Using
GIS to Characterize the Influence of the
Urban Heat
Island Effect on
Precipitation
Patterns in Albuquerque, NM
All cities of
the world share a characteristic difference from surrounding areas; air
temperatures within urban areas are measureably warmer. Visit a city on a hot summer day and the
heat emanating from the man-made surfaces is obvious. During the night, however, it is just as apparent that
surrounding rural areas cool off at a greater rate than the urban areas. Both cases result from a phenomenon called
the urban heat island effect, which refers to the difference between urban and
rural temperatures in a given area.
The causes of urban heat
islands (UHIs) are well established and include vegetation removal from
urban areas, impermeable man-made surfaces, large buildings, air pollution,
and anthropogenic activity (Akbari et
al., 1992).
The primary impacts of urban heat islands
are economic in nature and result from increased energy consumption and
degraded air quality. However, UHIs may
also impact climate, especially precipitation, to varying extents and at
different scales. Precipitation
modification is discussed in detail in the next section.
Use of the term 'heat island' becomes
clear when temperature contours within a city are compared with those of the
surrounding environment. Figure 1 shows the
'isolated' winter heat island of London, England, and Figure 2 illustrates
the temperature profile that is characteristic of UHIs. Typically, the temperature gradient rises
steeply at rural/urban boundaries, as great as 4°C/km (Oke, 1987), and
progressively eases off to form a 'plateau' of warmer air over urban
areas. Variations in land use and
terrain structure result in variations along the temperature curve where dips
correspond to less developed areas and peaks correspond to those that are more
highly built up. Although the peak
increase in urban temperature is usually in the range of 3-5°C (Akbari et al.,
1989), it is not uncommon to find heat islands in the range of 6-10°C in
tropical cities. (Akbari et al., 1992).
Urban heat islands also seem to affect formation and
distribution of precipitation. Strong
UHIs create uplift of water vapor and nuclei that are necessary for the
formation of convective thunderstorms.
However, water vapor and nuclei must be carried aloft to cloud level
before condensation occurs and this lag time between the upwelling of warm air
and subsequent condensation results in increased precipitation downwind of urban heat islands (Figures 3, 4). Studies suggest that this effect is
especially pronounced during high-intensity, convective thunderstorms (Huff,
Changnon, 1972). In the southwest,
where heat islands are large and high-intensity convective thunderstorms are
typical, it is not unreasonable to expect a large precipitation pattern response
to increased urban heating.
The high degree of precipitation spatial
variation in Albuquerque, NM became apparent while collecting and analyzing
data for the U.S. Geological Survey.
Two rain gages, in particular, located in far northern Albuquerque
consistently showed much greater precipitation than other gages.
As a result, the primary objective of
this study was to determine if this variation in rainfall was consistent in
character to the expected influence of the Albuquerque UHI. In particular, this study hoped to answer,
on a very preliminary basis, the question of whether or not Albuquerque shows
increased precipitation downwind of the urban area.
Due to time and resource constraints,
this project did not attempt to give quantitative answers to the project
objective, but instead hoped to simply find a precipitation pattern consistent
with typical urban heat islands.
The approach taken was simple:
The U.S. Geological collected rain data
at 29 locations throughout Albuquerque in the 2000 water year. These locations are shown in Figure 5.
This data was analyzed for the months of September-February
and March-August because these two periods correspond nicely with seasonal
differences in temperature and precipitation.
The period from September-February is typically the cool, dry season in
Albuquerque while March-August is typically warm and wet. The motivation for this comparison was to
further isolate UHI influences on convective thunderstorms that typically occur
during the months of March through August.
Precipitation contours were created with
ArcView GIS software. Although ArcView
provides multiple routines for creating contours, a
spline method seemed to yield the most ‘believable’ precipitation distribution based
on the available data.
Figure 6 shows the isohyets
for the period from September-February, and there is clearly an increasing
trend in the northern portion of Albuquerque.
Winds during this period are generally southerly (WRCC, 2001), and when
combined with the precipitation pattern could indicate an urban heat island
influence. However, due to weak urban
heat islands and minimal convective thunderstorm activity during these months,
it is unlikely that Albuquerque’s UHI is the primary cause for the precipitation
patterns.
Unexpectedly, elevation did not seem to show much of an
influence on precipitation. Elevation
in Albuquerque increases on either side of the Rio Grande, shown in Figure 5, but
increasing isohyets for this period do not always correspond with increasing
elevation.
March-August
Figure 7 shows the
isohyets for the period from September-February. Again, there is clearly an increasing trend in the northern
portion of Albuquerque. These months
typically create large urban heat islands and considerable convective thunderstorm
activity, and the precipitation pattern could indicate an UHI influence. However, because winds during this period
are generally northerly (WRCC, 2001), it is again unlikely that
Albuquerque’s UHI is the primary cause for the precipitation patterns.
Like the period from September-February, there was no strict
correspondence between increasing elevation and increasing precipitation.
It was determined that precipitation in
Albuquerque shows significant spatial variation on both seasonal and annual
scales. Average annual precipitation is
shown in Figure 8. However, precipitation patterns for
Albuquerque, NM during the 2000 water year did not indicate characteristics
typical of urban heat islands. In fact,
during the period of March-August precipitation patterns contradicted an
UHI influence.
The results of this paper indicate
neither the presence nor lack of Albuquerque’s urban heat island. Instead, it emphasizes the complexity of
urban climatology and the need to study urban storms individually and with more
detail. Without additional information,
such as temperature and wind data on a storm-by-storm basis, conclusions
regarding urban heat island influences on precipitation cannot be reached.
References
H. Akbari, J. Huang, P. Martien, L. Rainer, A. Rosenfeld,
and H. Taha. 1989. Saving Energy and Reducing
Atmospheric
Pollution by Controlling Summer Heat Islands. Proceedings of the Workshop on Saving Energy and
Reducing Atmospheric Pollution by Controlling
Summer Heat Islands. pp 31-37.
H. Akbari, S. Davis, S. Dosano, J. Huang, and S. Winnett
(eds.). 1992. Cooling Our Communities: A Guidebook on
Tree Planting and
Light-Colored Surfacing. Lawrence Berkelely
National Laboratory Report No. LBL-31587.
Berkeley, CA.
R.G. Barry and R.J. Chorley. 1998. Atmosphere, weather, and
climate, 7th ed. London: Routledge.
K. Garbesi, H. Akbari, P. Martien (eds). 1989. Editor's
introduction to the urban heat island. Proceedings of the
Workshop on Saving Energy and Reducing
Atmospheric Pollution by Controlling Summer Heat Islands. pp 2-6.
F.A. Huff and S.A. Changnon, Jr. 1972. Climatological
assessment of urban effects on precipitation. National
Science Foundation, Atmospheric Sciences
Section. NSF GA-18781.
E.P. Odum. 1997. Ecology: a bridge between science and
society. Sinauer Assoc.
T.R. Oke. 1987. Boundary Layer Climates, 2nd ed. London:
Routledge.
WRCC (Western Regional Climate Center). Local Climate Data
Summaries. Online. Internet. 23
April 2001.
Available: http://www.wrcc.sage.dri.edu/summary/lcd.html
For more information than you ever wanted to know about
Urban Heat Islands visit: eande.lbl.gov/HeatIsland