Background on Urban Growth
and Urban Heat Islands
By year 2025, 80% of the world’s population will live in cities according to a 1999 United Nation’s report. The blue line indicates the trend for the growth of cities.
As cities continue to grow, urban sprawl creates unique challenges related to land use planning, transportation, agriculture, housing, pollution, and development.
Urban expansion also has measurable impact on environmental process.
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Courtesy of UNPF
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Courtesy of Heat Island Group, LBNL
Urban areas modify boundary layer processes through the creation of "urban heat islands" or UHI. In cities, natural land surfaces are replaced by artificial surfaces that have very different thermal properties (e.g. heat capacity, specific heat, and thermal inertia). Such surfaces are typically more capable of storing solar energy and converting it to sensible heat. As sensible heat is transferred to the air, the temperature of the urban air tends to be 2-10 degrees higher than surrounding non-urban areas.
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In the past 30 years, several observational
and climatological studies have theorized that the UHI can have a
significant influence on mesoscale circulations and resulting convection.
Early investigations (Changnon 1968; Landsberg 1970; Huff and Changnon
1972a and 1972b; Huff and Changnon 1973) found evidence of warm seasonal
rainfall increases of 9 to 17% over and downwind of major urban cities.
The Metropolitan Meteorological Experiment (METROMEX) was an extensive
study that took place in the1970s in the United States (Changnon et al.
1977; Huff 1986) to further investigate modification of mesoscale and
convective rainfall by major cities. In general, results from METROMEX
have shown that urban effects lead to increased precipitation during the
summer months. Increased precipitation was typically observed within and
50-75 km downwind of the city reflecting increases of 5%-25% over
background values (Sanderson and Gorski 1978; Huff and Vogel 1978; Braham
and Dungey 1978; Changnon 1979; Changnon et al. 1981; Changnon et al.
1991). Using a numerical model, Hjemfelt (1982) simulated the urban heat
island of St. Louis and found positive vertical velocities downwind of the
city. He suggested that the enhanced surface roughness convergence effect
and the downwind shifting or enhancement of the UHI circulation by the
synoptic flow were the cause. METROMEX results also suggested that areal
extent and magnitude of urban and downwind precipitation anomalies were
related to size of the urban area (Changnon 1992).
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Space perspective of the Atlanta metropolitan area.
More recent studies have continued to validate and extend the findings from pre- and post-METROMEX investigations. Balling and Brazel (1987) observed more frequent late afternoon storms in Phoenix during recent years of explosive population growth. Analysis by Bornstein and LeRoy (1990) found that New York City effects both summer daytime thunderstorm formation and movement. Jauregui and Romales (1996) observed that the daytime heat island seemed to be correlated with intensification of rainshowers during the wet season (May-October) in Mexico City. Selover (1997) found similar results for moving summer convective storms over Phoenix, Arizona. Bornstein and Lin (2000) examined data from an Atlanta meso-network to show that the UHI induced a convergence zone that initiated storms during the summer of 1999. Thielen et al. (2000) used a meso-gamma scale model to address the extent of influence of urban surfaces on the development of convective precipitation. The results showed that sensible heat fluxes and enhanced roughness due to the urban heat island can have considerable influence on convective rainfall.
The literature indicates that the signature of the "urban heat island effect" may be resolvable in rainfall patterns over and downwind of metropolitan areas. However, a recent U.S. Weather Research Program panel concluded that more observational and modeling research is needed in this area (Dabberdt et al 2000).
[Research Background]