THE POTENTIAL OF CLIMATE MODELING IN GREENWAY PLANNING FOR PHOENIX, ARIZONA

Transcription

1 THE POTENTIAL OF CLIMATE MODELING IN GREENWAY PLANNING FOR PHOENIX, ARIZONA Crewe, K. School of Planning and Landscape Architecture, Arizona State University, Tempe, Arizona. ABSTRACT For the efficient design of parks and greenways in desert cities like Phoenix, Arizona, it is important to work with a range of alternatives. This is essential for finding optimal solutions to the area s challenging climate, and for helping local planners and communities make informed decisions about their existing and future green spaces. Many critical trade-offs affect outdoor space planning in Phoenix, however. With summer daytime temperatures over F, shade is critical, yet the scarcity of water limits vegetation and expansive lawns. Building densities provide cooling; yet can trap daytime heat and block cool wind currents at night. This study explores the adaptation of a 3D numerical model to assess multiple feedbacks in the microclimate system at the local scale. Using inputs for atmospheric conditions, surface materials, soils, vegetation and buildings, the model simulates changes from new vegetation patterns, building orientations and other factors. This paper describes the use of this model for planning a widespread linear park system for Phoenix, and how we adapted a European-based model to an extreme desert climate. Finally, we discuss how we tabulated the model outputs, and adapted these for public communication. This climate simulation model has the potential to enhance pedestrian life outdoors, and reducing auto dependency; moreover, a network of green space may substantially reduce heat island effects for the city overall. We argue that climate simulations of this kind are important not only for Phoenix, but for all fast-growing cities in challenging climatic regions. 1. INTRODUCTION Designing with climate in Phoenix, Arizona, presents a number of challenges. While the city's extreme heat requires shade for outdoor comfort, the low annual rainfall prohibits extensive tree planting and irrigation. While we know that compact building densities produce cooling through shade, compactness can also trap daytime heat and block cooling winds at night (Pearlmutter, l998; Pearlmutter, Bitan and Berliner, l999). Wind channels are highly desirable in the Phoenix area given the sluggish and polluted daytime atmosphere, yet dispersed settlement limits ventilation from urban canyons (Brazel et al., 2000). For these reasons, effective design for Phoenix calls for a knowledge of multiple alternatives for desert conditions, so as to explore practical trade-offs between cooling and irrigation, compact building and the heat island effect, and to find appropriate wind channels for a region of low-rise buildings and wide streets. Since computer modeling can test multiple conditions in one site, this becomes an important tool for evolving guidelines for outdoor planning and design. However, most available climate models call for a careful interface between conventional criteria and the extreme conditions, which determine Phoenix's outdoor environment. The Phoenix metropolitan area consists of 26 independent municipalities, and covers an area of 500 square miles (Figure 1). This auto-dominated region of 3 million is troubled by severe air pollution, traffic congestion, and an acknowledged lack of interaction at the neighborhood scale (Gammage, 1999; Gober and Burns, 2002). The area continues to grow at a rate of.5 miles per year. Phoenix planners, developers and architects have recognized for some time that an integrated system of greenways could encourage outdoor activity. Local planners favor greenways because these would strengthen neighborhoods and encourage activities (Steiner et al., 1999), while transportation planners argue pedestrian parks would reduce auto-dependence, cut infrastructure costs and reduce freeway pollution (Gomez Ibanez et al., 1999). Environmental planners favor greenways to encourage wildlife corridors and extend natural ecosystems within the city (Beatley and Manning, 1977). All these groups unite in favouring climate-sensitive design, since lowering outdoor temperatures by a few degrees could add hours of outdoor comfort to each days, and induce thousands more to ride their bikes to work. A second major benefit to urban greenways is their capacity to cool adjacent neighborhoods. This paper describes the adaptation of a European-based climate model to assist in informed decisions about the design and planning of greenways in Phoenix. The paper discusses how we provided interface with a model originally designed for a moderate, high-rainfall climate, adapting the model's criteria to desert conditions, and then testing for accuracy. We discuss the selection of a prototypical study area as a base, using an existing ten-mile greenway in the city of Scottsdale, the Indian Bend Wash (Figure 2). Proceedings of the 21 st International Cartographic Conference (ICC) Durban, South Africa, August 2003 Cartographic Renaissance Hosted by The International Cartographic Association (ICA) ISBN: Produced by: Document Transformation Technologies

