Printer Friendly Version
The heat is on ...
SwRI scientists draw on planetary and cometary knowledge to understand better why large cities are getting hotter
by Daniel Boice, Ph. D.
The cool serenity of the countryside can be an irresistible lure, especially after a week of congested traffic, irritable city dwellers, and unbearable heat. Rural areas, by their nature, have more vegetation and fewer cars and citizens, but how is it they seem cooler? Scientists at Southwest Research Institute are curious about this temperature difference, too, and have begun to develop a computer model to help them understand the special problems associated with urban environments such as air pollution, especially ozone concentration because of its sensitivity to air temperature; electric power consumption; airport operation; and overall quality of life issues. To bring the problem closer to home, the project team of Dr. Rosemary Killen, Dr. Walter Huebner, and Dr. Daniel Boice selected SwRI's headquarters city, San Antonio, as the prototype for their studies. Several attempts to describe the complex urban environment have been made over the past decades, but a consistent picture of the causes and importance of the many factors that influence urban air has not emerged.1,2
It has been known for more than a century that the environment of urban areas worldwide -- particularly that of large cities -- is warmer than the surrounding rural area. This is known as the Urban Heat Island (UHI) effect, which occurs as a result of anthropogenic, or human, alterations to the environment. Each city is unique, with its own blend of characteristics that influence the UHI, but some similarities have been found. Aside from the obvious difference that cities in general have less vegetative cover than rural areas, there are a number of human activities that contribute to the relative warmth of cities.3 Evaporative cooling is less in cities because buildings, streets, and sidewalks absorb the majority of solar energy input, and there is greater water runoff in cities because pavements are largely nonporous. Both processes can add to higher air temperatures. Waste heat from city buildings, power plants, industry, and vehicles also contributes to higher temperatures. Ironically, one remedy to beat the heat -- air-conditioning -- adds to the environmental problem with waste heat from the machinery that cools the air. Building and street materials -- tar, asphalt, brick -- generally have dark surfaces and thermal properties that cause them to absorb and hold heat during the day and release it during the night. And skyscrapers -- symbols of metropolitan prosperity -- create canyons that, depending on their geometry, trap solar energy and heat.
Within the past decade, the population of San Antonio and its surrounding communities surpassed one million, making it the ninth-largest metropolitan area in the United States, and its growth continues. An increase in population with accompanying urban development carries good news and bad news. Development can be a boon to the economy of a city, but unfortunate environmental effects may result because of a loss of vegetative cover; an increase in the number of buildings, paved areas, and cars; and the generation of more waste heat. Rising temperatures in cities also affect demand-side power management of electric utilities. On summer days in San Antonio, each degree above 70 degrees Farenheit requires an additional 60 megawatts of power from San Antonio's electric power utility. Most of the additional energy is used for air-conditioning, which produces waste heat that in turn contributes to the UHI.
The UHI effect itself may be an important contributor to the investigation of global warming. Most temperature measurements are made at weather stations located at airports, which are intense heat islands because of their long runways, terminals, large parking lots, and buildings. A question thus arises about the significance of these measurements influenced by the UHI in relation to global warming.
While studies have been conducted of the UHI effect and its role in other urban areas, no such study had been made for San Antonio. Urban environments differ, so one locale may exhibit a daytime heat excess while another may exhibit a nighttime excess, and the day/night temperature variation is decreasing. Scientists at SwRI who are experienced in planetary and cometary atmospheres undertook such a study using internal research funding to augment their existing knowledge of the Earth's atmosphere and to ascertain if the UHI effect could be modeled to help investigate important issues that affect the urban environment in general and San Antonio's environment in particular. By comparing similarities and differences between the detailed knowledge of the Earth's atmosphere with the variety of conditions that exist in the atmospheres of the planets and comets, scientists can learn more about both fields of study.
The study sought to address several questions. Does San Antonio have an increasing UHI effect and, if so, can the temperature increase that San Antonio has experienced relative to the surrounding rural area be quantified? Could the magnitude of the UHI effect caused by surface albedo (percent reflected sunlight; white materials have a high albedo, while black have low) of roads, parking lots, and buildings be better defined? What is the relationship between anthropogenic heat release and the UHI effect?
To answer the first question, researchers obtained and compared daily temperature records from the National Climatic Data Center in Asheville, North Carolina, for the years 1946 to 1990 for San Antonio and the small surrounding towns of New Braunfels, Boerne, and Poteet. These towns were selected because all are within 25 miles of San Antonio, surrounding the city on the northeast (New Braunfels), south (Poteet), and northwest (Boerne). Temperature differences between San Antonio and these surrounding towns were measured to cancel out the effects of any long-term climatic changes that have occurred in south central Texas over the past 45 years. The study showed minimum temperatures at the San Antonio International Airport (the location of the National Weather Station) are increasing at an average rate of about 0.5 degrees Farenheit per decade relative to the nearby towns.
