Urban Forestry & Urban Greening 14 (2015) 178–186

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Urban Forestry & Urban Greening
journal homepage: www.elsevier.com/locate/ufug

Urban forestry and cool roofs: Assessment of heat mitigation
strategies in Phoenix residential neighborhoods
Ariane Middel a,∗ , Nalini Chhetri a , Raymond Quay b
a
b

Julie Ann Wrigley Global Institute of Sustainability, Arizona State University, United States
Decision Center for a Desert City, Arizona State University, United States

a r t i c l e
Keywords:
Climate change
Cool roofs
ENVI-met
Heat mitigation
Trees and shade

i n f o

a b s t r a c t
The City of Phoenix (Arizona, USA) developed a Tree and Shade Master Plan and a Cool Roofs initiative
to ameliorate extreme heat during the summer months in their arid city. This study investigates the
impact of the City’s heat mitigation strategies on daytime microclimate for a pre-monsoon summer day
under current climate conditions and two climate change scenarios. We assessed the cooling effect of
trees and cool roofs in a Phoenix residential neighborhood using the microclimate model ENVI-met. First,
using xeric landscaping as a base, we created eight tree planting scenarios (from 0% canopy cover to 30%
canopy cover) for the neighborhood to characterize the relationship between canopy cover and daytime
cooling benefit of trees. In a second set of simulations, we ran ENVI-met for nine combined tree planting
and landscaping scenarios (mesic, oasis, and xeric) with regular roofs and cool roofs under current climate conditions and two climate change projections. For each of the 54 scenarios, we compared average
neighborhood mid-afternoon air temperatures and assessed the benefits of each heat mitigation measure
under current and projected climate conditions. Findings suggest that the relationship between percent
canopy cover and air temperature reduction is linear, with 0.14 ◦ C cooling per percent increase in tree
cover for the neighborhood under investigation. An increase in tree canopy cover from the current 10% to
a targeted 25% resulted in an average daytime cooling benefit of up to 2.0 ◦ C in residential neighborhoods
at the local scale. Cool roofs reduced neighborhood air temperatures by 0.3 ◦ C when implemented on residential homes. The results from this city-specific mitigation project will inform messaging campaigns
aimed at engaging the city decision makers, industry, and the public in the green building and urban
forestry initiatives.
© 2014 The Authors. Published by Elsevier GmbH. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Introduction
Rapid urbanization and the associated land cover changes from
natural or agricultural lands to built environments have transformed cities into urban heat islands (UHI), with increased air and
surface temperatures in the urban area as compared to its rural surroundings (Howard, 1833; Oke, 1982; Arnfield, 2003). Especially in
hot desert climates, the expanding UHI effect is of great concern,
because it increases outdoor water use (Guhathakurta and Gober,
2007, 2010) and the energy demand of cooling (Akbari et al., 2001;
Akbari, 2002; Golden, 2004; Grimmond, 2007), lowers air quality
(Stone, 2005; Sarrat et al., 2006), decreases thermal comfort (Hartz

∗ Corresponding author at: Julie Ann Wrigley Global Institute of Sustainability,
Arizona State University, P.O. Box 878009, Tempe, AZ 85287-8009, United States.
Tel.: +1 480 727 9228.
E-mail addresses: ariane.middel@asu.edu (A. Middel), nalini.chhetri@asu.edu
(N. Chhetri), ray.quay@asu.edu (R. Quay).

et al., 2006; Shashua-Bar et al., 2011), and increases illnesses and
mortality related to heat stress (Harlan et al., 2006; Golden et al.,
2006; Jenerette et al., 2011). In the face of global climate change,
the compounding effects of UHIs and more frequent and extended
heat waves further threaten the resilience of desert cities to cope
with increasing temperatures.
In 2008, the US Environmental Protection Agency (EPA) published a compendium of UHI mitigation strategies, promoting urban
forestry and albedo modification of hard surfaces, i.e., roofs and
pavement, as key strategies to reduce urban warming (EPA, 2008).
Trees can moderate climate through combined effects of (a) surface
shading, which reduces surface and air temperatures by intercepting incoming solar radiation; (b) evapotranspiration; and (c)
alteration of wind patterns (Oke et al., 1989; Akbari et al., 2001).
High albedo surfaces, e.g., cool (high reflectance) roofs, absorb
less heat during the day and reflect most of the incoming solar
radiation back into the atmosphere (Taha, 1997). In recent years,
several research studies that focused on UHI mitigation strategies in cities with hot summers assessed the thermal benefits of

http://dx.doi.org/10.1016/j.ufug.2014.09.010
1618-8667/© 2014 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/
by-nc-nd/3.0/).