2 Figure 1. Phoenix Metropolitan Area. Figure 2. Indian Bend Wash, Scottsdale Arizona. The paper explains the selection of relevant questions for modeling, and the exploration of hypothetical variations at the microclimate scale, evolving working design guidelines for practical use. Finally we discuss how the adapted model will serve as a module for a metropolitan-scale system whereby planners across the region can test local greenway plans for climate sensitive design. The paper responds to a call for a broader understanding of model application processes (Timmermans, 2002); it also contributes to a growing literature on desert climate issues, particularly desert cities. Finally, it contributes to knowledge of greenway planning and design (Little, 1990; Crewe, 2001). We used the three-dimensional model ENVI-met, by Michael Bruse of the University of Bochum, Germany ( We chose this model because it simulates microclimate in urban areas, testing interactions between building and natural surfaces, plants, water and air. The model also calculates the dispersion and sedimentation of particles and gases, calculates human outdoor comfort. We found some incompatibilities between the model's focus and local desert conditions, however. The model focuses strongly on vegetation moisture, such as transpiration and sensible heat flux from plants into the air; also moisture exchanges within the soil. Given the low transpiration rates of arid-adapted plants, and the extremely low water table of unirrigated desert landscape, this has proved a challenge. It was therefore important to test the model for basic accuracy before exploring hypothetical alternatives. ENVI-met s grid resolution is from 0.5m to 10m in space and 10 seconds in time; calculations are made during a diurnal cycle with a time frame from 24 to 48 hours. ENVI-met has been widely adapted for urban and environmental planning procedures in European cities, and is a part of the European Union funded research project, BUGS (Benefits of Urban Green Space), which is developing an integrated methodology to alleviate the adverse effects of urbanization ( ). 2. EVALUATING PHOENIX OUTDOOR SPACE Summer temperatures in the Phoenix valley reach daytime highs from 90 to 110 degrees between May and October, with a significant number of highs over 110 degrees in July, August and September. Night- time lows between 50 and 60 degrees. Winters are mild with daytime highs between 70 and 80 degrees, and lows in the 40s. Phoenix is increasingly subject to the heat island effect, as the increase of buildings, concrete and asphalt paving, and dust pollutants prevents daytime heat from escaping at night, obstructing the cooling winds which typically mitigate the Sonoran Desert. The average annual rainfall is 7 inches. To date, each of the 26 municipalities in the Phoenix metropolitan area has a greenway park system as part of its master plan. However, these parkways are rudimentary, diagrammatic and disconnected, and it is generally recognized that guidelines and evaluation tools are necessary for effective greenway design (Valley Forward, 2002), and that the entire greenway system should be refined and extended to incorporate an intricate fabric at the community level. Existing and proposed greenways fall within identifiable categories. Wider greenways of over 200m (found in parts of the Indian Bend Wash) typically include golf courses and other recreation, lakes and meandering trails. Narrower greenways (20-60m) might include cycle and hiking paths along the network of existing canals, extinct railway easements and easements for power lines and other infrastructures throughout the region; these form useful connections with important destinations such as city parks, downtowns and recreation grounds. Narrow greenways (20m) include back alleys, existing trails adjacent to main thoroughfares and freeways, or some desert "washes" or small arroyos which have survived development; these narrow corridors range typically from 10m to 15m and can serve local communities. All greenways move through a mix of residential, commercial and industrial neighborhoods. Planting along linear parks ranges from standard mix of turf and shade trees typical of park landscape in the U.S., to a simulated desert landscape using native trees and shrubs, with a groundcover of dry gravel. The use of native desert plants, and creation of a desert landscape is strongly advocated by architects and environmental planners for ecological and aesthetic reasons (Steiner et al., 1997).

