Copyright © 2005, Paper 09-012; 9,559 words, 11 Figures, 2 Animations, 0 Tables.
http://EarthInteractions.org
A Review of Current Investigations
of Urban-Induced Rainfall and
Recommendations for the Future
J. Marshall Shepherd*
Earth-Sun Exploration Division, NASA GSFC, Greenbelt, Maryland
Received 9 December 2004; accepted 16 February 2005
ABSTRACT: Precipitation is a key link in the global water cycle and a
proxy for changing climate; therefore, proper assessment of the urban envi-
ronment’s impact on precipitation (land use, aerosols, thermal properties) will
be increasingly important in ongoing climate diagnostics and prediction, Glob-
al Water and Energy Cycle (GWEC) analysis and modeling, weather forecast-
ing, freshwater resource management, urban planning–design, and land–
atmosphere–ocean interface processes. These facts are particularly critical if
current projections for global urban growth are accurate.
The goal of this paper is to provide a concise review of recent (1990–present)
studies related to how the urban environment affects precipitation. In addition
to providing a synopsis of current work, recent findings are placed in context
with historical investigations such as Metropolitan Meteorological Experiment
(METROMEX) studies. Both observational and modeling studies of urban-
induced rainfall are discussed. Additionally, a discussion of the relative roles of
urban dynamic and microphysical (e.g., aerosol) processes is presented. The
paper closes with a set of recommendations for what observations and capa-
bilities are needed in the future to advance our understanding of the processes.
KEYWORDS: Urban; Rainfall; Anthropogenic
* Corresponding author address: Dr. J. Marshall Shepherd, NASA GSFC, Code 613.1,
Greenbelt, MD 20771.
E-mail address: marshall.shepherd@nasa.gov
Earth Interactions • Volume 9 (2005) • Paper No. 12 • Page 1
1. Introduction
Urbanization is one of the extreme cases of land-use change. Although currently
only 1.2% of the Earth’s land is considered urban, the spatial coverage and density
of cities are expected to rapidly increase in the near future. A recent paperby
Elvidge et al. (Elvidge et al. 2004) indicated that the density of the impervious
surface area (ISA) for the conterminous United States is 112 610 km
2
, roughly the
size of the state of Ohio and slightly larger than the area of herbaceous wetlands
of the conterminous United States. It is estimated that by the year 2025, 60%ofthe
world’s population will live in cities (UNFP 1999). Movie 1 is an illustration of the
rapid growth of Phoenix, Arizona, over the last few decades. Though urban areas
Movie 1. A quick-time movie illustrating the rapid growth of Phoenix, AZ, over the
last few decades using a series of Landsat images.
See the online version of this paper to view animation.
Earth Interactions • Volume 9 (2005) • Paper No. 12 • Page 2
are local in scale, human activity in urban environments has impacts at local to
global scale by changing atmospheric composition, impacting components of the
water cycle, and modifying the carbon cycle and ecosystems. However, our un-
derstanding of urbanization on the total Earth–climate system is incomplete. Better
understanding of how the Earth’s atmosphere–ocean–land–biosphere components
interact as a coupled system and the influence of the urban environment on this
climate system is critical (Figure 1).
As an example of recent concerns about the role of urban environments on the
Earth system, several issues or questions raised in the United States’ Climate Change
Science Program plan (Climate Change Science Program and Subcommittee on
Global Change Research 2003) echo the aforementioned statement about the urban
environment–climate system linkage. A few examples include the following.
(i) How are land use and land cover linked to climate and weather?
(ii) How do climate variability and change affect land use and land cover,
and what are the potential feedbacks of changes in land use and land
cover to climate?
(iii) How do the primary and secondary pollutants from the world’s mega-
cities and large-scale, nonurban emissions (e.g., agriculture, ecosystems,
etc.) contribute to global atmospheric composition?
Figure 1. Various scales linking urban environments to the environmental system
[modified after Oke (Oke 1987): urban canopy layer (UCL) and urban
boundary layer (UBL)].
Earth Interactions • Volume 9 (2005) • Paper No. 12 • Page 3
(iv) How are estimates of atmospheric composition and related processes to
be used in assessments of the vulnerability of ecosystems to urban
growth and long-range chemical transport?
(v) What research is required on the climatic effects of temperature on air
quality, particularly in urban heat islands and other regional settings, and
the potential health consequences?
Urban areas modify boundary layer processes in several ways. One of the
primary mechanisms is through the creation of an urban heat island (UHI). In
cities, natural land surfaces are replaced by artificial surfaces that have different
thermal properties (e.g., heat capacity and thermal inertia). Such surfaces are
typically more capable of storing solar energy and converting it to sensible heat.