A. Middel et al. / Urban Forestry & Urban Greening 14 (2015) 178–186

urban forests (Emmanuel et al., 2007; Golden et al., 2007; ShashuaBar et al., 2009, 2010; Bowler et al., 2010; Chow and Brazel,
2012), cool roofs (Georgescu et al., 2013), or a combined strategy
(Sailor, 1995; Akbari et al., 2001; Rosenzweig et al., 2009; Ng et al.,
2012; Shahidan et al., 2012). At the same time, many state and
local governments across the US have taken steps to mitigate the
UHI through various activities, e.g., incentive programs, tree and
landscape ordinances, zoning codes, plans and design guidelines,
and urban forestry initiatives (EPA, 2008). In the arid southwest,
policies and programs to mitigate heat and reduce energy use
for summer cooling include Million Trees LA (McPherson et al.,
2011), TreeUtah (http://www.treeutah.org/statewide.htm), and the
Phoenix Tree and Shade Master Plan (City of Phoenix, 2010), among
others. Although research findings on UHI mitigation strategies
have informed ongoing activities, the local effectiveness of most
city-specific, targeted UHI mitigation strategies has not been fully
assessed, especially with regard to quantifying temperature reductions at the neighborhood scale.
This study was conducted in collaboration with the City of
Phoenix to quantify the thermal impact of two heat mitigation
activities currently undertaken by the City, i.e., the urban forestry
and cool roof initiatives. The stakeholder questions guiding our
research are: (a) In a typical residential neighborhood in Phoenix,
what is the relationship between percent tree cover and midafternoon air temperature during pre-monsoon summer? (b) What
are the cooling benefits achieved by increasing tree canopy from
10% (current) to 25% (2030 goal) and/or implementing cool roofs,
under existing conditions and projected warming? Our study used
a microscale model to simulate an ensemble of tree canopy cover,
cool roof, and climate scenarios for a representative residential
neighborhood in Phoenix. We assessed the thermal benefits of each
scenario for mid-afternoon on a pre-monsoon summer day – when
the warmest temperatures occur – and at the local and micro-scale,
the scales at which most UHI mitigation strategies are implemented
and the greatest relief is felt.

The City of Phoenix
The City of Phoenix (Fig. 1a) is the heart of the Phoenix
Metropolitan Area and situated at 33◦ 29 N, 112◦ 4 W in Maricopa County, AZ. Located in the arid southwest of the US and the
northeastern part of the Sonoran Desert, Phoenix experiences hot
summers with mean maximum temperatures of 40 ◦ C and above
from June to August and mild winters with average daily temperatures around 15 ◦ C. Precipitation is low and averages 208 mm
annually with only 1–2 mm in May and June (Western Regional
Climate Center, 2014). Incorporated in 1881 as a small agricultural
community of about 2500 people, Phoenix has grown rapidly since
then, despite the hot, dry climate, and has experienced extensive
anthropogenic land cover changes, predominantly from farmlands
and agricultural fields to residential developments prior to 1975
and from both agricultural and desert since 1975 (Knowles-Yánez
et al., 1999, Chow et al., 2012). The population of the City of Phoenix
has grown from 105,000 in 1950 to 1.5 million in 2012 and is currently the 6th largest city in the US (US Census Bureau, 2014). The
Maricopa Association of Governments (2013) projects that Phoenix
will be home to over 2 million residents by 2040.
Rapid and extensive urbanization has led to an intense UHI
in Phoenix that has increased nighttime temperatures steadily,
approximately 0.5 ◦ C per decade since 1910 (Brazel et al., 2000).
A time-trend analysis of air temperatures at Phoenix Sky Harbor
International Airport showed nighttime temperature differences
between rural and urban areas of up to 6 ◦ C in the summer (Brazel
et al., 2000). Winter mobile transect observations in Phoenix found
an average UHI intensity of 8 ◦ C (Sun et al., 2009), and a study in

