The structure of urban environments is known to alter local climate, in part due to changes in land cover. A growing subset of research focuses specifically on the UHI in terms of land surface ...temperature by using data from remote sensing platforms. Past research has established a clear relationship between land surface temperature and the proportional area of land covers, but less research has specifically examined the effects of the spatial patterns of these covers. This research considers the rapidly growing City of Phoenix, Arizona in the United States. To better understand how landscape structure affects local climate, we explored the relationship between land surface temperature and spatial pattern for three different land uses: mesic residential, xeric residential, and industrial/commercial. We used high-resolution (2.4 m) land cover data and an ASTER temperature product to examine 90 randomly selected sample sites of 240 square-meters. We (1) quantify several landscape-level and class-level landscape metrics for the sample sites, (2) measure the Pearson correlation coefficients between land surface temperature and each landscape metric, (3) conduct an analysis of variance among the three land uses, and (4) model the determinants of land surface temperature using ordinary least squares linear regression. The Pearson’s correlation coefficients reveal significant relationships between several measures of spatial configuration and LST, but these relationships differ among the land uses. The ANOVA confirmed that mean land surface temperature and spatial patterns differed among the three land uses. Although a relationship was apparent between surface temperatures and spatial pattern, the results of the linear regression indicate that proportional land cover of grass and impervious surfaces alone best explains temperature in mesic residential areas. In contrast, temperatures in industrial/commercial areas are explained by changes in the configuration of grass and impervious surfaces.
Increasing urbanization is likely to intensify the urban heat island effect, decrease outdoor thermal comfort, and enhance runoff generation in cities. Urban green spaces are often proposed as a ...mitigation strategy to counteract these adverse effects, and many recent developments of urban climate models focus on the inclusion of green and blue infrastructure to inform urban planning. However, many models still lack the ability to account for different plant types and oversimplify the interactions between the built environment, vegetation, and hydrology.
In this study, we present an urban ecohydrological model, Urban Tethys-Chloris (UT&C), that combines principles of ecosystem modelling with an urban canopy scheme accounting for the biophysical and ecophysiological characteristics of roof vegetation, ground vegetation, and urban trees. UT&C is a fully coupled energy and water balance model that calculates 2 m air temperature, 2 m humidity, and surface temperatures based on the infinite urban canyon approach. It further calculates the urban hydrological fluxes in the absence of snow, including transpiration as a function of plant photosynthesis. Hence, UT&C accounts for the effects of different plant types on the urban climate and hydrology, as well as the effects of the urban environment on plant well-being and performance.
UT&C performs well when compared against energy flux measurements of eddy-covariance towers located in three cities in different climates (Singapore, Melbourne, and Phoenix).
A sensitivity analysis, performed as a proof of concept for the city of Singapore, shows a mean decrease in 2 m air temperature of 1.1 ∘C for fully grass-covered ground, 0.2 ∘C for high values of leaf area index (LAI), and 0.3 ∘C for high values of Vc,max (an expression of photosynthetic capacity). These reductions in temperature were combined with a simultaneous increase in relative humidity by 6.5 %, 2.1 %, and 1.6 %, for fully grass-covered ground, high values of LAI, and high values of Vc,max, respectively. Furthermore, the increase of pervious vegetated ground is able to significantly reduce surface runoff.
A multi-method approach estimating summer waste heat emissions from anthropogenic activities (QF) was applied for a major subtropical city (Phoenix, AZ). These included detailed, quality-controlled ...inventories of city-wide population density and traffic counts to estimate waste heat emissions from population and vehicular sources respectively, and also included waste heat simulations derived from urban electrical consumption generated by a coupled building energy – regional climate model (WRF-BEM + BEP). These component QF data were subsequently summed and mapped through Geographic Information Systems techniques to enable analysis over local (i.e. census-tract) and regional (i.e. metropolitan area) scales. Through this approach, local mean daily QF estimates compared reasonably versus (1.) observed daily surface energy balance residuals from an eddy covariance tower sited within a residential area and (2.) estimates from inventory methods employed in a prior study, with improved sensitivity to temperature and precipitation variations. Regional analysis indicates substantial variations in both mean and maximum daily QF, which varied with urban land use type. Average regional daily QF was ∼13 W m−2 for the summer period. Temporal analyses also indicated notable differences using this approach with previous estimates of QF in Phoenix over different land uses, with much larger peak fluxes averaging ∼50 W m−2 occurring in commercial or industrial areas during late summer afternoons. The spatio-temporal analysis of QF also suggests that it may influence the form and intensity of the Phoenix urban heat island, specifically through additional early evening heat input, and by modifying the urban boundary layer structure through increased turbulence.
