Earth is increasingly an “urbanized ” planet. The “World Population Clock ” registered a Population of 7,175,309,538 at 8:30 pm (LST) on Oct. 6, 2013. Current and future trends suggest that this population will increasingly reside in cities. Currently, 52 percent of the world population is urban, which means we are a majority “urbanized ” society. Figure 1 indicates this trend will continue, wit

A. Horst

C. Mitra

Chris Strother

J. Marshall Shepherd

L. Bounoua

T. Andersen

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Urban Climate Archipelagos: A New 
Framework for Urban Impacts on Climate
By Shepherd, et all , posted on November 29th, 2013 in Articles, Climate 
J. Marshall Shepherd, T. Andersen, Chris Strother, A. Horst 
University of Georgia, Athens, Georgia
L. Bounoua 
NASA Goddard Space Flight Center, Greenbelt, Maryland
C. Mitra 
Auburn University, Auburn Alabama
Page 1 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...Figure 1 – Trends in urban and rural population growth. Source: UNPD 2012.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 2 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...Introduction 
Earth is increasingly an “urbanized” planet.  The “World Population Clock” registered a Population of 
7,175,309,538 at  8:30 pm (LST) on Oct. 6, 2013. Current and future trends suggest that this 
population will increasingly reside in cities. Currently, 52 percent of the world population is urban, 
which means we are a majority “urbanized” society.  Figure 1 indicates this trend will continue, with 
particular growth in less developed regions.
In terms of ecological impacts, urbanization is one of the more significant and long-lasting forms of 
land transformation, and its extent of increase is at least proportional to population growth and 
economic development.  Urbanization exerts environmental and climatic pressures on surrounding 
lands that are not fully understood.  Viewed from the perspective of the amount of space it currently 
occupies, urbanization appears to be a minor form of land transformation.  It is estimated that 
urbanization in the U.S. occupies about 3 percent of the land surface (Imhoff et al. 2004).
In terms of climate impacts, urbanization affects its environment through different physical 
mechanisms:  1) the reduction of the fraction of vegetation and the subsequent reduction in 
photosynthesis and plant’s water transpiration and interception, 2) the alteration of water infiltration 
and surface runoff and their impacts on soil moisture and the water table, 3) the alteration of surface 
albedo and its effect on the surface energy partitioning, and 4) the modification of the surface 
roughness and its implication for the turbulent exchanges of water, energy, and momentum fluxes, all 
of which affect climate.
The urban heat island (UHI) is a fairly well-understood urban-climate interaction (Grimmond et al. 
2010; Stone et al. 2010; Zhou and Shepherd 2010).  Urban areas are known to create the so-called 
UHI effect. On the other hand, using satellite observations over 38 of the most populous cities in the 
continental U.S., Imhoff et al. (2010) have shown that on a yearly average, urban areas are 
substantially warmer than the non-urban fringe by 2.9 degrees Celsius (C), except for urban areas 
within arid climates which behave as an urban heat sink.  They also show that the UHI amplitude is 
remarkably asymmetric with an average 4.3 degrees C in summer and only 1.3 degrees C in winter.  
Seto and Shepherd (2009), Mitra et al. (2011) and Shepherd (2013) described how the urban 
environment affects other major climate systems (e.g., the water cycle, biogeochemical cycles, and 
weather).
Figure 2 – “Ring of Asphalt” around 
the Southern Appalachia, circa 2006. 
Page 3 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...Red represents areas of urban 
impervious land cover as derived 
from the National Land Cover 
Dataset (NLCD). Image Credit: 
Generated by co-Author Strother 
using NLCD.
Motivation
Li and Bou-Zeid (2013) found, in a study of heat islands and heat waves, that the impact of cities is 
larger than the sum of its parts. Similarly, our recent work on urban rainfall effects in southern 
Appalachia and mid-Atlantic region have revealed a similar finding.  For example, our work with the 
National Science Foundation-funded Coweeta Long Term Ecological Research (LTER) site has 
