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Experimental study of temperature and airflow distribution inside an urban street canyon during hot summer weather conditions—Part I: Air and surface temperatures
ARTICLE IN PRESS
Building and Environment 43 (2008) 1383–1392 www.elsevier.com/locate/buildenv
Experimental study of temperature and airflow distribution inside an urban street canyon during hot summer weather conditions—Part I: Air and surface temperatures K. Niachou, I. Livada, M. Santamouris University of Athens, Physics Department, Section of Applied Physics, Building Physics-5, Laboratory of Meteorology, University Campus, Athens, Greece
Abstract This paper describes the measurements and analysis of an experimental campaign performed in an urban street canyon in Athens, Greece. A number of field and indoor experimental procedures were organized during summer 2002 aiming at the investigation of the impact of urban environment on the potential of natural and hybrid ventilation. The present study is focused on the experimental investigation of thermal characteristics of a typical street canyon, oriented in ESE–WNW direction, under hot weather conditions. The temporal and spatial distribution of air and surface temperatures is examined. Emphasis was given on the vertical distribution of air and surface temperatures and the air temperature profile in the centre of canyon under different weather conditions. The measured surface temperature differences across the street reached almost 30 1C and this favored the overheating of lower air levels. Buoyancy generated mainly from asphalt-street heating resulted in the development of the predominant recirculation inside the street canyon. r 2007 Elsevier Ltd. All rights reserved. Keywords: Urban street canyon; Surface temperature; Air temperature distribution; Stability conditions; Thermal effects
1. Introduction Air circulation and temperature distribution within urban canyons is of high significance for pedestrian comfort, pollutant dispersion [1], radiative and energy studies [2,3] and for the potential of natural and hybrid ventilation in urban buildings [4,5]. Several theoretical [6–8] but only few experimental [9–14] urban street canyon studies are reported in the literature. The thermal stratification and stability evolution of an East-to-West-oriented street in Kyoto, Japan, was investigated experimentally by Nakamura and Oke [15] during August 1983 and 1984. It has been observed that the thermal structure is strongly conditioned by the solar heating of the north floor and to a lesser extent of the northern part of the street. Relative high surface-to-air temperature differences were measured, upto 14 1C, especially close to surfaces exposed to direct solar radiation. Corresponding author. Tel.: +30 2107276847; fax:+30 2107295282.
E-mail addresses: [email protected] (K. Niachou), [email protected] (I. Livada), [email protected] (M. Santamouris). 0360-1323/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2007.01.039
Santamouris et al. [13] studied the thermal characteristics in a deep (H/W ¼ 2.5) pedestrian canyon with a NW–SE axis, under hot weather conditions in Athens, during summer 1997. It was revealed that there is an important surface temperature difference, up to 19 1C, between opposite building walls. Air temperature differences near the two opposite facades varied up to 4.5 1C due to the impact of convection phenomena from adjacent wall surfaces. Thermal effects were studied by Louka et al. [12] and were mainly discussed only when the windward wall was heated, where a thin thermal layer was found locally within a few centimeters from the heated wall. Bourbia and Awbi [16] performed air and surface temperature measurements in east–west and north–southoriented urban street canyons in EL-Oued city, Algeria, during cold and hot periods. The results showed that there were less air temperature variations compared with the surface temperatures due to street geometry and sky view factor. A measurement campaign [17] was performed in five different pedestrian streets, located in different
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Nomenclature g H W L h Vh Tair Ta Ta(3.5m) T0
acceleration due to gravity (m s2) mean height of buildings (m) canyon width (m) canyon length (m) height above the ground equal with 1.2H (m) mean horizontal wind speed above street canyon (ms1) mean air temperature near each building wall (1C) mean air temperature in the centre of street canyon (1C) mean air temperature at height z ¼ 3.5m above the ground (1C) mean air temperature at height z ¼ 0 (1C)
neighbourhoods in the centre of Athens, during summer 2001, in the framework of URBVENT European programme [18]. The field measurements consisted of temperature and wind velocity measurements for a number of five consecutive days, during day period. Notwithstanding, more measurements of temperature and wind distribution are needed in order to analyse the thermal and airflow characteristics inside urban canyons. A more detailed investigation of the previous study was performed by Georgakis and Santamouris [10] and it was focused on a deep urban pedestrian canyon during summer period in Athens. In the present study, a number of field experimental procedures were performed in an urban street canyon, in Athens, aiming at the investigation of the thermal and airflow characteristics during hot weather conditions. It is important to mention that the experimental period was characterized by the highest ambient temperatures of the last 5 years in Athens [19]. The aim is to present the detailed experimental data within a real urban street canyon during hot summer weather conditions. The results of this experiment can be very useful for the understanding of the impact of the urban street microenvironment on the potential of natural and hybrid ventilation in urban buildings, as well as, for the evaluation of street canyon models that focus on this field.
