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Built form thermal effects on the microclimate in the front and the rear open spaces. Case study: Four archetypes of street buildings in the Mediterranean Sea coast region in summer

LIMOR SHASHUA-BAR

Faculty of Architecture and Town Planning

Technion - Israel Institute of Technology

Haifa 32000

ISRAEL

http://www.technion.ac.il/~slimor

Abstract: - This subject is part of a study in progress on the strategic modeling of the urban microclimate. In this paper, the study explores the built form thermal effect and its impact on the climatic effects of other control variables, notably the cooling effect of shade trees. The open spaces considered in the analysis are the front and rear open spaces of four archetypes of street buildings in the Tel-Aviv metropolitan area with spacing between the built-up units. For each case, the diurnal pattern of the air temperature was simulated by the analytical Green CTTC model, using summer data from a representative meteorological station. The emphasis was on the midday values represented by 15:00 h. The simulation results indicate significant built form thermal effects throughout the four building archetypes, and between the front and the rear open spaces. The variation is large but can be satisfactorily explained by the differences in the building geometry as measured by the envelope ratio. The trees cooling effect was found to depend strongly on the built form effect. A strong negative relationship exists between these two climatic effects. In deep open spaces with small spacing distances, the trees cooling effect is considerably reduced. The continuous canyon street type used as a simplification for the actual generic street form may lead to significant underestimation of the actual urban canopy layer microclimate.

Key-Words: - Hot-humid climate, Urban microclimate, Built-form climatic effect, Vegetation climatic effect, Envelope ratio, Green CTTC model.

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1 Introduction

In recent studies, the built form has been found to have a significant effect on the microclimate behavior in different design alternatives of streets and courtyards as shown in Pavilions and courtyard houses [1] in inner courtyards [2], in semi-attached courtyards [3] and in colonnaded built-up units [4,5]. These studies indicate that the accepted geometry measure, the aspect ratio (height to width ratio) [6], shows different thermal impacts on the microclimate of different built-up design alternatives. An overall geometry measure, the envelope ratio, recently proposed in Shashua-Bar et. al [5] is found to have a uniform climatic impact in all the built-up forms studied.

The purpose of this paper is to illustrate, first and foremost, the difference in microclimate behavior in summer between the front and the rear of four archetypes of street buildings situated in the Tel-Aviv metropolitan area. The differences in the climatic effects among the four archetypes and between the front and rear open space of each archetype are explained satisfactorily by the envelope ratio, better than by the height-to-width ratio.

2 Method of Analysis

The analysis of the front and rear open space microclimates is conducted in this study on residential apartment buildings of the type found mostly along urban streets in cities in Israel, each with an attached open courtyard at the rear, and with spacing of a minimum of six meters between the built-up units. This type of buildings is derived from the street generic model [7], a schematic plan of which is shown in Fig. 1. Four residential sites in the Tel-Aviv metropolitan area representing this building type of generic form are shown diagrammatically in Fig. 2. The photographs of the four studied sites are given in Fig. 7, at the end of the paper.

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Fig. 1: Plan representation of the generic building form and definition of length components

Fig. 2 : Diagrammatic illustration of four sites in Tel-Aviv metropolitan area

(a) Rothschild Blvd. (b) Dizengoff Street (c) Recannati Street (d) Ben Zvi Street

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Sites (a) and (b) illustrate the situation in Tel-Aviv city center: Rothschild Boulevard and Dizengoff Street respectively. The number of stories is usually three to five, as indicated by the average height of the buildings (12 m). Site (c), in the Reccanati Street in Ramat-Aviv (a northern suburb of Tel-Aviv), illustrates the case of large high-rise buildings. Site (d), in Ben-Zvi Street in Givataim (an inland town near Tel-Aviv) illustrates the case of a medium high-rise of buildings, with a slight deviation from the generic form shown in Fig. 1.

The data on the building dimensions (H, L2, D), the spacing distance (L1) and the open space width (W) are given in Table 1. The data characterize the behavior of the related open spaces under analysis.

