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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.
Le
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 Huebnel~ 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
Background
It has been known for more than a
century that the environment of urban
areas worldwide - particularly that of
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large cities - is warmer than the surrounding rural a~ea. 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 b~cause 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-conditioningadds 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, depend-
ing 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 contilmes. 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 car,s; and the generation of more waste heat. Rising
temperatures in cities also affect demandside power management of electric utilities. On summer days in San Antonio,
each degree above 70°F 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
Warming Trend of Summer Minimum Temperatures
about the significance of these
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measurements influenced by the
Differences in the averUHI in relation to global warming.
age daily temperature
While studies have been conminima (nighttime) of San
ducted of the UHI effect and its
Antonio relative to New
role in other urban areas, no such
Braunfels for the summer
months from 1946 to
study had been made for San
1990 (dark line). The
Antonio. Urban environments difwarming trend due to the
fer, so one locale may exhibit a
UHI in San Antonio is
daytime heat excess while another
shown by the red line,
may exhibit a nighttime excess,
which represents a leastand the day /night temperature
squares fit to the data.
variation is decreasing. Scientists
at SwRI who are experienced in
·1
planetary and cometary atmos1946 1950 1954 1958 1962 1966 19701974 1978 1982 1986 1990
pheres undertook such a study
Year
using internal research funding to
augment their existing knowledge
experienced relative to the surrounding
are found in the maximum temperatures
of the Earth's atmosphere and to ascertain
in the winter months, which are increasrural area be quantified? Could the magif the UHI effect could be modeled to help
ing an average 1.4°F per decade compared
nitude of the UHI effect caused by surface
investigate important issues that affect the
to New Braunfels, and 0.9°F per decade
albedo (percent reflected sunlight; white
urban environment in general and San
materials have a high albedo, while black
compared to Poteet. No statistical change
Antonio's environment in particular. By
have low) of roads, parking lots, and
in the winter maximum temperatures
comparing similarities and differences
between San Antonio and Boerne was
buildings be better defined? What is the
between the detailed knowledge of the
relationship between anthropogenic heat
found. Despite many mitigating influEarth's atmosphere with the variety of
ences, such as much vegetation and little
release and the UHI effect?
conditions that exist in the atmospheres of
polluting industry, the temperature comTo answer the first question,
the planets and comets, scientists can
parisons indicate that San Antonio has an
researchers obtained and compared daily
learn more about both fields of study.
temperature records from the National
increasing UHI effect. 4 In other words,
Climatic Data Center in Asheville, North
San Antonio is hot, and it's getting hotter!
Approach
Carolina, for the years 1946 to 1990 for
Using a computer model based on
The study sought to address several
San Antonio and the small surrounding
physical and chemical principles, the
questions. Does San Antonio have an
thermodynamic effects of passive (such
towns of New Braunfels, Boerne, and
increasing UHI effect and, if so, can the
as building materials) and active (such
Poteet. These towns were selected because
temperature increase that San Antonio has
as vehicle emissions) sources of heat in
all are within 25 miles of San Antonio,
the urban environment were examined
surrounding the city on the northeast
(New Braunfels), south (Poteet), and
to determine their relative roles in pronorthwest (Boerne). Temperature difducing an urban heat island. Specifically, the role of surface albedo, relative
ferences between San Antonio and
these surrounding towns were mealightness or darkness of a surface, and
sured to cancel out the effects of any
the effect of anthropogenic heat release
long-term climatic changes that have
were examined within the context of
occurred in south central Texas over
the computer model.
the past 45 years. The study showed
Model Development
minimum temperatures at the San
After selecting San Antonio as a site
Antonio International Airport (the
for study of the UHI effect, the team modlocation of the National Weather Station) are increasing at an average rate
ified an existing computer code to
of about O.5°F per decade relative to
develop a model for solving problems
related to the urban atmosphere. In addithe nearby towns.
The difference in minimum temperation to studying temperature increase, the
tures is most pronounced during the
goal is to have a general tool for investisummer months. The average summer
gating a variety of important issues facing
modern cities: air quality, especially the
temperature minimum differences relative to New Braunfels indicated the
effects of the UHI on ozone concentration;
largest increase, 0.7 ± 0.2°F per decade.
changes in the local meteorology (winds,
Dr. Daniel Boice is a senior research scientist
humidity, clouds) due to a hot "bubble" of
In 1947, the minimum temperatures in
in the Planetary Science Section, Instrumentaair over the city; relationships of the UHI
New Braunfels were on average about
tion and Space Research Division. His previto ground and surface water; and long0.4°F warmer than San Antonio. In 1990,
ous experience includes numerical simulations
range city planning strategies to mitigate
summer
minimum
temperatures
were
of the dynamics and chemistry in the atmosmore than 3°F cooler in New Braunfels.
the negative effects of the UHI.
pheres of comets, planets, and cool stars. CurSimilar trends in the minimum summer
The Regional Atmospheric Modeling Sysrently he is analyzing the many observations
tem (RAMS), a mesoscale meteorological comtemperatures are seen in comparisons
taken of comets Hyakutake and Hale-Bopp to
puter program developed at Colorado State
with Poteet and Boerne. The same effects
yield a better global understanding of comets.
Technology Today. Fall 1997
7
The temperature above an idealized "city" is
shown in a cross section in which the only difference between the "city" (between about 1540 miles) and its surroundings is albedo. The
albedo of the city surface is 0.05 (nearly black),
while that of the surroundings is 0.95 (very
bright). A heat island of 18°F develops at noon.
This temperature difference closely matches differences measured between a parking lot and
undeveloped adjacent land.
Urban Heat Island Temperature - Albedo Test
0 .48
0 .42
U;
.!!
0 .36
GI
IJ
0.30
!
..
c
III
1/1
is
0.24
"i
IJ
1:GI
>
0.18
0 . 12
0 .06
.0
48
36
24
24
o
12
west east
Horizontal Distance (Miles)
12
~------------------------III
University and used by the m eteorology
community to study weather phenomena
(recently for the 1996 Olympic Games in
Atlanta), was selected for m odification.
Although mesoscale m eteorological 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 reemit 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 m odel 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 dif£usivity, 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
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•
48
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 d etailed 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 /GriggSkjellerup, and to understand recent
observations of comets Hyakutake and
Hale-Bopp. This comet research continu es to assimilate information from
ground-based observations and spacecraft m easurements 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 agreem ent w ith
measured UHI temperatures. The team
also demonstrated that the UEM could be
u sed 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 d aytime surface temperature,
as more sunlight is absorbed and retained
Technology Today · Fall 1997
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 d evelopmen t of the
UEM to local, state, and federal agencies interested in solving the many complex problems associa ted with modern
urban areas .•:.
References
1. M. Garstang, PD. Tyson, and GD.
Emmitt, "The structure of heat islands," Rev.
Geophys. Space. Phys. 13,1975, pp. 139 -165.
2. T.R. Oke, "The Heat Island of the
Urban Boundary Layer: Characteristics,
Causes and Effects," in Wind Climate in
Cities, J.E. Cermak et al. (eds.), Kluwer Acad.
Publ., Dordrecht, 1995, pp. 81-107.
3. W.B. Myer, "Urban heat island and
urban health: early American perspective,"
Professional Geographer, Vol. 43, No. 1, 1991,
pp.38-48.
4. D.C. Boice, W.E Huebner, and N. Garcia, 'The urban heat island of San Antonio,
Texas (USA)," in Air Pollution IV, B. Caussade et
al. (eds.), Computational Mechanics Publications, Southhampton, 1996, pp. 649 - 656.