Th
mean air temperature above the top of street canyon (1C) Tamb mean ambient air temperature taken from the nearest meteorological station (1C) Tasphalt mean surface temperature on the asphalt street in the centre of canyon (1C) DTasphalt-air temperature difference between asphalt street and the air layer at 3.5m above the ground ¼ Tasphalt–Ta(3.5m) (1C) Rb bulk Richardson number ¼ gh(Th–To)/ {(Ta+273)(Vh)2} s.d. standard deviation (1C) CV coefficient of variability (%) ( )z quantities at height z ( )h quantities at height h
regard to the surrounding urban scale. The canyon is oriented with its long-axis in an ESE–WNW direction (1001 from the North). The canyon walls were continuous for a 45 m block and the average wall heights were 22 m on the southern fac- ade and 18 m on the northern fac- ade (Fig. 2). The building walls are made of concrete
2. Description of field measurements In the framework of the RESHYVENT measurement campaign [20], a number of experimental procedures were organized in an urban street canyon, located at a highdensity residential area, near the centre of Athens. The experiments were performed on a 24 h basis, inside and outside the canyon during the period 19:20 LT of 19 July to 08:20 LT of 23 July 2002. The studied canyon (Fig. 1), having a height to width ratio (H/W) of 1.7, is located very near to a major high circulation axis and it is typical with
Fig. 1. A photo of the studied urban street canyon with the mobile meteorological station.
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covered with white plaster, while the centre of the canyon is an asphalt road with grey pavement tiles on each side of it. Meteorological data consisting of air and surface temperature measurements and wind velocity were re-
corded on a continuous basis. A detailed description of the field measurements is given in Table 1. The temperature measurements consisted of:
SSW Facade
22m
NNE Facade
P(1)
15.5 m
P(2)
11.5 m
P(16)
7.5 m
P(15)
P(3)
P(17)
P(4)
18m
P(14)
3.5 m P(5)
P(13)
WNW
45m P(6)
P(7)
P(8)
P(9)
P(10) P(11) P(12)
ESE 12m Surface Temperature
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Air Temperature
Fig. 2. A schematic representation of temperature measurements performed inside the urban street canyon during July 2002 in Athens, Greece.
Air temperatures in the centre of canyon: The air temperature was measured at four different heights in the centre of canyon, every 30 s. Four circular design thermometers were attached on a telescopic mast appended onto a mobile meteorological station. The miniature screen formed a housing for a range of temperature sensing elements, providing weather protection while allowing the free passage of air. The accuracy of the thermometers is 70.5 1C (max) under normal meteorological conditions, whilst at low wind speeds and radiation, it is 72 1C (max). Air temperatures near building walls: Outdoor air temperature was also measured inside the canyon and near opposite building walls, every 30 s. Small ambient temperature data loggers were shielded inside a white painted wooden cylinder that permitted air circulation. The sensors have an accuracy of 70.2 1C and they were completely protected from solar radiation. Surface temperatures at street level: Surface temperatures were conducted on the ground by an infrared thermometer equipped with a laser beam, every one hour. The surface temperatures were recorded across the canyon axis on the asphalt street and on the sidewalks on each side of the road. The emissivity value used for the measured materials was considered equal with 0.90.
Table 1 Description of the field experiments in an urban street canyon during July 2002 in Athens, Greece Air temperatures
Surface temperatures
Wind velocities
Inside canyon
At a cross section from SSW to NNE building at street level and on the building walls.
Inside canyon
In the center At 3.5, 7.5, 11.5 and 15.5 m from the ground. Near building walls At a distance of 0.5 m from the exterior walls and at a height of 14 m (near SSW fac- ade) and 10 m (near NNE fac- ade)
SSW Fac- ade Point 1 [4th floor (white plaster)] Point 2 [3rd floor (white plaster)] Point 3 [2nd floor (white plaster)] Point 4 [1st floor (white plaster)] Point 5 [ground (white plaster)]
In the center Horizontal wind speed and direction was measured at 3.5, 7.5, 11.5 and 15.5 m from the ground. Near building walls u, v, w wind velocity components were measured at a distance of 2 m from the exterior building walls and at a height of 15.5 m (near SSW fac- ade) and 11.5 m (near NNE fac- ade) from the ground. Outside canyon Horizontal wind speed and direction was measured at a distance of 24 m from the ground.
