Integrated thermal effects of generic built forms and vegetation on the UCL microclimate

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Abstract

This paper presents a tool for quantifying the integrated thermal effect of built forms and of vegetation on the urban canopy layer (UCL) climate in design built-up alternatives. Three generic models were studied, representing the most common types of residential urban street: (a) the street form, a conventional type, with spacing between the houses, (b) the canyon form—a limiting case of the street form, (c) the courtyard house form. Recessed colonnades in streets and courtyards were considered in this study as the fourth generic model. The four models were analyzed hierarchically from shallow open spaces to deep ones. For each studied case, the built form effect, the vegetation effect and the colonnade effect were estimated using simulated data generated by the analytical Green CTTC model recently developed by the authors. Emphasis in this study is on the UCL air temperature variation at midday, in summer, in a hot-humid region, near the Mediterranean Sea coast (31–32 °N). Eighty-six simulations were generated for estimating the various thermal effects. In addition, 100 experimental observations at 11 urban wooded sites were analyzed to confirm the simulated effect of the trees. Statistical analysis indicates that each of the thermal effects of the built form, vegetation and of the colonnade can be explained each by one linear relationship, common to all the studied built-up generic models, to a high degree of accuracy and confidence level. This provides a useful general design tool, as opposed to the analysis of a particular simulated case, to assess the potential thermal effects of control variables in different building configurations. The study also considers the extent of the thermal effects of built form, vegetation and colonnades, in streets and in courtyards. These effects are shown to depend, each on the envelope ratio, an overall geometry factor, and thus are interdependent.

Introduction

The relevance of urban geometry to microclimatic features of the urban canopy layer (UCL) has been demonstrated in studies on urban climate. Oke [1] proposed the height-to-width ratio of street canyons—the aspect ratio—now generally accepted as a quantitative surrogate for the UCL geometry. In recent studies, the built form was also found to have a significant effect on the microclimate behavior in different design alternatives of streets and courtyards, as shown in pavilions and enclosed courtyard houses [2], in semi-enclosed courtyards [3], and in the generic street form [4]. The latter represents houses along an urban street with spacing between the units. As an example taken from the study on the generic street form [4], the extent of the thermal impact of the studied configurations in a hot-humid region was about 6.8 K at midday in summer, ranging from 4.7 K above the reference meteorological station, in shallow open spaces with wide spacing, to −2.1 K in deep open spaces with narrow spacing. The detailed mechanism of the built-form effect needs further systematic study.

Besides the built form effect, the most important control variable, which affects the UCL microclimate, is the vegetation, especially shade trees. Apart from providing shade for pedestrians, the evaporative cooling effect of trees in parks and streets accounts for about 3–4 K at midday in summer in temperate and hot regions [5], [6], [7]. The above figure refers to shade trees in large open spaces such as gardens and parks and in streets and open spaces of moderate depth—aspect ratios of about 0.5 [8]. Preliminary study of the effect of urban geometry on the trees cooling effect [7] indicates a significant negative relationship, as illustrated by the fact that the effect of a given area of trees is reduced by deepening the open space.

This paper presents a systematic study of the hierarchical thermal effects of the built form and that of shade trees through analysis of four major generic forms, specifically, the street form of separate buildings along an urban street, the canyon form, the courtyard house form, and the colonnade form. The interrelationship between the variables and generic forms is resolved in this study through the use of a geometry factor—the envelope ratio—that sums up the general features of the units’ open-space geometry.

The vegetation effect was first estimated on the basis of data (100 measurements) collected in a previous study at eleven wooded urban sites in the Tel-Aviv metropolitan area [8]. The empirical evidence indicates a statistically significant relationship between the thermal effect, the density of the trees, and the built-up geometry. The built-form and vegetation effects were also estimated through simulations. The cases studied were: sixty for the street form, five for the canyon form, five for courtyard house form and sixteen for the colonnade form in canyon streets and in courtyards. The diurnal air temperature values for each case were generated using the analytical “Green CTTC model” developed by the authors [9] and applied on summer data for urban sites, in a hot-humid region near the Mediterranean Sea coast (31–32°N). The analytical CTTC model as originally formulated by Swaid and Hoffman [10] and the extended version, the Green CTTC model [9], have been used successfully in predicting the UCL air temperature in various sites. It has been validated in many studies with good agreement with measured data in situ, with and without vegetation [7], [10], [11], [12].

Section snippets

Methodology

The accepted opinion, as evidenced by facts and well noted in vernacular architecture, is that the geometry of an open space has a distinct effect on its microclimate. In hot regions, for example, deep open spaces in alleys and streets are significantly cooler than in shallow ones [7]. The cooling effect in the UCL average air temperature resulting from high aspect ratios is usually credited to the proportional increase in the UCL shade areas, which will be shown to be a misconception (see

Experimental study: sites and observations

The data used in this paper for estimating the vegetation effect are drawn from the authors’ previous field study [8], carried out on calm days in the summer (July–August) of 1996 on eleven urban green areas with Ficus trees, in the Tel-Aviv metropolitan area near the Mediterranean Sea coast. They represent a variety of geometric configurations such as small gardens and courtyards, avenues with and without traffic and canyon streets with trees. About 100 observations were located in the eleven

Setup plan

Statistical analysis of the experimental data which are summarized in Table 2, and the simulated cooling effects in Table 3, Table 4, Table 5, Table 6, were used to determine a general relationship relating the thermal effect in question to a common factor in all the generic forms. In this way, the prediction process is simple to understand and apply.

The statistical analysis for investigating the relationships was performed with linear regressions. The common factor used as explanatory variable

Summary and conclusions

The paper studies hierarchically the mechanism of quantifying the thermal effects of built forms and vegetation on the UCL microclimate. Statistical analysis of a large number of cases in four generic models yielded to general solutions and provided correct guidelines with accurate results. The case under analysis deals with the UCL microclimatic variations at 15:00 h in summer, in a hot-humid region near the Mediterranean Sea coast (31–32 °N). The built form and the vegetation effects were

Acknowledgment

The authors are indebted to E. Goldberg for editorial assistance and comments.

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