179

the spring observed a range of UHI intensity of 9.4 ◦ C to 12.9 ◦ C
(Hawkins et al., 2004). The Phoenix UHI has been well documented
and has been the focus of many research studies at various spatial and temporal scales (Stabler et al., 2005; Brazel et al., 2007;
Georgescu et al., 2012; Grossman-Clarke et al., 2010; Chow and
Brazel, 2012; Middel et al., 2014). This is due to several extrinsic factors, including the clear and calm weather conditions in
Phoenix, projected future droughts, and ongoing urban expansion,
as well as location-specific factors, such as partnerships between
academic institutions and private and public sector agencies, a
well-established extensive network of urban weather stations, and
strong media-coverage (Chow et al., 2012). In recent years, the
City of Phoenix has collaborated increasingly with researchers
from Arizona State University to adopt and implement policies
that incorporate UHI research findings and advance the City’s sustainability goals. Examples for these collaborative efforts are the
Downtown Phoenix Urban Form Project (City of Phoenix, 2008),
which informed zoning ordinances to mitigate the UHI through
optimized arrangement and design of urban features; a study to
investigate the outdoor water use efficiency of urban greening scenarios (Gober et al., 2010); the Phoenix Tree and Shade Master Plan
(City of Phoenix, 2010) to mitigate heat through an urban forest;
and the 2013 Phoenix cool roofs initiative, as part of the Green
Construction Code (City of Phoenix, 2006). This collaborative study
focuses on the two most recent heat mitigation efforts of the City
of Phoenix to address urban warming, i.e., the cool roofs and urban
forestry initiatives.
The Green Construction Code (City of Phoenix, 2006) was adopted
by the Phoenix City Council in 2005 and included purchasing guidelines for Energy Star Reflective Roof Certified Products to encourage
cool roofs on publicly owned buildings. In addition, the City adopted
a requirement for the use of high reflective cool roofs on all new
city-owned buildings. In October 2012, Mayor Greg Stanton started
the Phoenix Cool Roofs initiative to coat 70,000 square feet of the
City’s existing rooftops with reflective paint. As of January 2014,
approximately 52,000 square feet of public rooftops have been
coated (Fig. 1c), but the private and residential sectors have not
yet been included in this effort.
The Tree and Shade Management Task Force, a crossdepartmental committee led by the City of Phoenix Parks and
Recreation Department, developed a master plan that serves as
roadmap to incrementally achieve the goal of having a tree canopy
cover of 25% for the entire city by 2030 (City of Phoenix, 2010). The
plan outlines three goals: (1) educate the public on the benefits
of trees, as there is limited understanding of the importance of
the urban forest; (2) increase canopy cover to 25% and protect
existing trees, because more trees are currently being lost than
planted; and (3) improve planting, maintenance, and irrigation
practices, e.g., use drought-resistant, low-water use trees. Stabler
et al. (2005) estimated the Phoenix Metropolitan Area tree cover
to be 13%, but the tree canopy cover within the City of Phoenix is
currently estimated to be only 8 to 10%. Historic neighborhoods
near the urban core have the highest percentage of mature tree
canopy (Fig. 1b), because lush vegetation was traditionally used
here for cooling before air-conditioning became widely available
(Gober, 2006). In general, the Phoenix has a diverse tree palette,
ranging from native species, such as palo verde, ironwood, and
mesquite trees, to non-native species that were introduced from
similar climates, e.g., eucalyptus, ash, elm, olive, palm, and citrus
trees. A tree inventory recently completed by the City of Phoenix
lists a total of 92,845 trees on publicly owned land, i.e., parks,
right-of-ways, and around municipal buildings (City of Phoenix,
2014); however, the city-maintained urban forest covers less than
1% of the total land area in the City of Phoenix. In order to reach
the 2030 tree canopy goal of 25%, the public needs to be engaged
in the City’s urban forestry initiative and increase canopy cover

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A. Middel et al. / Urban Forestry & Urban Greening 14 (2015) 178–186

Fig. 1. (a) Aerial view of the City of Phoenix, including downtown; (b) Phoenix neighborhood with high tree canopy cover and low tree canopy cover; and (c) cool roofs and
street trees in Phoenix downtown.

