Given increasing utility of numerical models to examine urban impacts on meteorology and climate, there exists an urgent need for accurate representation of seasonally and diurnally varying anthropogenic heating data, an important component of the urban energy budget for cities across the world. Incorporation of anthropogenic heating data as inputs to existing climate modeling systems has direct societal implications ranging from improved prediction of energy demand to health assessment, but such data are lacking for most cities. To address this deficiency we have applied a standardized procedure to develop a national database of seasonally and diurnally varying anthropogenic heating profiles for 61 of the largest cities in the United Stated (U.S.). Recognizing the importance of spatial scale, the anthropogenic heating database developed includes the city scale and the accompanying greater metropolitan area.

Our analysis reveals that a single profile function can adequately represent anthropogenic heating during summer but two profile functions are required in winter, one for warm climate cities and another for cold climate cities. On average, although anthropogenic heating is 40% larger in winter than summer, the electricity sector contribution peaks during summer and is smallest in winter. Because such data are similarly required for international cities where urban climate assessments are also ongoing, we have made a simple adjustment accounting for different international energy consumption rates relative to the U.S. to generate seasonally and diurnally varying anthropogenic heating profiles for a range of global cities. The methodological approach presented here is flexible and straightforwardly applicable to cities not modeled because of presently unavailable data. Because of the anticipated increase in global urban populations for many decades to come, characterizing this fundamental aspect of the urban environment – anthropogenic heating – is an essential element toward continued progress in urban climate assessment.

Conversion of natural to urban land forms imparts influence on local and regional hydroclimate via modification of the surface energy and water balance, and consideration of such effects due to rapidly expanding megapolitan areas is necessary in light of the growing global share of urban inhabitants. Based on a suite of ensemble-based, multi-year simulations using the Weather Research and Forecasting (WRF) model, we quantify seasonally varying hydroclimatic impacts of the most rapidly expanding megapolitan area in the US: Arizona's Sun Corridor, centered upon the Greater Phoenix metropolitan area. Using a scenario-based urban expansion approach that accounts for the full range of Sun Corridor growth uncertainty through 2050, we show that built environment induced warming for the maximum development scenario is greatest during the summer season (regionally averaged warming over AZ exceeds 1 °C).

Warming remains significant during the spring and fall seasons (regionally averaged warming over AZ approaches 0.9 °C during both seasons), and is least during the winter season (regionally averaged warming over AZ of 0.5 °C). Impacts from a minimum expansion scenario are reduced, with regionally averaged warming ranging between 0.1 and 0.3 °C for all seasons except winter, when no warming impacts are diagnosed. Integration of highly reflective cool roofs within the built environment, increasingly recognized as a cost-effective option intended to offset the warming influence of urban complexes, reduces urban-induced warming considerably. However, impacts on the hydrologic cycle are aggravated via enhanced evapotranspiration reduction, leading to a 4% total accumulated precipitation decrease relative to the non-adaptive maximum expansion scenario. Our results highlight potentially unintended consequences of this adaptation approach within rapidly expanding megapolitan areas, and emphasize the need for undeniably sustainable development paths that account for hydrologic impacts in addition to continued focus on mean temperature effects.