3 Climate studies about cooling desert cities through vegetation stress the relative effectiveness of small pockets of vegetation in cooling a neighborhood. A study of parks in Tel Aviv and Southern Israel, claims that small vegetated sites can function as "semi-solitary micro-oases" in high-temperature environments, providing more effective cooling per square foot than larger parks; maximum cooling was reported in green sites of approximately 60m, which were not only cool on site, but cooled up to 100m beyond the boundary (Shashua Bar, 2000). A Phoenix study of the irrigated Indian Bend Wash shows significant cooling along the borders (Brazel et al., 2000), while a study of the nearby downtown Tempe revealed significant nighttime cooling from irrigated areas 200m away (Crewe et al., 2000). Work in Tucson and Phoenix also advocates the use of trees to mitigate heat from low albedo asphalt paving (Brazel, et al.; McPherson et al., l989; McPherson, l994). Aerial photos have revealed substantially cooler microclimates in well-vegetated residential neighborhoods in the Phoenix area. However, McPherson points to irrigation costs in cooling with vegetation, noting that the most effective cooling comes from heavily irrigated trees such as ficus and mango, rather than native mesquites and acacias which require little water. Research also calls for accurate performance estimates of optimum tree species and turf grass. Wind is an untapped climate resource in the Phoenix area, both to cool the outdoor environment and to flush pollutants. However most prevailing ventilation studies address winds in urban canyons, where building geometry and orientation accelerate ventilation (Oke, l981). There are many examples of successful urban planning which facilitates wind corridors. Planners the city of Stuttgart in Southwestern Germany, regulate building patterns to encourage the nightly flow of cool clean air down ravines into the city; similar city regulations control wind in the Canadian cities of Calgary and Edmonton. In all cases, major development applications require a model simulation of ventilation effects on all adjacent pedestrian paths. Studies of building morphology are relevant to greenways chiefly because they shed light on the spread of cool air into surrounding neighborhoods. A number of studies recommend moderate building density for maximum daytime shading during the day (Pearlmutter, l998; Pearlmutter et al., l999; Stone and Rodgers, 2001). However, in his study of compact building formations in the Negev desert, Israel, Pearlmutter draws attention to some night time summer overheating (Pearlmutter, 1998). In a local study of building densities in Tempe's commercial Mill Avenue, Crewe et al. (2002) found that compact commercial densities promoted significant summer cooling in Phoenix's Mill Avenue, but also some night-time warming. Overall, climatology literature on desert cities has called for a finer resolution of climate information at the urban scale, particularly as affects vegetative cooling, cooling from buildings and wind. At the same time, the literature modeling calls for further studies about the specific applications of climate models to the built environment at the micro-scale (Timmermans, 2002; Brazel and Crewe, 2002). 3. THE MODEL The modeling program has four user interfaces. First it requires input of the basic layout from digital maps; then it involves generating baseline data from other geographic information systems and/or has to generate the data herself/himself in ENVI-met s own cartographic format. The program allows for varied resolutions, from 0.5 to.10 m at the neighborhood scale to 1 km in a regional study. The program consist of modules of working areas such as 130 by 130 maximum, allowing a study area of approximately 1300 m by 1300m. The second interface is database-editing. Physical properties such as soil types, humidity, temperature, and other temporal data are placed in the program for calculation. The third one is the modeling interface, where additional parameters are inserted for a specific modeling run. The output data can be interpreted and visualized through the software program, LEONARDO. It is also possible to convert the data in ENVI-met to other programs since the program structure is public. The first phase of the model was to accurately model the existing data along our chosen section of Indian Bend Wash. A digital map was obtained for the City of Scottsdale Information Systems. Since ENVI-met uses its own graphic interface, all cartographic information had to be redrawn in ENVI-met`s own graphic editor. Likewise all climatic data plus data about existing buildings, plants, soils, and surfaces had to be entered through the ENVI-met interface, conforming to a given list of properties; this involved some comparative matching to the nearest equivalent. The second phase included changing certain variables, such as width, wind direction, planting and surface character. The outputs are basic 3D information on atmosphere, surface and fluxes and soils. Choosing a live site enabled testing of the model in complex ways. This was necessary given the climatic differences between Phoenix and moderate European climates.

4 We chose the Indian Bend Wash, Scottsdale; a 10-mile corridor of open space designed around a natural desert arroyo, and functioning both as public recreational space and a natural drainage channel. The Wash is bordered by houses on either side, and further to the east the 6-lane Hayden Road, a typical Scottsdale arterial and a significant source of traffic pollutants and heat from low-albedo asphalt. We chose this site since it includes a variety of urban prototypes, from a wide swathe of 300m that includes golf courses, lakes, playing fields and wide open lawn, to a narrower belt for cycling, walking and quiet relaxation. To the north the Wash accommodates a single footpath. Building patterns alongside the Wash are prototypical of the Phoenix area, including:! Medium density two to three story condominiums and apartments,! Low density single-family homes,! Commercial complexes typically including a small shopping center with parking lot. For the purposes of this model we selected a study area of 1km by 1 km, which included a greenway of 100m, and some medium and low density housing on either side, with Hayden Road to the east (Figure 3). Figure 3. Study area Indian Bend Wash. Photo 1. Indian Bend Wash character. Photo 2. Natural Linear Park character. Since the ENVI-met model allowed us to prepare a number of basic scenarios, we created additional databases from the existing one by narrowing the existing greenway (Figure 4). 800m Existing; full-scale greenway 100 Typical railroad and power easement 15' Typical small neighborhood trail 0' No greenway; asphalt road between houses Since corridor width is an important factor in greenway planning, given the scarcity of city land, it was useful to test varying impacts depending on width. Since the greenway surroundings remained the same for each width on the model, we could make valid width-related comparisons. Scenarios for testing on the model included:! Gravel ground surface on the Wash (typical of "desert landscaping"! Heavy tree cover, heavy watering! Heavy tree cover, light watering! Tree/ shrub mix (light watering)! Greenway runs east/west. Additional scenarios might include: higher density buildings (through adding buildings to the model), taller buildings (through re-classifying existing buildings on the model), lakes along the greenway (by altering an existing classification of "turf" or "tree cover" to "water body"). Time periods throughout the day are important too, since these affect periods of high use. Heavy use periods in the Phoenix area are from 6-9 am and 5-10 pm, both for commuting and recreation.