Other contributing factors to the onset of the UHI may be attributed to differences
in surface albedo and anthropogenic heat release in the urban area. As sensible heat
is transferred to the air, the temperature of the air in urban areas tends to be
2°–10°C higher than surrounding nonurban areas. It is fairly well established that
the positive heat anomaly is most evident on a clear and windless night with peaks
in the late evening to early morning hours (Kim and Baik 2002). Arid urban areas
like Phoenix can also exhibit a relatively weak UHI or and sometimes an urban
heat sink because large amounts of energy are converted into latent heat rather than
sensible heat because of the prevalence of irrigated lands (Diem and Brown 2003).
The UHI intensity can exhibit diurnal and seasonal cycles and is modulated by
cloud and wind conditions. Anthropogenically generated heat also affects UHI
intensity. Although the magnitude (e.g., mean urban temperature – mean rural
temperature) of the UHI is typically proportional to the city size (Oke 1981) and
most apparent after sunset (Oke 1987), the UHI circulation is more clearly ob-
served during the daytime than nighttime because of the urban–rural pressure
gradient and vertical mixing during daytime hours (Shreffler 1978; Fujibe and
Asai 1980). This fact has implications for why urban-forced convection is not
simply a night–early morning phenomenon. To further understand the origins of
the UHI, it is instructive to examine a surface heat budget equation:
Q
SW
+ Q
LW
+ Q
SH
+ Q
LE
+ Q
G
+ Q
A
= 0. (1)
In Equation (1), the terms are Q
SW
(net shortwave irradiance), Q
LW
(net long-
wave irradiance), Q
SH
(surface sensible heat flux), Q
LE
(latent turbulent heat flux),
Q
A
(anthropogenic heat input), and Q
G
(ground heat conduction).
An equilibrium surface temperature is required for (1) to balance. At the surface,
if no heat storage is permitted, differential heating results from horizontal gradients
in one or more of the terms in (1). Spatial gradients in this equilibrium temperature
in conjunction with the overlying thermodynamic and moisture stratification will
dominate the upward or downward flux of heat for thermally forced systems,
which results in horizontal temperature gradients required to drive a mesoscale
circulation. In the case of the UHI, the difference in surface properties of urban and
rural areas leads to the differences in the thermal fluxes in (1). Figure 2 isa
qualitative description of the surface energy balance processes. Rural and urban
systems obtain energy from radiative processes in which energy is gained from the
sun and lost to the upper atmosphere and space. Shortwave radiation from the sun
is absorbed only during the daytime, but the longwave radiation emitted by the
Earth Interactions • Volume 9 (2005) • Paper No. 12 • Page 4
Earth system is lost all the time. For example, in the figure, the incident solar
radiation, QI, is 7.6 kW h m
2
day
−1
for both locations because the same sun shines
on both environments with equal intensity. With an albedo of 0.25 typical ofa
rural forest ecosystem, the reflected solar radiation, QR, is 1.9 kW h m
2
day
−1
in
the country and 0.4 kW h m
2
day
−1
in the city, which illustrates the urban envi-
ronment’s capacity to absorb more of the sun’s energy than the rural ecosystem.
Clearly, the differences in the reflected solar radiation, anthropogenic, latent heat,
and outgoing infrared terms lead to heat islands and resulting thermal circulations.
Figure 3 is an infrared image at 4
m identifying numerous urban heat island
signatures (darker areas) associated with major cities in the eastern United States.
Vukovich and Dunn (Vukovich and Dunn 1978) used a three-dimensional primi-
tive equation model to show that heat island intensity and boundary layer stability
have dominant roles in the development of heat island circulations. Additionally,
Huff and Vogel (Huff and Vogel 1978) found that the urban circulation is pri-
marily enhanced by the increased sensible heat fluxes and surface roughness of the
urban area.
There is renewed debate on how the urban environment might affect precipi-
tation variability. Possible mechanisms for urban environments to impact precipi-
tation or convection include one or a combination of the following: 1) enhanced
convergence due to increased surface roughness in the urban environment (e.g.,
Changnon et al. 1981; Bornstein and Lin 2000; Thielen et al. 2000), 2) destabi-
lization due to UHI-thermal perturbation of the boundary layer and resulting
downstream translation of the UHI circulation or UHI-generated convective clouds
(e.g., Shepherd et al. 2002; Shepherd and Burian 2003), 3) enhanced aerosols in
the urban environment for cloud condensation nuclei (CCN) sources (e.g., Diem
Figure 2. Typical rural and urban surface energy balance. The values are in units of
kWhm
2
day
−1
(courtesy of R. Sass, Rice University, online at http://
www.ruf.rice.edu/∼sass/UHI.html).
Earth Interactions • Volume 9 (2005) • Paper No. 12 • Page 5
![Figure 1. Various scales linking urban environments to the environmental system [modified after Oke (Oke 1987): urban canopy layer (UCL) and urban boundary layer (UBL)].](/figures/figure-1-various-scales-linking-urban-environments-to-the-pdun8f41.webp)