This research evaluates the climatic summertime representation of the diurnal cycle of near-surface temperature using the Weather Research and Forecasting System (WRF) over the rapidly urbanizing and water-vulnerable Phoenix metropolitan area. A suite of monthly, high-resolution (2 km grid spacing) simulations are conducted during the month of July with both a contemporary landscape and a hypothetical presettlement scenario. WRF demonstrates excellent agreement in the representation of the daily to monthly diurnal cycle of near-surface temperatures, including the accurate simulation of maximum daytime temperature timing. Thermal sensitivity to anthropogenic land use and land cover change (LULCC), assessed via replacement of the modern-day landscape with natural shrubland, is small on the regional scale. The WRF-simulated characterization of the diurnal cycle, supported by previous observational analyses, illustrates two distinct and opposing impacts on the urbanized diurnal cycle of the Phoenix metro area, with evening and nighttime warming partially offset by daytime cooling. The simulated nighttime urban heat island (UHI) over this semiarid urban complex is explained by well-known mechanisms (slow release of heat from within the urban fabric stored during daytime and increased emission of longwave radiation from the urban canopy toward the surface). During daylight hours, the limited vegetation and dry semidesert region surrounding metro Phoenix warms at greater rates than the urban complex. Although prior work has suggested that daytime temperatures are lower within the urban complex owing to the addition of residential and agricultural irrigation (i.e., “oasis effect”) we show that modification of Phoenix's surrounding environment to a biome more representative of temperate regions eliminates the daytime urban cooling. Our results indicate that surrounding environmental conditions, including land cover and availability of soil moisture, play a principal role in establishing the nature and evolution of the diurnal cycle of near-surface temperature for the greater Phoenix, Arizona, metropolitan area relative to its rural and undeveloped counterpart.

Evaluation of built environment energy demand is necessary in light of global projections of urban expansion. Of particular concern are rapidly expanding urban areas in environments where consumption requirements for cooling are excessive. Here, we simulate urban air conditioning (AC) electric consumption for several extreme heat events during summertime over a semiarid metropolitan area with the Weather Research and Forecasting (WRF) model coupled to a multilayer building energy scheme. Observed total load values obtained from an electric utility company were split into two parts, one linked to meteorology (i.e., AC consumption) which was compared to WRF simulations, and another to human behavior. WRF-simulated non-dimensional AC consumption profiles compared favorably to diurnal observations in terms of both amplitude and timing. The hourly ratio of AC to total electricity consumption accounted for ~53% of diurnally averaged total electric demand, ranging from ~35% during early morning to ~65% during evening hours. Our work highlights the importance of modeling AC electricity consumption and its role for the sustainable planning of future urban energy needs. Finally, the methodology presented in this article establishes a new energy consumption-modeling framework that can be applied to any urban environment where the use of AC systems is prevalent.

A web-based software tool has been developed to assist urban planners and air quality management officials in assessing the potential ofurban heat island mitigation strategies to affect the urban climate, air quality, and energy consumption within their cities. The user of thetool can select from over 170 US cities for which to conduct the analysis, and can specify city-wide changes in surface reflectivity and/or veg-etative cover. The Mitigation Impact Screening Tool (MIST) then extrapolates results from a suite of simulations for 20 cities to estimate airtemperature changes associated with the specified changes in surface characteristics for the selected city. Alternatively the user can simply definea nominal air temperature reduction that they hope to achieve with an unspecified mitigation scenario. These air temperature changes are theninput to energy and ozone models to estimate the impact that the mitigation action may have on the selected city. The results presented by MISTinclude a high degree of uncertainty and are intended only as a first-order estimate that urban planners can use to assess the viability of heatisland mitigation strategies for their cities. As appropriate, MIST analyses should be supplemented by more detailed modeling.

Creating a Healthier, More Livable and Prosperous Phoenix

Phoenix is poised to become the next great American City. The Tree and Shade Master Plan presents Phoenix’s leaders and residents a roadmap to creating a 21st Century desert city. The urban forest is a keystone to creating a sustainable city because it solves many problems with one single solution. By investing in trees and the urban forest, the city can reduce its carbon footprint, decrease energy costs, reduce storm water runoff, increase biodiversity, address the urban heat island effect, clean the air, and increase property values. In addition, trees can help to create walkable streets and vibrant pedestrian places. More trees will not solve all the problems, but it is known that for every dollar invested in the urban forest results in an impressive return of $2.23 in benefits.