on private land. Assessing the impact of an increased tree canopy
cover on neighborhood climate is the first step towards educating
the public on the thermal benefits of trees and creating incentives
for the residential sector to engage in the initiative.
In 2012, the Center for Integrated Solutions to Climate Challenges (CISCC) at Arizona State University partnered with the
Climate Assessment for the Southwest (CLIMAS) program at the
University of Arizona and the Decision Center for a Desert City
(DCDC) at Arizona State University in a joint research project to
examine what information or services were needed to help local
communities with their climate adaptation planning and implementation. To establish case studies, cities in Arizona were solicited
to submit proposals for assistance in their climate adaptation programs. The City of Phoenix requested assistance in assessing the
benefits of their Cool Roof initiative and Tree and Shade Plan
under current and future climate change conditions. The intent
of this assessment was initially characterized as a way to provide
documentation of benefits to justify expenditure of City funds to
promote and manage these programs.
Methods
To assess the impact of the City of Phoenix urban forestry and
cool roofs initiatives on near ground air temperatures, we simulated
an ensemble of residential neighborhood scenarios using ENVI-met
V3.1 Beta (Bruse, 2014). ENVI-met is a three dimensional atmospheric model that has been successfully used by many scholars to

simulate microclimate in Phoenix after supplementing the model’s
plant database with native species (Chow and Brazel, 2012; DecletBarreto et al., 2013; Hedquist and Brazel, 2014; Middel et al., 2014).
For our simulations, we used the model configuration parameters
from a recent study on the impact of urban form and landscaping types on mid-afternoon microclimate in Phoenix (Middel et al.,
2014). The employed model parameters were evaluated for June 23,
2011, a typical pre-monsoon summer day, using observed atmospheric initial conditions from weather stations in the Central
Arizona-Phoenix Long-Term Ecological Research (CAP LTER) North
Desert Village (NDV) experiment (Martin et al., 2007). The model
evaluation yielded a Root Mean Square Error (RMSE) of 1.41 ◦ C
(mesic), 1.81 ◦ C (oasis), and 2.00 ◦ C (xeric) for 2 m air temperatures;
the Mean Bias Error (MBE) was −0.02 ◦ C (mesic), 0.43 ◦ C (oasis),
and 1.20 ◦ C (xeric); and the Mean Absolute Error (MAE) was 1.18 ◦ C
(mesic), 1.58 ◦ C (oasis), and 1.74 ◦ C (xeric). As model domain, we
chose a Phoenix residential neighborhood scenario from Middel
et al. (2014), representative of a typical single-family home subdivision. The neighborhood is classified as Open Lowrise Local Climate
Zone after Stewart and Oke (2012), and features uniformly arranged
detached 2-story buildings. The ENVI-met area input file for the
neighborhood has a horizontal and vertical grid resolution of 1 m
and a total of 215 × 195 × 30 grid cells plus 5 nesting grids.
We assessed the two city-specific passive cooling strategies
for the mid-afternoon of June 23, 2011 in two steps. First, we
focused on the urban forest initiative to investigate the relationship
between percent tree canopy cover and temperature reduction at

A. Middel et al. / Urban Forestry & Urban Greening 14 (2015) 178–186

181

Fig. 2. (a) Tree cover scenarios from 0% to 30% for xeric residential neighborhood under initial conditions; and (b) ensemble of scenarios with varying tree cover, landscaping,
roof type, and climate.

the neighborhood scale. We designed eight scenarios with varying
tree canopy cover, ranging from 0% to 30% (Figs. 2a and 3), using
a mix of native and non-native trees (Table 1) evenly distributed
across the available space. To eliminate compounding effects of
grass and other vegetation, we chose inorganic mulch as ground
cover, reflective of xeric landscaping. By keeping the arrangement
of buildings and streets constant, we eliminated the effects of urban

form on the simulated microclimate and therefore isolated the
impact of trees.
In the second step, we assessed the combined impact of tree
canopy cover and cool roofs on mid-afternoon air temperatures for
June 23, 2011 and for two climate change scenarios (Fig. 2b). We
designed three landscaping scenarios for the residential subdivision that reflect the predominant yard styles in Phoenix: mesic

Fig. 3. Residential neighborhood in Phoenix with xeric yards and varying tree canopy cover.

