to noon, noon to 6 p.m., 6 p.m. to midnight and midnight to 6 a.m. — and four seasons.

"We found that there are nearly two times as many occurrences of rainfall from noon to midnight in the urban area compared to the control area," said Burian. "There was an increase of rainfall during the noon to midnight time period of approximately 110 percent in the urban area and 52 percent in the downwind region, compared to the upwind region. This anomaly has significant implications for flood control in Houston."

Because Houston is a coastal city, the researchers wanted to rule out any effects from the coastline curvature. To do this, they divided the entire Texas coast into seven zones that extended 100 kilometers inland and included four or five major inlets or bays. Their analysis of rainfall data in these zones for 52 months shows that the rainfall anomaly only occurs over and downwind from Houston.

Burian suggests that the Urban Heat Island (UHI) effect may be one factor influencing the change in rainfall. First observed in 1833, UHI occurs when the temperature in an urban area is higher than its rural environment. UHI affects local and regional temperature distributions, wind patters and air quality

"The urban impact on rainfall is due to one or a combination of four factors: UHI, the increased roughness created by tall buildings, changes in atmospheric moisture and increased cloud condensation from automobiles and industry," said Burian.

In extending this research, Burian and Shepherd will use numerical modeling to determine how these factors interact to produce rainfall anomalies. They are also conducting a climate change study that uses long-term rain gauge data and population density data to examine the relationship between urban/industrial development and the rainfall anomalies over time."

"Precipitation is a key link in the global water cycle," Burian explained. "A proper understanding of its temporal and spatial character will have broad implications for ongoing climate diagnostics and prediction."