Street level Point 6 [grey pavement tiles] Point 7 [grey pavement tiles] Point 8 [asphalt] Point 9 [asphalt] Point 10 [asphalt] Point 11 [grey pavement tiles] Point 12 [grey pavement tiles] NNE Fac- ade Point 13 [Ground (white plaster)] Point 14 [1st floor(white plaster)] Point 15 [2nd floor(white plaster)] Point 16 [3rd floor (white plaster)] Point 17 [4th floor (white plaster)]
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Surface temperatures on building walls: Surface temperatures were also measured on opposite building walls with the infrared thermometer. Measurements were performed from the ground level up to the fourth floor, every 1 h.
In order to study further, the impact of thermal effects on airflow inside the canyon, simultaneous measurements of wind velocity inside and above the street canyon were performed on a consecutive basis during the 24 h period (Table 1). 3. Analysis of surface temperatures 3.1. Street temperatures The street temperatures were measured on an hourly basis, at a number of seven points across the asphalt street and the sidewalks (Fig. 2). The maximum surface temperatures reached almost 64 1C, during midday, due to the vertical incidence of solar radiation, as shown in Fig. 3 (point 10). The pavement near the SSW facade (point 6) presented maximum temperatures up to 55 1C, which were mostly observed 1 h before the maxima on the street centre. On the contrary near the NNE fac- ade (point 12), the street temperatures were much lower and rarely exceeded 30 1C. The maximum daily temperature amplitudes across the street reached 31 1C near the south facing wall (point 6) and 35 1C on the asphalt street (point 8) due to the incident solar radiation, however near the north facing wall (point 12) these amplitudes hardly ever exceeded 6 1C. The surface temperature differences between the daily maximum values across the canyon reached 33 1C, as a function of the optical and physical properties of street materials and the variability of solar radiation. However, for the same construction materials, the corresponding maximum temperature differences ranged from 9 1C on the sidewalks, up to 28 1C on the asphalt street.
During daytime period and until 9:00 LT, the temperature distribution was uniform as during night. At noontime period and around 15:00 LT, the surface temperatures of the north part of the street showed a maximum increase of 25 1C in comparison with the corresponding temperature during the morning (9:00 LT). The highest surface temperatures were always observed near the centre and the north part of the asphalt street, while the lowest values were measured on the sidewalk near the south part of the canyon floor. This favoured the overheating of the lower air levels and thus resulted in the development of upward movements near the south facing building wall. During the night, the distribution of the surface temperatures across the canyon axis began to stabilize after 22:00 LT. The maximum surface temperatures (up to 38 1C) were observed on the asphalt street and near its centre, whilst the lowest temperatures were found on the grey pavement tiles near the NNE fac- ade. The F-Anova test [21] was applied on the means of the surface temperatures across the street and showed that the temperature differences between them are statistically significant at the significance level (s.l.) of 0.05. 3.2. Wall temperatures Surface temperatures were measured on the opposite building walls from the ground up to the fourth floor, on an hourly basis during the 24 h period. As expected, the thermal behaviour of the two opposite walls is more complex due to parameters affecting the thermal balance of building materials (physical properties and canyon geometry) and due to incident solar and emitted infrared radiation. Fig. 4 shows the distribution of wall temperatures, with the form of boxplots, during day (07:00–21:00 LT) and night (22:00–06:00 LT) period. In general, it has been observed that during day period, the SSW fac- ade presented higher temperatures than the opposite wall. The absolute maximum surface temperature amplitudes along the canyon walls during daytime were up to 13 1C on the
Surface Temperatures (°C)
65 60 55 50 45 40 35 30 25 20 0:00
Point 6 6:00
12:00
20/07/2002
18:00
Point 8 0:00
6:00
12:00
18:00
21/07/2002
Point 12 0:00
6:00
12:00
18:00
0:00
22/07/2002
Fig. 3. Hourly distribution of street temperatures (point 8: on the asphalt street and points 6 and 12: on the sidewalks near SSW and NNE facades).
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Wall Temperatures (°C)
a
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40
35
30
25 SSW Facade
NNE Facade
20 P(1)
b
P(2)
P(3)
P(4)
P(5) P(13) P(14) P(15) P(16) P(17)
Measured Points 40
Wall Temperatures (°C)
Outliers 35
Max 75th percentile
30
Average 25th percentile
25 Min
NNE Facade
SSW Facade 20 P(1)
P(2)
P(3)
P(4)
P(5) P(13) P(14) P(15) P(16) P(17)
Measured Points Fig. 4. Boxplots of wall temperatures on SSW and NNE fac- ades (from the ground up to the fourth floor) for day (a) and night (b) during the experimental period (points are numbered as shown in Fig. 2).