Phoenix has a strong foundation on which to build the future. Phoenix residents value natural resources and have voted repeatedly to invest in the living infrastructure. For instance, the Phoenix Parks and Preserve Initiative was passed twice with over 75 percent voter approval. This modest sales tax has purchased land for the Sonoran Preserve, funded habitat restoration efforts along Rio Salado, built new parks and planted hundreds of new trees. These projects and others like it provide the base for a healthy urban forest. Trees and engineered shade have the potential to be one of the city’s greatest assets and the Tree and Shade Master Plan provides the framework for creating a healthier, more livable and prosperous Phoenix.

The Urban Forest – Trees for People

The urban forest is a critical component of the living infrastructure. It benefits and attracts residents and tourists alike to live, work, shop and play in the city. Phoenix’s urban forest is a diverse ecosystem of soils, vegetation, trees, associated organisms, air, water, wildlife and people. The urban forest is found not only in parks, mountain preserves and native desert areas, but also in neighborhoods, commercial corridors, industrial parks and along streets. The urban forest is made up of a rich mosaic of private and public property that surrounds the city and provides many environmental, economic, and social benefits.

In order for the urban forest to be a profitable investment, Phoenix must do more than just plant trees. The entire lifecycle of the tree must be addressed because the current planting, maintenance, and irrigation practices are preventing many trees from providing their maximum return on investment. The Tree and Shade Master Plan provides a detailed roadmap to address these issues, as well as many others, with realistic and incremental steps. To succeed, this plan requires a long-term investment from the residents and leaders of Phoenix.

Trees are Solution Multipliers

Solution multipliers solve numerous problems simultaneously. Trees are a perfect example of a solution multiplier because when planted and maintained correctly, they can provide many economic, environmental, and social benefits. According to the US Forest Service, trees benefit the community by: providing a cooling effect that reduces energy costs; improving air quality; strengthening quality of place and the local economy; reducing storm water runoff; improving social connections; promoting smart growth and compact development; and creating walkable communities (US Forest Service and Urban & Community Forestry). Trees are high-yield assets; for example, the City of Chicago values its trees at $2.3 billion dollars. Trees have a documented return on investment (ROI) in Arizona of $2.23 for every $1 invested (US Department of Agriculture Forest Service). This demonstrates the important role that trees have within the city's economy. This is why it is critical to manage and invest in the urban forest; the health of the urban forest is closely linked to the economic health of the city.

Maintainable Infrastructure

Phoenix is a desert city that has a history of several decades of drought. In order to achieve a healthy urban forest we must use water wisely. Currently, 60 percent of Phoenix’s water is used outdoors, mainly for landscape irrigation. According to the City of Phoenix’s Water Services Department, Phoenix has an adequate sustainable water supply to meet the State of Arizona’s 100-year assured water supply standard. This includes growth in Phoenix’s system water demand over the next 20 years or more. Nonetheless, to achieve a maintainable urban forest, water must be used more efficiently. This is done with high-efficiency irrigation systems, use of drought-tolerant plant material, strategic placement of shade corridors and continued education. In order for a healthy urban forest to exist, it must be coupled with strong water management.

Implementation

The Urban Forest Infrastructure Team and the Parks and Recreation Department are charged with coordinating and maintaining the Tree and Shade Master Plan. Many City departments will implement the plan as they work to fulfill their own missions. The Tree and Shade Master Plan will not only provide a framework to achieve an average 25 percent tree canopy coverage by 2030 but will also help to achieve many goals and policies from the Green Phoenix Initiative and the voter ratified General Plan.

The plan proposes incremental steps to achieve the 2030 vision and canopy goal. The City of Phoenix is beginning to put a process in place to preserve, maintain, and redevelop the urban forest. This plan intends to increase the quality of life and economic vitality of the city by recommending ways to create a sustainable urban forest for future generations.