The difference in minimum temperatures is most pronounced during the summer months. The average summer temperature minimum differences relative to New Braunfels indicated the largest increase, 0.7 ± 0.2 degrees Farenheit per decade. In 1947, the minimum temperatures in New Braunfels were on average about 0.4 degrees Farenheit warmer than San Antonio. In 1990, summer minimum temperatures were more than 3 degrees Farenheit cooler in New Braunfels. Similar trends in the minimum summer temperatures are seen in comparisons with Poteet and Boerne. The same effects are found in the maximum temperatures in the winter months, which are increasing an average 1.4 degrees Fahrenheit per decade compared to New Braunfels, and 0.9 degrees Fahrenheit per decade compared to Poteet. No statistical change in the winter maximum temperatures between San Antonio and Boerne was found. Despite many mitigating influences, such as much vegetation and little polluting industry, the temperature comparisons indicate that San Antonio has an increasing UHI effect.4 In other words, San Antonio is hot, and it's getting hotter!
Using a computer model based on physical and chemical principles, the thermodynamic effects of passive (such as building materials) and active (such as vehicle emissions) sources of heat in the urban environment were examined to determine their relative roles in producing an urban heat island. Specifically, the role of surface albedo, relative lightness or darkness of a surface, and the effect of anthropogenic heat release were examined within the context of the computer model.
After selecting San Antonio as a site for study of the UHI effect, the team modified an existing computer code to develop a model for solving problems related to the urban atmosphere. In addition to studying temperature increase, the goal is to have a general tool for investigating a variety of important issues facing modern cities: air quality, especially the effects of the UHI on ozone concentration; changes in the local meteorology (winds, humidity, clouds) due to a hot "bubble" of air over the city; relationships of the UHI to ground and surface water; and long-range city planning strategies to mitigate the negative effects of the UHI.
The Regional Atmospheric Modeling System (RAMS), a mesoscale meteorological computer program developed at Colorado State University and used by the meteorology community to study weather phenomena (recently for the 1996 Olympic Games in Atlanta), was selected for modification. Although mesoscale meteorological models have been used previously to simulate the transport and deposition of air pollution, none has coupled meteorology with air pollution chemistry in an urban setting. This is important because many chemical reactions are sensitive to air temperature; as an example, a change of a few degrees can affect reaction rates greatly and, in turn, the concentrations of pollutants. Further, the interaction of sunlight with the atmosphere is crudely accounted for in existing models. Many air pollutants, including aerosols, absorb and re-emit radiation in the infrared as heat. Thus, heat can be trapped by urban air, creating a positive feedback on the UHI and, in turn, on air pollutants. On the other hand, strong winds may dissipate pollutants and urban heat.
Several modifications to RAMS had to be made. First, the soil model was adapted to include urban "soil" and "vegetation" classes. The code was manipulated such that the characteristics of the urban parameters could be varied in the model. These parameters include heat capacity, thermal diffusivity, thermal conductivity, moisture capacity, hydraulic conductivity, soil porosity, surface albedo, emissivity for long-wave radiation (heat), and surface roughness. A capability was added to allow the inclusion of internal heat sources to simulate all anthropogenic activity (vehicles, air-conditioners, power plants, etc.) in the city. Lastly, a computer code developed with NASA funding to study chemical reactions in the atmospheres of comets was modified to include urban air pollution chemistry, and coupling to the adapted meteorological computer program has begun.
The NASA comet model describes the detailed gas-phase chemistry in an expanding cometary atmosphere and has been used successfully for more than a decade to interpret spacecraft data from encounters with comets P/Halley, P/Giacobini-Zinner, and P/Grigg-Skjellerup, and to understand recent observations of comets Hyakutake and Hale-Bopp. This comet research continues to assimilate information from ground-based observations and spacecraft measurements to develop a better overall understanding of the physical and chemical properties of comets.
Results and Conclusion
Using the modified code, named the Urban Environment Model (UEM), the project team reproduced the development of a heat island by modeling the complex interaction between winds, temperature and humidity, percent surface water, internal heat rate, and albedo and other surface properties. The preliminary model results were in general agreement with measured UHI temperatures. The team also demonstrated that the UEM could be used to study the effects of passive and active heat sources. Two conclusions were reached from these results. Dark-colored urban surfaces (low albedo) mainly increase the daytime surface temperature, as more sunlight is absorbed and retained and a modest convective cell (updraft of warm air with associated downdraft of cooler air) is formed in the air above the hot surface. Secondly, anthropogenic heat release has little effect on surface temperature, but is very effective in increasing wind speed and vertical convection updraft as heat is deposited directly into the atmosphere and the resulting hot air rises above the city.
Initial results are encouraging, and further development and extension of the UEM continues. The resulting three-dimensional, time-dependent model will be used to investigate important issues in modern cities, such as the effects of building and paving materials, park lands, and patterns of urban growth on heat island intensity, as well as their effects on the concentration of ground-level ozone and other urban air pollutants. The effects of the UHI on air pollution are a particularly important question for San Antonio, especially as they relate to ozone concentration. San Antonio currently is in compliance with Environmental Protection Agency ozone regulations; however, it is close to being classified as an ozone nonattainment city, which can lead to serious impacts on quality of life and the economy.
SwRI scientists are promoting the potential for further development of the UEM to local, state, and federal agencies interested in solving the many complex problems associated with modern urban areas.
Published in the Fall 1997 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.