5 Figure 4. Adjusted greenway widths. 4. PRELIMINARY RESULTS This section discusses some preliminary results from a succession of model readings from climate data on June 15, Table I shows a sample tabulation. As might have been expected, we have found the most significant cooling within and around the greenway to come from heavy tree cover along a wide corridor (Figure 5). However, narrow well-planted greenways made a significant impact on the neighborhood downwind, particularly in the early evening hours. In all scenarios, when there was significant cooling this affected the area bordering the greenway (not the sidewalk however), the space around neighboring houses, and the corridors leading to the adjacent block. With less cooling, only the greenway borders and the space in front of houses was affected. Cooling was only found on the side affected by wind. Figure 5. Greenway with full tree cover Figure 6. Greenway with gravel landscape Greenway width clearly affected the degree of cooling in the downwind neighborhoods, varying in distance in proportion to the breadth of the corridor and the vegetation. With the full corridor width, the existing turf vegetation and the prevailing wind from east to west, areas of cooling spread some 80 meters from the corridor edge, surrounding three rows of downwind buildings. With the greenway diminished by 50%, there was some heat gain along the adjacent path and at the rear of the adjacent houses. With no corridor, the entire area experienced temperatures at the level of the Hayden sidewalk (Figure 4).

6 Prevailing wind patterns (typically east to west) apply. Model readings from June 15, 2002 data revealed some major wind channels between buildings; here the wind currents were stronger than the wind along the greenway itself. Although the greenway itself was calm, it appeared to generate wind currents within the neighborhood downwind, since there was notably more wind to the west of the 100% corridor than the 50% corridor (Figure 7). However, for all readings there was considerably less wind around the greenway than that adjacent to Hayden Road. Wind activity on the east side of the greenway equaled wind from the 50% corridor. With a simulated change in wind direction from east/west to north/south there were no lateral cooling effects on the neighborhoods (Figure 8). Figure 7. Wind data. Figure 8. Simulation of North-South Wind Direction. As one might expect, maximum cooling was achieved with total tree cover, with lowering of temperature within the greenway and over the entire adjacent neighborhood. Semi-forest produced lesser cooling along the corridor itself, yet almost the same level of cooling within the downwind neighborhood. Tree cover had no apparent effect on the neighborhoods to the west at all. Irrigated turfgrass along the greenway produced some cooling on the downwind neighborhood, but temperatures remained high along the greenway itself. Gravel surfaces along the greenway (minus trees) produced no cooling (Figure 6). For presentation purposes, we have superimposed model data on digital aerial maps (Figure 9) Figure 9. Model data on aerial map (Purple hotter, brown cooler). For tabulating, we selected key pedestrian spots along the greenway that could serve as a planning tool. These included:! Within greenway: recreation, walking dogs! Path along greenway: cycling, short walks! House surroundings! Paths connecting greenway to streets: important greenway access