The City of Phoenix’s Cool Urban Spaces Report (2014) investigated the impact of the Phoenix Cool Roofs and Tree and Shade Master Plan initiatives on the city. The study evaluated how these heat mitigation efforts affect microclimates and human thermal comfort in the Phoenix metropolitan area. These findings are especially relevant as rapid and extensive urbanization has led to an urban heat island (UHI) effect that has increased steadily at approximately 0.9°F per decade. The city’s questions guiding this research were: 1. 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? 2. What is the diurnal thermal benefit of tree canopy shade for a typical heat wave day during pre-monsoon summer?

The impacts of cool roofs and trees on near-ground air temperatures were modeled through 54 scenarios for a typical residential neighborhood in Phoenix. We ran the model for a combination of three tree-planting scenarios (no trees, current canopy cover and 2030 canopy goal) and three landscaping scenarios (mesic, oasis and xeric) with regular roofs and cool roofs under current climate conditions and two climate change projections. Two significant results of the tree and shade initiative are: 1. Increasing tree canopy cover to 25% leads to an additional temperature reduction of 4.3°F, which is a total cooling benefit of 7.9°F as compared to a bare neighborhood, and 2. Switching landscaping from xeric to oasis, i.e., adding grass patches to residential backyards, reduces average neighborhood temperatures by 0.4°F to 0.5°F.

The scenario with the lowest air temperatures is the residential neighborhood with mesic landscaping, 25% tree canopy cover and cool roofs under current climate conditions with an average neighborhood temperature of 99.5°F. In contrast, the xeric neighborhood with no tree cover and regular roofs under the high-emissions climate change scenario is the hottest. This indicates that the combination of increased tree canopy cover and cool roofs does lower temperatures as well as reduce the demand for air conditioning, thereby reducing anthropogenic heat. However, trees and cool roofs are only part of the solution and need to be included in a broader, more comprehensive mitigation and adaptation plan.

Across all climate and tree scenarios, the effect of cool roofs alone on local daytime temperatures is relatively low. Air temperature reduction only amounts to 0.5°F in the neighborhood. Regarding the city’s cool roofs initiative, results show little benefit for extending this project to commercial and residential properties based on its cooling impacts alone. Our research thus far indicates that there is no simple solution to mitigating the UHI, but a complex balance of strategies will be necessary so that efforts to lower the daytime temperatures do not increase nighttime temperatures or shift UHI impacts to more vulnerable populations.

Shade plays an important role in designing pedestrian-friendly outdoor spaces in hot desert cities. This study investigates the impact of photovoltaic canopy shade and tree shade on thermal comfort through meteorological observations and field surveys at a pedestrian mall on Arizona State University’s Tempe campus. During the course of 1 year, on selected clear calm days representative of each season, we conducted hourly meteorological transects from 7:00 a.m. to 6:00 p.m. and surveyed 1284 people about their thermal perception, comfort, and preferences. Shade lowered thermal sensation votes by approximately 1 point on a semantic differential 9-point scale, increasing thermal comfort in all seasons except winter. Shade type (tree or solar canopy) did not significantly impact perceived comfort, suggesting that artificial and natural shades are equally efficient in hot dry climates. Globe temperature explained 51 % of the variance in thermal sensation votes and was the only statistically significant meteorological predictor. Important non-meteorological factors included adaptation, thermal comfort vote, thermal preference, gender, season, and time of day. A regression of subjective thermal sensation on physiological equivalent temperature yielded a neutral temperature of 28.6 °C. The acceptable comfort range was 19.1 °C–38.1 °C with a preferred temperature of 20.8 °C. Respondents exposed to above neutral temperature felt more comfortable if they had been in air-conditioning 5 min prior to the survey, indicating a lagged response to outdoor conditions. Our study highlights the importance of active solar access management in hot urban areas to reduce thermal stress.

This established model is applied here to show the relative effects of four common mitigation strategies: increasing the overall (1) emissivity, (2) percentage of vegetated area, (3) thermal conductivity, and (4) albedo of the urban environment in a series of percentage increases by 5, 10, 15, and 20% from baseline values.