SSW fac- ade (at the fourth floor) and 7 1C on the NNE fac- ade (at all floors), as a result of the incident solar radiation. During the night, these values were reduced for both building walls and were measured up to 4 1C at the ground floor mainly due to reduced wind speeds near the street level. The vertical distribution of the surface temperatures on the opposite buildings was studied during day and night, since it is important to understand the transfer phenomena between them and the adjacent air. During daytime period, the temperature differences on the SSW fac- ade reached almost 5 1C between the ground and the fourth floor. The highest temperatures occurred on the ground floor around 16:00–18:00 LT. This was due to reduced wind speeds at the lower levels and heat transfer phenomena from the asphalt heating. On the NNE fac- ade and during the day period, the temperature differences varied up to 4 1C again between the ground and the fourth floor, while the differences between the middle height surfaces were not significant, either for the SSW or NNE facade.
Table 2 Simultaneous wall temperature differences (TSSW–TNNE) from the ground level up to the forth floor during day and night period (TSSW: wall temperature on SSW fac- ade, TNNE: wall temperature on NNE fac- ade) TSSW–TNNE
Ground floor
1st floor
2nd floor
Day period Mean s.d. Max
1.46 1.42 5.00
1.15 1.44 4.00
0.50 1.15 5.00
0.50 1.68 4.00
1.00 2.16 6.00
Night period Mean s.d. Max
0.63 0.65 2.00
0.50 0.66 2.00
0.25 1.19 2.00
0.04 1.12 3.00
0.68 1.09 3.00
3rd floor
4th floor
In order to investigate the impact of canyon orientation on the wall temperatures, the differences of the instantaneous wall temperatures of the same height surfaces of the two opposite buildings, as well as, the differences of the maximum wall temperatures during day and night period
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have been compared (Table 2). This comparison analysis permits to evaluate at a certain degree the investigation of the impact of the absorbed solar radiation on the thermal balance of the measured surfaces, considering that the other terms of the thermal balance are almost similar. The simultaneous temperature differences between the opposite canyon walls during the day were higher at the fourth floor, up to 6 1C, because of the increased solar radiation and at the ground floor, up to 5 1C, as a result of the higher ground temperatures and the reduced wind effect. During the night period, these temperature differences are seriously reduced and the maximum differences were observed between the ground floors. Nevertheless, these differences reached 3 1C as a result of the direct solar heating. In almost all cases, where a significant temperature difference was observed, the SSW facing wall was warmer than the opposite building wall and especially on the ground floor. This could be explained by the fact that lower level surfaces have lower sky view factors and thus radiative losses to the sky are smaller. The comparison between the surface temperatures on the ground level and on the building walls led to the following results: during the day period, the maximum surface temperatures were higher on the street level and the absolute maximum temperature difference was 25 1C. This was expected considering that the incident solar radiation on a horizontal surface is greater during summer. However, the physical properties of the compared materials are different, since the asphalt street has a higher absorptivity than the white plaster of the building walls. During the night period, the absolute maximum surface temperature difference between the ground and the building walls was 6 1C. The positive radiative balance at the street level is directly related to the canyon geometry and the thermal capacities of the materials on the street and on the building walls. These observations will be determinant for the interpretation of air circulation inside the street canyon.
4. Analysis of air temperatures In order to understand the mechanisms that determine the distribution of air temperature inside a canyon, air temperature measurements have been performed in the centre of canyon and close to opposite building facades. The air temperature distribution has been analysed in order to investigate the impact of the street layout and orientation, as well, of the surface temperatures due to convection heat transfer phenomena. The ambient air temperature values were taken by the nearest meteorological station, namely National Observatory of Athens (NOA).
4.1. Air temperature distribution in the centre of canyon The air temperature distribution, based on the 5 min mean air temperature values, does not seem to show significant variations with height near the centre of canyon (Fig. 5). However, the estimation of air temperature lapse rates (1C/100 m) between 3.5 and 15.5 m (Fig. 6) resulted in a number of interesting remarks. During the day, a temperature inversion was observed around 8:00–10:00 LT reaching almost 7.2 1C/100 m, which is much higher than the average normal gradient (0.6 1C/100 m). Besides, this temperature inversion coincided with the minimum of temperature differences between the asphalt street and the air layer at 3.5m above the ground (DTasphalt-air). Later in the morning, the atmosphere became unstable, as a result of the direct solar heating of asphalt street and of building walls. As shown in Fig. 6, the instability presents its maximum intensity, up to 8 1C/100 m, around noon-time period, when the maximum surface-air temperature differences reached almost 30 1C inside the canyon. Moreover, air temperature inversions were also observed during noon (around 14:00–15:00 LT), when higher air temperatures were measured above the canyon and for almost isothermal conditions near the bottom of canyon.