7 For each model run we noted climatic conditions along these pedestrian spots, noting also basic physical parameters such as corridor width, gravel or grass, time of day, wind direction etc. Processing the data from the model has involved assigning a numerical code from one to three, based on air-cooling and wind; this code will be extended in time to include air pollution, humidity and other qualities. For air temperature, we measure degrees of cooling relative to the Hayden Road sidewalk, taken at the same latitude. We chose this sidewalk as a base measurement because it represents the pedestrian experience without greenway benefits; any cooling below street sidewalk temperatures might be attributed to the greenway. Wind is measured in meters per second. The rating scale follows ratings on the model key, with one notch (or one color change on the model) representing per three degrees Kelvin, or two meters per second in wind speed. For temperature, we assigned a rating of zero (0) for temperatures equaling the sidewalk, one (1) for one notch (three Kelvin) cooler, two (2) for two notches cooler, three (3) for three notches cooler, and so on. We maintained a similar rating system for wind speeds (a numerical count per notch or color change on the model). Table 1. Sample temperature readings from climate data June 15, 2002, at 6.00 pm/. WIDTH, TURF WIDTH, 100% TREES WIDTH, GRAVEL 100% TREE, WIND N/S* House front House rear Corr. Ctr Thru paths Corr. path Hayden sidewalk *Prevailing wind patterns apply 5. CONCLUSION WIDTH, 50% TREES At this preliminary stage, the ENVI-met model has yielded some significant results pertaining to greenways in the Phoenix metropolitan area. The veracity of model results depends on further testing of the Indian Bend Wash during the summer of A second stage in this study is to use the Indian Bend Wash model site as a module for a metropolitan scale model, whereby future development sites can be tested for optimum design. This will draw on a wide range of available data covering present and proposed land uses, demographics, transportation patterns and air pollution levels, as well as the policies of individual cities and neighborhood groups. For further information about this metropolitan-scale model application, please refer to the paper presented at this conference by Izzet Ozkeresteci, Use and Evaluation of the ENVI-met Model for Environmental Design and Planning: an Experiment on Linear Parks. 6. REFERENCES [1] T. Beatley and K. Manning, The Ecology of Place, pub; Island Press, Washington, D.C. (1997). [2] Brazel, N. Selover, R. Vose and G. Heisler, Climate Research, 15 p (2000). [3] Brazel and K. Crewe, Journal of the Arizona-Nevada Academy of Science, 34 p (2002). [4] K. Crewe, Journal of Urban Design, 6 3, p (2001). [5] K. Crewe, Journal of the American Planning Association, 67 4 (2001). [6] K. Crewe, J. Blair and A. Brazel, New Urbanism comes to Phoenix, under review. [7] G. Gammage, Phoenix in perspective : reflection on developing the desert, pub; Herberger Center for Design Excellence, Arizona State University, Tempe, AZ (1999). [8] Girling and K. Helphand, Yard, street, park: the design of suburban open space., pub; J. Wiley, New York (1994). [9] P. Gober and E. K. Burns, Journal of Planning Education Research 21 4, p (2002). [10] J. Gomez-Ibanez, W. Tye and C. Winston, C. (ed.), Essays in transportation economics and policy: a handbook in honor of John R. Meyer, pub; Brookings Institution Press, Washington D.C. (1999). [11] E. Little, Greenways for America, pub; Johns Hopkins University Press, Baltimore (l990). [12] E.G. McPherson, J.R. Simpson and M. Livingston, Energy and Buildings 13, p (1989). [13] E.G. McPherson, E. G., Urbanization and Terrestrial Ecosystems, 24, p (1994). [14] T.R. Oke, Journal of Climatology 1, p (1981). [15] Pearlmutter, D., Environmentally Friendly Cities, pub; Proceedings of Planning for Low Energy Architecture (PLEA), Portugal, p (1998). [16] Pearlmutter, A. Bitan and P. Berliner, Atmospheric Environment, 23, p (1999). [17] D. Pearlmutter and P. Berliner, Atmospheric Environment, 36, p (2000).

8 [18] L. Shashua-Bar, L. and M.F. Hoffman, Geometry and orientation aspects in passive cooling of canyon streets with trees. Architecture, City, Environment, pub, Proceedings of Planning for Low Energy Architecture (PLEA), Cambridge, UK, p (2000). [19] Steiner, L. McSherry, D. Brennan, M. Soden, J. Yarchin, D. Green, J. McCarthy, C. Spellman, J. Jennings, K. Barre, Journal of the American Planning Association 65 2, p (1999). [20] Steiner, The living landscape: an ecological approach to landscape planning, pub; McGraw Hill, New York (2000). [21] Stone and M. O. Rodgers, Journal of the American Planning Association 67 2, p (2001). [22] Timmermans, Decision support systems in urban planning (ed.) pub; E & F. Spon, London (l997).

9 THE POTENTIAL OF CLIMATE MODELING IN GREENWAY PLANNING FOR PHOENIX, ARIZONA Crewe, K. School of Planning and Landscape Architecture, Arizona State University, Tempe, Arizona. Biography Katherine Crewe was born in South Africa, but moved to California in the late l970s. After completing a Masters degree in Landscape Architecture (MLA) at the University of California, Berkeley, she moved to the east coast and practiced landscape architecture in Baltimore and Boston. In l997 she completed a Ph.D. in Planning and Landscape Architecture at the University of Massachusetts, Amherst. Since l998, Crewe has taught at Arizona State University in Tempe, Arizona. Her study areas include the planning and design of linear parks, and also cities and small towns on the urban fringe. She is, in addition, working with the Pima Maricopa Indian Community to re-establish a tribal village on reservation territory.