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Table 1: Dimension data of the studied sites

Sites / Height
H
[m] / Facade
L2
[m] / Depth
D
[m] / Spacing distance, L1 [m] / Open space width, W [m]
1. Rothschild Blvd.
Rothschild Blvd. / Front, 1a
Rear, 1b / 12
12 / 14
14 / 15
15 / 6
6 / 48
12
2. Dizengoff Street
Dizengoff Street / Front, 2a
Rear, 2b / 12
12 / 11
11 / 20
20 / 6
6 / 24
12
3. Reccanati Street
Reccanati Street / Front, 3a
Rear, 3b / 50
50 / 30
30 / 30
30 / 20
20 / 50
50
4. Ben-Zvi Street
Ben-Zvi Street / Front, 4a
Rear, 4b / 30
30 / 75
75 / 15,30
15,30 / 22
22 / 30
70

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The object of this study is the prediction of the diurnal air temperature variations in the sites open spaces (front and rear). The simulation tool for estimating the thermal effects is the analytical Green CTTC model [8]. Emphasis in the simulation analysis is on the midday air temperature values, represented by 15:00 h in summer when the heat is the heaviest. Diurnal climatic summer data (for July) on solar radiation, wind velocity, surface and air temperature and humidity were obtained from a nearby meteorological station at Beit-Dagan, representing the Tel-Aviv Mediterranean Sea coast climate. The average values for July air temperature and relative humidity (RH) at this meteorological station were max of 30.2 ºC with RH of 60% at 15:00 h, and min of 24 ºC with RH of 90% at 6:00 h.

The microclimate air temperature is defined in this study as the value averaged over all surface elements of the related open space under analysis. The built form thermal effect is calculated in this study as the difference between the predicted air temperature value of the built-up unit to that at the reference meteorological station (30.2 ºC at hour 15:00 h).

The overall geometry measure used to explain the variation of the thermal effects in different urban sites is the envelope ratio (V). This geometry measure is calculated for each site and is equal to the open ground area divided by the total envelope area of the built-up unit (ground and bordering walls). The measure V is a novel concept developed recently by the author, the rationale behind it is discussed in [5].

3 Results

The climatic effects of the built form are predicted by simulations using the Green CTTC model. The results are shown in Table 2. The figures of the built form effect at 15:00 h indicate a large variation between the site's front and rear open spaces in the four studied building units. The regression analysis of the simulated data in Table 2 shows that the variation is satisfactorily explained by the envelope ratio V (correlation of 0.982) but less than by the aspect ratio H/W (correlation of 0.938). The higher the envelope ratio V (indicating a low aspect ratio or a wide spacing distance relative to the facade length), the warmer the related microclimate.

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Table 2: Estimates of thermal effects of built form and 70% tree coverage at 15:00 h, July data

Sites / Aspect ratio
H / W / Envelope ratio
V / Built form
effect [K] / Trees effect
[k]
1. Rothschild Blvd.
Rothschild Blvd. / Front, 1a
Rear, 1b / 0.25
1.00 / 0.601
0.322 / 2.78
0.67 / -2.61
-1.81
2. Dizengoff Street
Dizengoff Street / Front, 2a
Rear, 2b / 0.50
1.00 / 0.415
0.303 / 1.40
0.36 / -2.08
-1.71
3. Reccanati Street
Reccanati Street / Front, 3a
Rear, 3b / 1.00
1.00 / 0.341
0.341 / 0.58
0.58 / -1.78
-1.78
4. Ben-Zvi Street
Ben-Zvi Street / Front, 4a
Rear, 4b / 1.00
0.43 / 0.377
0.503 / 1.27
1.81 / -2.02
-2.41

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= -1.930 + 7.772 V

St. error (0.252) , (0.612)

Fig. 3: The relationship of the built form effect on air temperature (), at 15:00 h in summer, with the site's envelope ratio (V).

= 0.664 - 1.417 ln (H / W)

St. error (0.133) , (0.214)

Fig. 4: The relationship of the built form effect on air temperature (), at 15:00 h in summer, with the site's aspect ratio (H / W).