38
Air Temperatures (°C)
36 34 32 30 28 26 Ta (3.5m) 24 0:00
6:00
12:00 18:00 20/7/02
Ta (15.5m) 0:00
6:00
12:00 18:00 21/7/02
Ta (NOA) 0:00
6:00
12:00 18:00
0:00
22/7/02
Fig. 5. Evolution of air temperature in the centre of canyon at different heights (at 3.5 and 15.5 m) and in the ambient air (from NOA station).
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5
0
-5
Lapse Rate -10 0:00
6:00 12:00 18:00 0:00 20/7/02
6:00 12:00 18:00 0:00 Time (h) 21/7/02
Δ Tasphalt-air 6:00 12:00 18:00
35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 0:00
Asphalt-Air Temperature Differences (°C)
Lapse Rates (°C/100m)
10
1389
22/7/02
Fig. 6. Temperature differences between asphalt street and air (at 3.5 m) and lapse rates (1C/100 m, between 3.5 m and 15.5 m) in the centre of canyon.
1.2 Rb=-0.42 20:00-20:10LT
1.0
Rb=-0.48 19:00-19:10LT
z/H
0.8 0.6
Rb=-0.63 18:00-18:10LT
0.4
Rb=-0.64 16:00-16:10LT
0.2
Rb=-0.69 15:00-15:10LT
0.0
Rb=-0.85 17:00-17:10LT
0.0
0.2
0.4 0.6 0.8 (Ta-Tasphalt)/(Tamb-Tasphalt)max
1.0
1.2
Fig. 7. Normalized temperature profiles, in the centre of street canyon during 15:00–20:10 LT on 22nd of July 2002 for different instability values (Rb).
In order to decide whether the ambient temperatures from NOA station are representative of the air temperatures above the canyon, the mean air temperature values at 15.5 m inside the canyon were compared with the corresponding ambient temperatures of NOA station during day and night period. The statistical t-test of the means was applied on air temperatures and it was found that either during the day (jtj ¼ 0.629ot0.05E2) or night (jtj ¼ 0.397ot0.05E2), the temperature differences are not statistical different at the s.l. of 0.05. As a result, the ambient temperatures from NOA station can be considered equal with the air temperatures above the canyon. From the comparison analysis of the ambient temperatures of NOA station with the simultaneous air temperatures at the lowest measured level (at 3.5 m) inside the canyon during day (jtj ¼ 0.262ot0.05E2) and night (jtj ¼ 0.006ot0.05E2), it has been found that the differences are not also statistical significant. However, when the simultaneous air temperature differences (Fig. 5) are examined, it has been calculated that
ambient temperatures are higher, in almost 83%, during daytime period (with the maximum differences up to 1.9 1C), than the air temperature above the street (at 3.5 m). Similar results are reported by Georgakis and Santamouris [10], where the ambient temperatures on the top of a deep canyon in the centre of Athens were approximately 5 1C than that measured air temperatures inside the canyon. In the present canyon, the measured air temperatures during night period were found higher near the ground than in the ambient air, in almost 82% of the cases, though the maximum differences did not exceed 1 1C. This is in agreement with Santamouris et al. [13] who measured that air temperatures inside a deep pedestrian canyon were higher up to 1.5 1C compared with ambient temperatures above the canyon. 4.1.1. Air temperature profile In spite of the small vertical air temperature differences inside the canyon, the temperature profile was studied
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for different stability weather conditions. Stability was indicated by the bulk Richardson number [22], Rb ¼ gh(Th–T0)/{(Ta+273)(Vh)2}, where T0 was approximated by the mean surface temperature on the asphalt street, Tasphalt, that was estimated as the mean value of the surface temperatures at points 8, 9 and 10 (Fig. 2). Th was considered equal with the ambient temperature from NOA station, while Ta was estimated as the mean value of air temperatures at the four different heights in the centre of canyon. According to Oke [23], Rb number becomes negative for unstable weather conditions. During day period, unstable conditions were measured in 85% of the cases, while during night this value was reduced to 64%. Thus, it is interesting to study the vertical air temperature distribution under unstable weather conditions. Fig. 7 shows the temperature profiles in centre of street canyon, during unstable weather conditions (0.85pRbp0.42), normalized with the absolute maximum temperature difference between the ambient air and the asphalt street. It can be observed that instability increased (Rb ¼ 0.85) when the street-air (at 3.5 m) temperature differences (Tasphalt–Ta), in the central part of the street canyon increased. However, the vertical air temperature differences inside the canyon are very small for all Rb values. This was expected since when the atmosphere becomes unstable, mainly due to ground heating, the mixing inside the canyon is increased and the vertical temperature gradient is very small. 4.2. Air temperature near canyon facades The air temperature distribution across the canyon is of great interest. Roth et al. [24] and Stoll and Brazel [25] reported that in the middle of canyon and near the ground level air temperature was more dependent upon the flux divergence in air volume including that of the horizontal transport. Air temperature measurements were measured near the opposite building walls, as shown in Fig. 1. The measured air temperature differences between the two facades vary, as a function of the canyon layout and the