revealed an emerging “Ring of Asphalt” around the Southern Appalachians (Figure 2).  Using the 
National Land Cover Database (NLCD), we found that this region has experienced a 15.5 percent 
increase in urban land cover from 1992-2006.  Within this “ring,” chains or systems of urban areas are 
apparent, particularly from Atlanta to Charlotte along the Interstate 85 corridor.
The primary objective of this paper is to introduce a new framework and terminology for thinking 
about and studying urbanization and its effects on the climate system. The secondary objective is to 
provide a set of “starter” research questions under the framework of “urban climate archipelagos” or 
UCAs. The literature is rich with papers on UHI or individual impacts of cities. However, the vantage 
point of space and with models, we can now see that “chains of cities” provide a different type and 
scale of forcing on the climate system.
What is an Urban Climate Archipelago (UCA)?
Figure 3 – Global urban footprints. 
Image Credit: Matt Harzell.
Page 4 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...Clearly, urban land cover can act as a forcing function on the climate system.  Most studies have 
focused on individual UHIs or their impacts on rainfall, health, and other indirect processes.  Figure 3 
is a compelling geographic representation of global urban footprints generated by Geographer Matt 
Harzell. Clearly, the shapes and sizes vary, but what happens when systems or chains of urban 
footprints begin to aggregate or align? Increasingly aggregated urban regions are becoming apparent 
on the landscape.  The concept of aggregate urban landscapes is not new.  French geographer Jean 
Guttman’s book “Megalopolis” described the urbanized region from Boston to Washington, D.C. 
Derived from the Greeks, Megalopolis means “very large city.”  In contemporary times, the Oxford 
Dictionary of Geography describes a Megalopolis as “any many-centered, multi-city, urban area of 
more than 10 million inhabitants, generally dominated by low-density settlement and complex 
networks of economic specialization.”   According to the Indian Census, an “urban agglomeration” is 
a continuous urban spread constituting a town and its adjoining outgrowths or two or more physically 
contiguous towns together with or without outgrowths of such towns. A total of 160.7 million persons 
(or 42.6 percent of the urban population) live in Million Plus UAs/Cities, according to the 
Government of India.
Figure 4 is a satellite image from the Suomi NPP satellite of “Lights at Night” in the U.S.  At many 
geographic locations, a “chain” of urban landscapes is apparent.  Guttman’s Megalopolis is also 
apparent.  An archipelago is defined as “a chain of islands.” It stands to reason that a chain or 
collection of urban areas would produce an aggregate impact on temperature, moisture, or 
precipitation.  The overarching idea, which emerged from our contribution to a NASA-funded 
Interdisciplinary Science project on urban climate, is an extension of the concept of an “Urban Heat 
Island.
Figure 4 – Lights at night in the United 
States. Data is from the day/night band 
of the Visible Infrared Imager 
Radiometer Suite (VIIRS) on the Suomi 
National Polar-orbiting Partnership 
(NPP) satellite. Image Credit: NASA.
Urban Climate Archipelagos (UCAs) are hypothesized to have a scale magnification effect on climate 
processes. A good analogy is the effect of one mountain on cloud-precipitation processes versus a 
chain or range of mountains, which extends the “scale of impact” on cloud or precipitation processes. 
Rosenzweig et al. (2005) mentioned UHI archipelagos in a discussion of heat islands in New Jersey.  
However, they were referring to individual “hot spots” within the overall heat island of a single 
metropolitan area, such as Philadelphia.  Ohashi and Kida (2002) explored large cities in proximity to 
each other and how they worked to produce an atmospheric “chain” flow. They found that the urban-
sea breeze circulation of a Japanese coastal city affects air pollution in a nearby inland city. In Figure 
Page 5 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...5, a cross section from weather model run depicts how the wind flow (represented by the arrows) 
interacts when two cities (Case 3) are present in the simulation.
 