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A. Middel et al. / Urban Forestry & Urban Greening 14 (2015) 178–186

Table 1
Common native and introduced tree species in Phoenix.
Tree

Species

Height (m)

Afghan Pine
Blue palo verde
Bottle tree
Chinese elm
Chinese pistache
European olive
Ironwood
Mexican fan palm
Phoenix thornless mesquite
Arizona ash
Shoestring acacia
Southern live oak

Pinus eldarica
Parkinsonia florida
Brachychiton populneus
Ulmus parvifolia
Pistacia chinensis
Olea europaea
Olneya tesota
Washingtonia robusta
Prosopis hybrid Phoenix
Fraxinus velutina
Acacia stenophylla
Quercus virginiana

6
8
11
9
4
9
6
20
9
12
12
9

(sprinkler-irrigated grass and lush vegetation), xeric (decomposing granite mulch, low-water use vegetation), and oasis (a mix
between mesic and xeric). Then, we virtually planted trees to create a 10% canopy cover and 25% canopy cover in the neighborhood,
using common tree species in Phoenix (Table 1). To simulate high
reflective cool roofs, we set the roof albedo value in the ENVImet configuration file to 0.88, an Environmental Protection Agency
(EPA) standard for Energy Star roof coating after a 3-year wear and
tear period (Georgescu et al., 2013). We based our projected climate scenarios runs on results from the statistically downscaled
CMIP3 climate model outputs for the Southwestern US, as outlined in the Assessment of Climate Change in the Southwest United
States report (Cayan et al., 2013). Minimum and maximum average
annual warming projections were, respectively, 1.1 ◦ C and 3.3 ◦ C,
both for the low (B1) emission scenario from 2070 to 2099 and the
high (A2) emission scenario from 2041 to 2070. In total, we ran 54
ENVI-met model simulations, i.e., for each of the 9 combined tree
canopy cover and landscaping scenario with regular roofs and cool
roofs under current conditions and for 1.1/3.3 ◦ C warming, using
the parameters listed in Table 2.
Results
Relationship between tree canopy cover and air temperature
For the each of the eight xeric neighborhood scenarios with
varying tree canopy cover, we extracted 2 m air temperature values at 3:00 pm from the ENVI-met simulations of June 23, 2011.
Table 2
ENVI-met building and climate parameters, evaluated for June 23, 2011, for base
conditions, and roof/climate scenarios.
Building data

Regular roofs

Inside temperature (◦ C)
Heat transmission walls (Wm−2 K)
Heat transmission roofs (Wm−2 K)
Albedo walls
Albedo roofs
Meteorological data
Wind speed, 10 m above
ground (ms−1 )
Wind direction (0:N, 90:E) (◦ )
Roughness length at reference
point (m)
Initial temperature
atmosphere (K)
Specific humidity in 2500 m
[water/air] (g kg−1 )
Relative humidity in 2 m (%)
Cloud cover (x/8)

Cool roofs
23.00
1.60
6.00
0.55

0.20
Base case

0.88
Climate
scenario 1

Climate
scenario 2

1.50

300.10

302.30
2.39

23.00
0.00

We chose 3:00 pm, because mid-afternoon is the hottest time of
day, and, thus, shade trees are the most beneficial in terms of
human comfort. To significantly reduce the edge effect that occurs
at the western border of the model domain due to the inflow profile, we excluded 65 boundary grids at the western border of the
model domain from the temperature averages. Box plots summarize the neighborhood temperatures for the eight tree planting
scenarios (Fig. 4). Modeling results exhibit a linear relationship
between percent tree canopy cover and near ground air temperature, with R2 = 0.88 and 0.14 ◦ C air temperature reduction per
percent increase in canopy cover. These findings are in line with
results from other studies investigating tree canopy cover impacts
on air temperatures. Hamada and Ohta (2010) found a significant
negative correlation between summer afternoon air temperature
and percent tree cover within a 200 m radius from an observation site in Japan. Myint et al. (2013) investigated the relationship
between surface temperatures and land cover types in Phoenix and
found a linear relationship between tree cover and daytime temperatures for July 2005. Simpson (1998) reported a mid-day air
temperature reduction of 0.04 ◦ C to 0.20 ◦ C for every one percent
increase in canopy cover.
The relationship between canopy cover and cooling becomes
less clear when examined in combination with varying neighborhood designs. Grass and other vegetative cover have an impact on
air temperature, mainly through evapotranspiration. Modifications
of land surface cover changes the heat storage capacity of the urban
environment and urban form further affects microclimate through
a change in wind patterns and shading. Finally, the arrangement of
trees (dispersed vs. clustered) and tree species may have an impact
on the cooling benefit. These compounding effects need further
investigation through a more comprehensive, observational study.
Cooling benefit of trees and cool roofs at the neighborhood scale