surface characteristics. As it was expected, the air temperature close to SSW facade was higher than near NNE facade. The mean value of the instantaneous temperature differences between the two opposite facades during the day period was close to 3 1C, while the absolute maximum air temperature difference was 5.4 1C. The highest differences were observed during the early afternoon period around 16:00–17:00 LT and the lowest in the early morning around 7:00 LT (Fig. 8). Comparison of surface and air temperatures (Table 3) during daytime shows that in almost all cases near the NNE fac- ade, the surface temperature is higher than the adjacent air temperature up to 3.3 1C. The SSW fac- ade presented higher temperature values than the air close to it, up to 3.7 1C, around 14:00 LT. However, in some cases, the air temperature is higher than the corresponding surface temperatures by almost 2.7 1C (during noon period). Similar conclusions are reported by Nakamura and Oke [15] in an east-west urban canyon when higher air than wall temperatures were observed during midday period, which was explained probably due to the warming of the air volume by the combined effects of turbulent sensible heat
Table 3 Surface and air (near the wall) temperature differences (TSSW–Tair and TNNE–Tair) during day and night period TSSWTair
TNNETair
Day period Mean s.d. CV (%) Max Min
0.87 1.41 162 3.30 2.70
2.02 1.22 60 3.70 0.40
Night period Mean s.d. CV (%)* Max Min
1.35 0.58 43 0.40 2.70
0.59 0.81 137 0.60 2.20
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Air Temperatures (°C)
36 34 32 30 28 26 Tair (SSW Facade) 24 0:00
6:00
12:00 18:00 20/7/02
0:00
6:00
Tair (NNE Facade) 12:00 18:00 21/7/02
0:00
6:00
12:00 18:00 22/7/02
Fig. 8. Evolution of air temperature near opposite building facades (SSW and NNE).
0:00
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transfer from the north wall and its mixing via the air circulation. The wall temperatures were lower than the adjacent air temperatures during night (Table 3). Near SSW fac- ade, the air presented higher temperatures up to 2.7 1C in comparison with the wall temperatures. Similarly, near NNE fac- ade the absolute maximum wall–air temperature differences were measured equal with 2.2 1C. The maximum surface temperature differences either on the SSW or the NNE facade were observed in the early midnight hours and around 1:00 LT. 5. Conclusions The analysis of temperature distribution in an ESE–WNW-oriented urban street canyon has been performed on a 24 h basis, during a short measurement campaign in Athens under hot summer conditions. The analysis of the field measurements has led to the following observations: The air temperature distribution in the centre of canyon was not characterized by statistical significant variations with height. A strong temperature inversion, up to 7.2 1C/100 m, was observed during the morning that coincided with the minimum of temperature differences between the asphalt street and the air layer at 3.5 m above the street. During the day the atmosphere became unstable, as a result of the direct solar heating of the asphalt street and building walls and thus the normalized temperature profile in the centre of street canyon remains the same under different unstable conditions. Generally speaking, under different weather stability conditions, the air temperature differences inside and outside the canyon were found very small. Regarding the air temperatures near the opposite building walls the highest values were measured, as it was expected, close to SSW than near NNE facade. The highest differences up to 5.4 1C, were observed during the early afternoon hours around 16:00–17:00 LT and the lowest in the early morning around 7:00 LT. Comparison of the hourly surface temperatures measured on the opposite building walls from the ground level up to the fourth floor has shown that the SSW facade presented greater temperatures than the opposite. The measured temperature differences between the opposite canyon walls during the day were higher at the fourth floor, up to 6 1C, because of the increased solar radiation and at the ground floor, up to 5 1C, as a result of the higher ground temperatures and the reduced wind effect. Analysis of the street temperatures across a section from North to South facing walls revealed the existence of maximum surface temperatures on the asphalt street up to 60 1C, during midday, due to the vertical incidence of solar radiation. The pavement near the SSW facade presented maximum temperatures equal with 55 1C, which was measured 1 h before the maxima on the street centre. On the contrary near the NNE fac- ade, the street temperatures ranged between 25 and 30 1C. The measured hourly surface