The regression lines in Fig. 3 and Fig. 4 describe the built form relationship, estimated by the regression analysis. The extent of the built form effects in these four sites is about 0.4 K at the rear courtyard of Dizengoff site and rises to 2.8 K, warmer than the air temperature at the meteorological station, at the frontal side of the Rothschild Blvd. The range of the thermal built form effects was found to reach much higher values corresponding to variations in the envelope ratio, ranging from ratio of 0.2 to ratio of 0.7, than to H/W between 0.25 to 2.0, and spacing distance relative to frontal length of 0.33 to 1.0 [5].

4 Implications

As illustrated in this study, the building form directly affects the urban open space microclimate. Indirectly, it has been shown in recent studies that the geometry of an urban open space affects the related microclimate through its impact on the thermal effects of other variables in the system of the microclimate mechanism [5,9]. In this respect, vegetation, especially shade trees, play an important role in the mechanism of the microclimate formation. The evaporative cooling of trees in parks and in shallow open spaces of streets accounts for about 3-4 K at midday hours, in summer, in temperate and hot regions [e.g., 10-16], but these estimates can turn to be considerably smaller in deep open spaces, especially in closed courtyards. The present study makes examines this interdependency.

In this study, the simulated trees cooling effect is defined as the difference between the predicted air temperature at the built-up unit with no trees to that with trees. The trees used in the analysis are of well-developed trees of the Ficus species. The parameters used for the Ficus trees: 0.05 for the canopy solar radiation transmissivity and 40% of global solar radiation for evapotranspiration and daily changes in the trees' heat storage [8].

= -1.556 - 0.397

St. error (0.041) , (0.029)

Fig. 5: The relationship of the Ficus trees cooling effect (), at 70% tree coverage, at 15:00 h in summer, with the built form effect ().

= -0.763 - 3.153 V

St. error (0.084) , (0.205)

Fig. 6: The relationship of the Ficus trees cooling effect (), at 70% tree coverage, at 15:00 h in summer, with the site's envelope ratio (V).

The cooling effects of Ficus trees are shown in Table 2 for the 70% tree coverage (this climatic effect is directly proportional to the tree coverage). The estimates are predicted by the Green CTTC model taking into consideration all the variables affecting the microclimate. The variation of the trees cooling effect is also relatively large, ranging from -1.7 K in the courtyard of the Dizengoff building, to -2.4 K in the Ben Zvi courtyard. The range was found to be considerable, from -1.1 K for envelope ratio of 0.2 to -3.3 K for envelope ratio of 0.7 [5].

The regression line in Fig. 5 shows a strong negative relationship (correlation of -0.987) between the trees cooling effect and the built form effect. The warmer the microclimate of the urban open space, the stronger the trees cooling effect for the same tree coverage. Fig. 6 shows the relationship of the trees cooling effect with the envelope ratio. The fit is as good as in Fig. 5 (correlation of -0.984), also showing a negative relationship. The smaller the envelope ratio (as in high H/W), the smaller the trees climatic effect.

5 Conclusions

The study explores the thermal effect of built form and its impact on the climatic effects of other control variables, notably the cooling effects of shade trees. The analysis uses simulations to predict the thermal effects in the front and in the rear open spaces of four archetypes of street buildings. The analysis is conducted on summer data in a hot-humid region.

The main findings are:

1 - The built form effects are significant. They are positively related to the site's envelope ratio, which depends on the open space geometry and on the spacing distance between the units. The deeper the open space (high H/W) and the smaller the spacing distance, the smaller the built form effect.

2 - Significant differences were found to exist between the front microclimate and that at the attached courtyard at the rear of the building. The continuous canyon street type, usually used as a simplification to explore the actual street generic form of buildings with spacing may thus lead to significant underestimation of the actual urban canopy layer microclimate. Particularly, the bias is due to the spacing.

3 - A noticeable negative relationship was found to exist between the built form effect and the trees cooling effect. The stronger the built form effect, the smaller the trees cooling effect. Thus, in deep open spaces (small envelope ratio), the trees cooling effect is low, and it may not pay to use shade trees, especially when in such cases the shade area from the bordering walls is relatively considerable.

Fig. 7: Photographs of the four studied sites

(a) Rothschild Blvd. (b) Dizengoff Street (c) Recannati Street (d) Ben Zvi Street