280
0.01
299.00

Fig. 4. Relationship between percent tree canopy cover and near ground air temperatures at the neighborhood scale as modeled by ENVI-met for June 23, 2011, at
3:00 pm. The upper and lower bounds of the box plots indicate the 25th and 75th
percentile of the 2 m air temperature values in the model domain, the whiskers
represent the 10th and 90th percentiles, and the dots stand for the 5th and 95th
percentile.

We ran an ensemble of 54 neighborhood scenarios with ENVImet, combining three landscaping options (mesic, oasis, and xeric),
three tree planting scenarios (no trees, current canopy cover, 2030
canopy goal), two roof scenarios (regular roofs, high reflective cool
roofs) and three climate scenarios (June 23, 2011; +1.1 ◦ C; +3.3 ◦ C).
For each of the 54 scenarios, we extracted average neighborhood
mid-afternoon air temperatures to assess the cooling benefits of

A. Middel et al. / Urban Forestry & Urban Greening 14 (2015) 178–186

each heat mitigation measure under current and projected climate
conditions. Again, 65 grids at the western border of the model
domain were excluded from the calculation to eliminate edgeeffects.
The simulation results for 2 m air temperature at 3 pm in the residential neighborhood are illustrated in Fig. 5. Results are arranged
in a matrix and grouped by climate scenario (rows) and roof type
(columns). A 9 by 9 sub-matrix displays neighborhoods by percent tree canopy cover (rows) and landscaping style (columns).
As expected, the scenario with the lowest air temperatures is the
residential neighborhood with mesic landscaping that has a combination of 25% tree canopy cover and cool roofs under current
climate conditions (row 3, column 4) with an average neighborhood temperature of 37.4 ◦ C. In contrast, the hottest scenario is
the xeric neighborhood with no tree cover and regular roofs under
the high emission climate change scenario (row 7, column 3). Near
ground air temperatures for this scenario average 46.7 ◦ C. Across
all columns of the matrix, an increase in tree canopy cover from
0% to 10% decreases average neighborhood temperatures by 2.0 ◦ C.
Increasing tree canopy cover further to 25% leads to an additional
temperature reduction of 2.4 ◦ C – a total cooling benefit of 4.4 ◦ C, as
compared to the bare neighborhood. Switching landscaping from
xeric to oasis, i.e., adding grass patches to residential backyards,
reduces neighborhood temperatures by 0.2 ◦ C to 0.3 ◦ C on average. Replacing all inorganic mulch by turf has a local cooling effect
of 1.7 ◦ C to 1.9 ◦ C. These results are consistent with observational
studies in hot and dry climates. In Athens, Greece, Shashua-Bar et al.
(2010) evaluated passive cooling scenarios in urban streets having
tree canopy cover. They found a cooling benefit of 2.2 ◦ C at 15:00 h
for a tree coverage of 35% and an additional 1.2 ◦ C cooling effect
for a 70% tree cover scenario. Microclimate observations during a
controlled experiment in the arid Negev Highlands, Israel, yielded a
daytime temperature reduction of up to 2.5 ◦ C for a bare courtyard
that was planted with trees and grass (Shashua-Bar et al., 2009).
Srivanit and Hokao (2013) investigated the local cooling effects of
trees using on-site measurements and ENVI-met simulations for
a hot dry summer day in Japan. An increase of tree canopy by
20% reduced air temperatures by 2.3 ◦ C. In Phoenix, Golden et al.
(2007) investigated the cooling potential of trees located on parking
lots and found that the tree canopy reduced 2 m air temperatures
by 3.5 ◦ C at noon. Chow and Brazel (2012) suggested xerophytic
shade trees as sustainable UHI mitigation measure for Phoenix and
estimated a daytime cooling benefit of 1.1 ◦ C at the local scale,
with cooling benefits of 2.5 ◦ C at the microscale. These findings
emphasize the importance of strategic tree location in heat mitigation.
Compared to the cooling impact of trees, the impact of cool
roofs on local daytime temperatures modeled by ENVI-met is
relatively low. Across all climate and tree scenarios, 2 m air temperature reduction through the implementation of cool roofs only
amounts to 0.3 ◦ C for the neighborhood. Results from other studies assessing the impact of cool roofs on air temperatures range
from minor cooling benefits to significant benefits, due to varying
modeling assumptions and parameterizations, geography, and scenarios. Our results are consistent with the low end range of these
studies.
Li et al. (2014) simulated cool roofs with the Weather Research
and Forecasting (WRF) model in the Baltimore-Washington
metropolitan area, using an albedo value of 0.7. They found a 2 m
air temperature reduction of up to 0.6 ◦ C at 3:00 pm. Using WRF
for Bakersfield, California, Ban-Weiss et al. (2014) found an afternoon temperature decrease of 0.2 ◦ C when replacing all existing
roofs with cool roofs. At the global scale, Oleson et al. (2010) estimated an urban daily maximum decrease of 0.6 ◦ C by implementing
white roofs. At the regional scale, Georgescu et al. (2012) simulated the effects of cool roofs on the Arizona Sun Corridor’s regional