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temperature differences reached almost 30 1C between the north and south part of the street and this favoured the overheating of lower air levels and thus resulting in the development of upward movements near the south facing canyon wall. The above results indicate that the air temperature distribution inside a street canyon is function of canyon geometry and orientation, as well as, of the optical and thermal properties of building and street materials and ambient weather conditions. Furthermore, the understanding of the specific thermal characteristics is essential for the understanding of airflow inside the canyons and for the studies of natural and hybrid ventilation in the urban environment. Acknowledgments The present research has been performed in the framework of the research project RESHYVENT, which was financed by the Fifth Framework Programme of the European Commission, Directorate General for Science, Research and Technology under the Contract ENK6CT2001-00533. The contribution of the Commission is gratefully acknowledged. References [1] Vardoulakis S, Fisher BEA, Perikleous K, Gonzalez-Flesca N. Modelling air quality in street canyons: a review. Atmospheric Environment 2003;37:155–82. [2] Numez M, Oke TR. Long wave radiative flux divergence and nocturnal cooling of the urban atmosphere II. Within an urban canyon. Boundary-Layer Meteorology 1976:121–35. [3] Numez M, Oke TR. The energy balance of an urban canyon. Journal of Applied Meteorology 1977;16:11–9. [4] Niachou K, Hassid S, Santamouris M, Livada I. Comparative monitoring of natural, hybrid and mechanical ventilation systems in urban canyons. Energy and Buildings 2005;37(5):503–13. [5] Santamouris M. Energy and climate in the urban environment. London: James and James; 2001. [6] Hunter LJ, Watson ID, Johnson GT. Modelling air flow regimes in urban canyons. Energy and Buildings 1990/1991;15–16:315–24. [7] Hunter LJ, Johnson GT, Watson ID. An investigation of threedimensional characteristics of flow regimes within urban canyon. Atmospheric Environment 1992;26B:425–32. [8] Oke TR. Street design and urban canopy layer climate. Energy and Buildings 1988;11:103–13. [9] Georgakis C, Santamouris M. On the airflow in urban canyons for ventilation purposes. International Journal of Ventilation 2004; 3(1):1–9. [10] Georgakis C, Santamouris M. Experimental investigation of air flow and temperature distribution in deep urban canyons for natural ventilation purposes. Energy and Buildings 2006;38(4):367–76. [11] Longley ID, Gallagher MW, Dorsey JR, Flynn M, Barlow JF. Shortterm measurements of airflow and turbulence in two street canyons in Manchester. Atmospheric Environment 2004;38(1):69–79. [12] Louka P, Vachon G, Sini JF, Mestayer PG, Rosant JM. Thermal effects on the airflow in a street canyon—Nantes’ 99 experimental results and model simulations. Journal of Water, Air and Soil Pollution 2002;2:351–64. [13] Santamouris M, Papanikolaou N, Koronakis I, Livada I, Assimakopoulos DN. Thermal and air flow characteristics in a deep
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K. Niachou et al. / Building and Environment 43 (2008) 1383–1392 pedestrian canyon and hot weather conditions. Atmospheric Environment 1999;33:4503–21. Vachon G, Rosant JM, Mestayer P, Louka P, Sini JF, Delaunay D, et al. Experimental investigation of pollutant dispersion within a street in a low wind conditions, the experiment Nantes’99. In: Ninth international symposium ‘Transport and Air Pollution’, Avignon, France. INRETS 2000, p. 95–102. Nakamura Y, Oke TR. Wind, temperature and stability conditions in an E–W oriented urban canyon. Atmospheric Environment 1988; 22(12):2691–700. Bourbia F, Awbi HB. Building cluster and shading in urban canyon for hot dry climate. Part 1: air and surface temperature measurments. Renewable Energy 2004;29:249–62. Allard F, Ghiaus C. Natural ventilation in the urban environment. London: James and James; 2005. URBVENT. European Research Project on Natural Ventilation in Urban Areas for Optimal Openings Design. WP2 Final report, Joule Programme, Project Contract No. ENK 6-CT-2000-00316; 2000. Livada I, Niachou K, Santamouris M. The impact of wind on air temperature distribution in Athens and in Santorini. Proceedings of
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passive and low energy cooling for the built environment (PALENC) conference, vol. 1. 2005. p. 139–143. RESHYVENT. Cluster project on demand controlled hybrid ventilation in residential buildings with specific emphasis of the integration of renewables. European Joule Project, Contract No. ENK6-CT2001-00533; 2002. Zar JA. Biostatistical Analysis. Enjlewood Cliffs, New York: Prentice-Hall Inc.; 1974. Uehara K, Murakami S, Oikawa S, Wakamatsu S. Wind tunnel experiments on how thermal stratification affects flow in and above urban street canyons. Atmospheric Environment 2000;34: 1553–62. Oke TR. Boundary layer climates. 2nd ed. UK: Methuen; 1987. Roth M, Oke TR, Emery WJ. Satellite—derived urban heat islands from three coastal cities and the utility of such data in urban climatology. International Journal of Remote Sensing 1989;10: 1699–720. Stoll MJ, Brazel AJ. Surface air–temperature relationships in the urban environments of Phoenix. Arizona, Physics Geography 1992;13:160–79.