•Urban anthropogenic heat (QF) was estimated over different spatial-temporal scales.•We utilised a novel multi-method approach (inventory and BEM) to estimate QF.•Our approach shows improved QF sensitivity to weather vs. previous methods.•Strong regional variations in QF exist, especially notable over space and time.
Abstract
Global climate is changing as a result of anthropogenic warming, leading to higher daily excursions of temperature in cities. Such elevated temperatures have great implications on human ...thermal comfort and heat stress, which should be closely monitored. Current methods for heat exposure assessments (surveys, microclimate measurements, and laboratory experiments), however, present several limitations: measurements are scattered in time and space and data gathered on outdoor thermal stress and comfort often does not include physiological and behavioral parameters. To address these shortcomings, Project Coolbit aims to introduce a human-centric approach to thermal comfort assessments. In this study, we propose and evaluate the use of wrist-mounted wearable devices to monitor environmental and physiological responses that span a wide range of spatial and temporal distributions. We introduce an integrated wearable weather station that records (a) microclimate parameters (such as air temperature and humidity), (b) physiological parameters (heart rate, skin temperature and humidity), and (c) subjective feedback. The feasibility of this methodology to assess thermal comfort and heat stress is then evaluated using two sets of experiments: controlled-environment physiological data collection, and outdoor environmental data collection. We find that using the data obtained through the wrist-mounted wearables, core temperature can be predicted non-invasively with 95 percent of target attainment within ±0.27 °C. Additionally, a direct connection between the air temperature at the wrist (
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,
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) and the perceived activity level (PAV) of individuals was drawn. We observe that with increased
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, the desire for physical activity is significantly reduced, reaching ‘Transition only’ PAV level at 36 °C. These assessments reveal that the wearable methodology provides a comprehensive and accurate representation of human heat exposure, which can be extended in real-time to cover a large spatial distribution in a given city and quantify the impact of heat exposure on human life.
We examined the horizontal and vertical nocturnal cooling influence of a small park with irrigated lawn and xeric surfaces (∼3 ha) within a university campus of a hot arid city. Temperature data from ...0.01- to 3-m heights observed during a bicycle traverse of the campus were combined with modeled spatial temperature data simulated from a three-dimensional microclimate model (ENVI-met 3.1). A distinct park cool island, with mean observed magnitudes of 0.7–3.6°C, was documented for both traverse and model data with larger cooling intensities measured closer to surface level. Modeled results possessed varying but generally reasonable accuracy in simulating both spatial and temporal temperature data, although some systematic errors exist. A combination of several factors, such as variations in surface thermal properties, urban geometry, building orientation, and soil moisture, was likely responsible for influencing differential urban and non-urban near-surface temperatures. A strong inversion layer up to 1 m over non-urban surfaces was detected, contrasting with near-neutral lapse rates over urban surfaces. A key factor in the spatial expansion of the park cool island was the advection of cooler park air to adjacent urban surfaces, although this effect was mostly concentrated from 0- to 1-m heights over urban surfaces that were more exposed to the atmosphere.
ABSTRACT
Observations of local‐scale urban surface energy balance (SEB), which include fluxes of net all‐wave radiation (Q*), and eddy covariance measurements of sensible (QH) and latent heat (QE) ...were collected in an arid Phoenix, AZ suburb from January to December 2012. We studied diurnal variations in SEB partitioning over four distinct seasons: winter, equinoxes, and summer; the latter period is further subdivided into (1) months prior to and (2) months occurring during the North American Monsoon. Largest flux densities were observed in summer, with most available energy partitioned into QH. Much less energy is partitioned into QE, but this term is strongly affected by monsoonal precipitation, where greater‐than‐average QE can be discerned for several days after storm events. The presence of a positive daily flux residual (RES) i.e. Q* − (QH + QE) for most of the summer indicates that anthropogenic heat (QF) from residential cooling is likely a significant factor influencing SEB. Analysis of hourly ensemble SEB fluxes during all seasons also indicates that RES is largest in the morning, but QH dominates in the afternoon. Results of SEB trends and magnitudes from Phoenix were also compared with other urban sites, especially in (sub)tropical cities. When normalized with net radiation terms, a consistent diurnal hysteresis between ensemble QH and RES occurs, suggesting a robust parameterization of this relationship for model development during clear‐sky conditions. SEB dynamics also appear to be affected by local surface characteristics, with regular nocturnal negative QH associated with a high urban sky‐view factor. Measured QE fluxes during dry seasons were larger than expected based on the small proportion of irrigated plan area vegetated surfaces. A probable explanation could be an enhanced micro‐scale advective forcing of evapotranspiration arising from leading‐edge effects over patchy residential lawns, which has possible implications for modelling evapotranspiration in hot arid cities.