 
 
 
Figure 5 – Chain flow. Source: Ohashi and Kida, 2012, American Meteorological 
Society.
 
Figure 6 – U.S. carbon dioxide emissions in 
2002 from various industrial, commercial, 
and residential sources for 2002. Units are 
Page 6 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...log base 10 of metric tonnes of 
carbon/gridcell/year. Image Credit: Vulcan 
Project, Purdue University.
Herein, we define Urban Climate Archipelagos (UCAs) as a chain of distinct urban entities with 
discernible aggregate impacts on at least one segment of the climate system. The idea builds upon the 
concepts of Megalopolis or “Urban Aggregations.“  However, it is distinctly unique because it 
incorporates a clear linkage to climate processes.  The definition is more expansive than Rosenzweig 
et al. (2005)’s mention of archipelago, which focused on fractured elements within one larger UHI.  
While archipelagos have distinct independent members, UCAs are likely to have independent and 
synergistic affects on the climate. For this reason, we used the term “climate” rather than “heat” (i.e., 
UHA’s).
UCA’s are apparent in carbon dioxide assessments produced by the Vulcan Project. Figure 6 
represents U.S. carbon emissions in  2002. Several UCAs are apparent and their combined 
contributions are likely not trivial.  The implications of carbon emissions from UCAs affect carbon 
dome-health relationships, regional climate-emission action plans, and biogeochemical cycling.
We conducted a preliminary analysis of trends in temperature in the eastern U.S. (1951-2006) using 
the Nature Conservancy’s Climate Wizard Tool. The data is from the gridded PRISM dataset. Figure 
7 reveals UCA signatures in the temperature trends and growth of the UHI chain in the region of the 
50-year period of time.  Ongoing work by our group, led by NASA postdoctoral associate Dr. Theresa 
Anderson, explores whether the “Mid-Atlantic” UCA has an impact on cloud and precipitation 
processes. Preliminary results, using the Weather Research and Forecast (WRF) model, suggest the 
UCA from “Richmond to Philadelphia” can have an impact on convective precipitation systems.  In 
Figure 8, the precipitation totals are clearly enhanced along the archipelago, particularly in the 
Washington, D.C., and Baltimore area. Li et al. (2013) have found similar results for the region. 
 Shepherd (2013) provides an overview of the physical mechanisms related to this “urban rainfall 
effect,” and it stands to reason that a “chain of urban land cover” would exert broader influence on the 
hydroclimate system.
Figure 7 – Trends in temperature (1951-2006) for the 
eastern U.S. Units are degrees Fahrenheit/year. Image 
Credits: PRISM and Climate Wizard as generated by 
Author Shepherd.
Page 7 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...UCA issues are apparent in rapidly urbanizing global nations as well.  China and India, with their ever
-growing urban populace, also are seeing an increase of these urban climate archipelagos. The India 
Census Report 2011 has noted an increase of urban agglomerations from 384 to 475 from 2001 to 
2011.  These numbers clearly highlight the need to access the climatic impacts of the UCAs on local, 
regional and global scale.
Future UCA Research Questions: A Starter List
The concept of Urban Climate Archipelagos is emerging and represents fertile ground for research 
going forward. In closing, we offer a set of potential scholarly research opportunities centered on the 
UCA concept:
What are the spatio-temporal definitions of UCAs? •
Are there different classes or sub-divisions of UCAs? •
What is the global or regional distribution of UCAs and are there discernible space-time trends? •
What are the thermal impacts of UCAs on weather, climate, and related applications? •
Do UCAs increase the likelihood of an Urban Rainfall Effect reported by Shepherd et al (2010) 
and others?
•
How are aspects of the biogeochemical cycle affected by UCAs? •
What is the UCA’s scale of influence on the broader global climate system, and what are the 
implications for representing them in climate models?
•
Are there “chain flows” or other interactions that emerge because of UCAs, and do they have 
implications for pollution, land surface hydrology, or weather processes?
•
What are the policy and stakeholder implications of UCAs? •
What observational and modeling frameworks will be required to study UCAs? •
 
Figure 8 – Mesoscale model results illustrating how the convective precipitation field (in mm) 
varies with and without urban land cover in the WRF model simulation. Image Credit: Co-
Author Andersen.
Page 8 of 10 Urban Climate Archipelagos: A New Framework for Urban Impacts on Climate | Earthzine
5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur... 
Acknowledgements: We would like to thank Dr. Thomas Mote for his insightful comments on the 
paper. We would also like to acknowledge support from the NASA IDS Program (NNX12AO24G), 
NASA Precipitation Measurements Mission Program (NNX10AH65G) and the NSF Coweeta LTER 
(DEB-0823293).
References
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5/21/2014 http://www.earthzine.org/2013/11/29/urban-climate-archipelagos-a-new-framework-for-ur...Stone, B., J. Hess, and H. Frumkin (2010) Urban Form and Extreme Heat Events: Are Sprawling 
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By Shepherd, et all, posted on November 29th, 2013 in Articles, Climate

http://www.srs.fs.usda.gov/pubs/ja/2013/ja_2013_shepherd_002.pdf

By Shepherd, et all, posted on November 29th, 2013 in Articles, Climate

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