183

temperatures for various urban expansion scenarios and found
that highly reflective roofs could offset urban-induced warming by
about 50%, but also changed precipitation patterns in the region.
This study shows that implementing cool roofs for heat mitigation
comes with tradeoffs that need to be considered.

Discussion
An increase in tree canopy cover from the current 10% to the City
of Phoenix goal of 25% resulted in a 2.0 ◦ C temperature reduction
at the local scale and could offset the amount of urban warming
predicted by the more conservative climate change scenario. However, the City’s goal of 25% tree canopy cover by 2030 can only
be achieved with public support. Further research is necessary to
systematically assess the impact of the arrangement and type of
trees on the heat mitigation potential of the urban forest, especially in the context of various urban forms and compared to or
combined with artificial shading structures. Tree layout, spacing,
and location are key factors for optimal cooling, because shading
is localized. Trees provide more cooling benefits at the microscale
than the local scale, therefore, tree placement at strategic locations
is important to increase pedestrian comfort and energy savings
from buildings.
There is a wide range of research results on the impact of cool
roofs on air temperatures, from 0.2 ◦ C up to 1.5 ◦ C, mainly because
studies were conducted at small to large scales in various climates
using a variety of models, parameterizations and assumptions. Our
ENVI-met simulations for cool roofs are in agreement with the
lower end range of these studies and show no significant daytime
temperature reduction at 2 m height across all tree and climate scenarios. However, we would expect an impact on building energy
performance, which could be quantified through a building energy
simulation. The impact of cool roofs on pedestrian thermal comfort also needs further investigation, since albedo modification of
urban materials changes the radiation balance of the urban environment. A full economic benefit analysis of cool roofs should not
only assess the costs and diurnal cooling benefit of high reflective
roofs, but also include an analysis of how the cooling benefit might
decline over time due to dust and dirt, how the roofs might impact
precipitation patterns in the region.
The impact of trees and cool roofs on nighttime temperatures,
and thus on the UHI, also needs further investigation. ENVI-met 3.1
has several shortcomings that make the model unsuited for nocturnal cooling analyses and limit its use to daytime situations. First, the
ENVI-met meteorological forcing parameters cannot be adjusted
during the simulation, resulting in a conservative diurnal temperature curve that underestimates maximum temperatures during
the day and overpredicts minimum temperatures at night. Second,
in the model, walls and roofs do not store heat and consequently
do not release heat at night. Therefore, ENVI-met overestimates
emitted long-wave radiation during the day and underestimates it
at night. Third, previous research found that the rate of nighttime
cooling is lower under tree canopies due to a reduced sky-view
factor (Akbari et al., 2001). This heat retention cannot be modeled
with ENVI-met. Despite these limitations, ENVI-met yields reasonable results for daytime simulations if the model is calibrated well.
Lastly, ENVI-met does not account for anthropogenic heat. Waste
heat release from A/C systems in some Phoenix neighborhoods was
found to increase 2 m air temperatures by 1 ◦ C and more during
the night using the WRF model (Salamanca et al., 2014). The mean
warming effect was found to be negligible for near surface temperatures during the day at the regional scale, but heat emission
reductions due to reduced energy use of buildings with cool roofs
should be investigated further at the microscale using building
energy models.