Metropolitan Phoenix has been amongst the most rapidly urbanizing cities in the USA, and is also subject to an urban heat island (UHI) of significant intensity and extent. There is a need to mitigate ...its detrimental effects through sustainable methods, such as through the application of low-water demand, xerophytic trees with broad canopies within residential yards (i.e. xeriscaping). Urban xeriscaping has the potential to reduce urban water use, urban temperatures and outdoor thermal discomfort, but evaluating its effectiveness has not been widely researched. In this study, we used a micro-scale urban climate model (ENVI-met) to generate xeriscaping scenarios in two residential areas with different existing surface vegetation cover (mesic vs. xeric). We subsequently examined the resulting impacts of xeriscaping on near-surface temperatures and outdoor thermal comfort over different spatial scales and temporal periods. Compared to existing conditions, xerophytic shade trees have strong UHI mitigation potential in existing xeric residential areas in Phoenix, with greater cooling occurring at (i.) microscales (∼2.5 °C) vs. local-scales (∼1.1 °C), and during (ii.) nocturnal (0500 h) vs. daytime periods (1700 h) under high xeriscaping scenarios. Conversely, increased xeriscaping resulted in net warming and increased thermal discomfort over mesic residential neighborhoods over all spatial scales and temporal periods. These varying results over different residential land cover in Phoenix therefore must be considered by stakeholders when considering xeriscaping as a UHI mitigation method.
► We examine how xeriscaping scenarios affected residential temperatures and thermal comfort. ► Increased xeriscaping reduced temperatures and thermal discomfort in xeric neighborhoods. ► Cooling magnitudes were greater at (1) microscales and (2) at night vs. local-scales and daytime. ► However, greater xeriscaping increased warmth and thermal discomfort in mesic neighborhoods. ► Its effectiveness as a sustainable UHI mitigation method depends on residential surface cover.
Urban green spaces offer vital ecosystem services such as regulating elevated temperatures in cities. Less information exists, however, on how urban green spaces influence outdoor thermal comfort ...(OTC), which is dependent on people’s perceptions of the complex interactions amongst ambient humidity, wind and both air and radiant temperatures. In this study, we analysed an existing OTC dataset compiled within a large Singapore urban park and calibrated OTC thresholds for physiological equivalent temperatures (PET) by analysing PET against thermal perception survey responses from the park visitors (
n
= 1508). We examined OTC according to (i) neutral, (ii) acceptable and (iii) preferred temperatures, where respondents felt ‘comfortable’ outdoors in the park. We estimated that neutral temperature, when all respondents experience neither heat nor cold stress, is 26.2 °C; acceptable temperatures, when only slight heat or cold stress is experienced, range between 21.6 and 31.6 °C; and preferred (‘ideal’) temperature for all respondents is 24.2 °C. Respondents residing for more than 6 months in Singapore achieved thermal neutrality, suggesting that a greater degree of thermal adaptation likely developed during acclimatisation to local climate through a combination of physiological, behavioural and psychological circumstances. Comparisons with other OTC studies showed differences in synoptic climates are linked to variations in the magnitude and ranges of perceived PET. Lastly, respondents in this study perceived lower neutral and preferred temperatures compared to respondents surveyed over a variety of urban land use categories in another local study. The differences in neutral and preferred temperatures between studies suggest that lower park temperatures and different environmental attitudes influence perceived OTC.
Low-latitude areas generally experience relatively large precipitation totals, but droughts/dry spells do occur periodically and are potentially hazardous in these regions - especially within rapidly ...developing urban settlements. These areas typically have high water demand and therefore may potentially be subjected to water scarcity. Effective local water resource management lowering risks and vulnerabilities to drought is thus paramount, and these policies may be affected in regions with national borders sharing a common transboundary water resource. In this study, we (a) quantify and identify drought episodes using the Palmer Drought Severity Index in the neighbouring equatorial regions of Singapore and Johor, Malaysia, and (b) qualitatively examine each region's drought impacts and consequent responses through archival research over the past fifty years. The data indicate that both frequencies and intensities of drought episodes in both Singapore and Johor have increased over time, suggesting greater exposure to this hazard. However, there are distinct variations in drought impacts in Singapore and Johor, and how each region addresses water resource management to drought with varying degrees of success. Despite the close geographical proximity, significant variations in regional adaptive capacities suggest that different drought vulnerabilities exist. We discuss the efficacy of drought responses over different time scales, and suggest that a combination of demand- and supply-side policies, especially in the long-term, should be considered to reduce vulnerability to this hazard.
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