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Fig. 5. ENVI-met simulations for an ensemble of 54 scenarios that combine landscaping styles and heat mitigation strategies for current and projected pre-monsoon summer
climate in Phoenix; simulations for June 23, 2011, 3:00 pm.

A. Middel et al. / Urban Forestry & Urban Greening 14 (2015) 178–186

In addition to shade benefits during the day, trees provide
various ecosystem services and socio-economic benefits, e.g., air
quality improvement, higher property value, reduced building
energy-use, noise mitigation, reduction of storm water runoff and
flooding, reduced street maintenance costs, carbon sequestration,
creation of wildlife habitats, and recreational opportunities for residents (McPherson et al., 2005; Nowak and Dwyer, 2007). However,
maintaining a healthy urban forest requires maintenance costs and,
in desert environments, irrigation. A cost–benefit analysis and full
economic assessment of the Phoenix urban forest, assessing benefits against costs, was beyond the scope of this project, but should
be conducted for a comprehensive assessment of all tree benefits
and management costs. Recent studies attempted to determine the
environmental and socio-economic value of urban forests for the
cities of Toronto, Canada (Millward and Sabir, 2011) and Los Angeles, California (McPherson et al., 2011) using the iTree software. The
City of Phoenix recently completed an inventory for trees on publicly owned land (City of Phoenix, 2014), which could provide the
basis for a cost–benefit analysis, but needs to be expanded to private
properties first. An economic analysis of the Phoenix urban forest
should also include an assessment of environmental costs that are
associated with increased irrigation demands. With the prospect of
more frequent and extended droughts in the southwestern US due
to climate change, increasing tree canopy cover in Phoenix to 25%
will put additional stress on the water supply. For sustainable heat
mitigation in the desert, design strategies need to be developed that
maximize the collective benefits from the urban forest in balance
with water use and costs, e.g., water-sensitive urban design with
low water use trees and artificial shading structures.
Conclusions
This study has several implications for heat mitigation measures
and climate adaptation in Phoenix. First, our model results show
that increased tree coverage in Phoenix neighborhoods reduces air
temperatures, but the magnitude of this impact, even at a 25% tree
canopy cover, may not be sufficient to offset increased temperatures due to climate change. However, trees can be one component
of a climate adaptation strategy. Second, further research is
needed to quantify the cooling, social, economic, and environmental benefits; the promotion and management costs; and water
use implications of a tree program in a comprehensive economic
assessment. Finally, the benefit of cool roofs for heat mitigation
and climate change adaptation needs to be further investigated
for the entire Phoenix region. Cool roofs have benefits that extend
beyond their impact on ambient temperatures. To assess their role
in UHI mitigation and climate change adaptation, a comprehensive
analysis of implementation and maintenance costs, property values, building energy savings, and thermal comfort implications is
needed.
Acknowledgements
This research was supported by the National Oceanic and Atmospheric Administration (NOAA) under the Sectoral Applications
Research Program (SARP), Grant NA10OAR4310185, the Walton
Sustainability Solutions Initiatives (WSSI), a unit of the Julie Ann
Wrigley Global Institute of Sustainability at Arizona State University, and the Decision Center for a Desert City II: Urban Climate
Adaptation, Grant SES-0951366. Any opinions, findings, and conclusions or recommendations expressed in this material are those
of the authors and do not necessarily reflect the views of the
sponsoring agencies. We would like to thank Philip McNeely, City
of Phoenix Environmental Program Manager; Joe Gibbs, Environmental Quality Specialist; and Richard Adkins, Phoenix Parks and

185

Recreation Department Forestry Supervisor for their assistance
with this project. We would also like to thank WSSI and Aaron Lance
in the Decision Theater at Arizona State University for providing the
hardware to run the microclimate simulations. Finally, we would
like to thank Sally Wittlinger from the Decision Center for a Desert
City (DCDC) for her editorial support.
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