Building and Environment 43 (2008) 1383–1392 www.elsevier.com/locate/buildenv
Experimental study of temperature and airflow distribution inside an urban street canyon during hot summer weather conditions—Part I: Air and surface temperatures K. Niachou, I. Livada, M. Santamouris University of Athens, Physics Department, Section of Applied Physics, Building Physics-5, Laboratory of Meteorology, University Campus, Athens, Greece
Abstract This paper describes the measurements and analysis of an experimental campaign performed in an urban street canyon in Athens, Greece. A number of field and indoor experimental procedures were organized during summer 2002 aiming at the investigation of the impact of urban environment on the potential of natural and hybrid ventilation. The present study is focused on the experimental investigation of thermal characteristics of a typical street canyon, oriented in ESE–WNW direction, under hot weather conditions. The temporal and spatial distribution of air and surface temperatures is examined. Emphasis was given on the vertical distribution of air and surface temperatures and the air temperature profile in the centre of canyon under different weather conditions. The measured surface temperature differences across the street reached almost 30 1C and this favored the overheating of lower air levels. Buoyancy generated mainly from asphalt-street heating resulted in the development of the predominant recirculation inside the street canyon. r 2007 Elsevier Ltd. All rights reserved. Keywords: Urban street canyon; Surface temperature; Air temperature distribution; Stability conditions; Thermal effects
1. Introduction Air circulation and temperature distribution within urban canyons is of high significance for pedestrian comfort, pollutant dispersion [1], radiative and energy studies [2,3] and for the potential of natural and hybrid ventilation in urban buildings [4,5]. Several theoretical [6–8] but only few experimental [9–14] urban street canyon studies are reported in the literature. The thermal stratification and stability evolution of an East-to-West-oriented street in Kyoto, Japan, was investigated experimentally by Nakamura and Oke [15] during August 1983 and 1984. It has been observed that the thermal structure is strongly conditioned by the solar heating of the north floor and to a lesser extent of the northern part of the street. Relative high surface-to-air temperature differences were measured, upto 14 1C, especially close to surfaces exposed to direct solar radiation. Corresponding author. Tel.: +30 2107276847; fax:+30 2107295282.
E-mail addresses: [email protected] (K. Niachou), [email protected] (I. Livada), [email protected] (M. Santamouris). 0360-1323/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2007.01.039
Santamouris et al. [13] studied the thermal characteristics in a deep (H/W ¼ 2.5) pedestrian canyon with a NW–SE axis, under hot weather conditions in Athens, during summer 1997. It was revealed that there is an important surface temperature difference, up to 19 1C, between opposite building walls. Air temperature differences near the two opposite facades varied up to 4.5 1C due to the impact of convection phenomena from adjacent wall surfaces. Thermal effects were studied by Louka et al. [12] and were mainly discussed only when the windward wall was heated, where a thin thermal layer was found locally within a few centimeters from the heated wall. Bourbia and Awbi [16] performed air and surface temperature measurements in east–west and north–southoriented urban street canyons in EL-Oued city, Algeria, during cold and hot periods. The results showed that there were less air temperature variations compared with the surface temperatures due to street geometry and sky view factor. A measurement campaign [17] was performed in five different pedestrian streets, located in different
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Nomenclature g H W L h Vh Tair Ta Ta(3.5m) T0
acceleration due to gravity (m s2) mean height of buildings (m) canyon width (m) canyon length (m) height above the ground equal with 1.2H (m) mean horizontal wind speed above street canyon (ms1) mean air temperature near each building wall (1C) mean air temperature in the centre of street canyon (1C) mean air temperature at height z ¼ 3.5m above the ground (1C) mean air temperature at height z ¼ 0 (1C)
neighbourhoods in the centre of Athens, during summer 2001, in the framework of URBVENT European programme [18]. The field measurements consisted of temperature and wind velocity measurements for a number of five consecutive days, during day period. Notwithstanding, more measurements of temperature and wind distribution are needed in order to analyse the thermal and airflow characteristics inside urban canyons. A more detailed investigation of the previous study was performed by Georgakis and Santamouris [10] and it was focused on a deep urban pedestrian canyon during summer period in Athens. In the present study, a number of field experimental procedures were performed in an urban street canyon, in Athens, aiming at the investigation of the thermal and airflow characteristics during hot weather conditions. It is important to mention that the experimental period was characterized by the highest ambient temperatures of the last 5 years in Athens [19]. The aim is to present the detailed experimental data within a real urban street canyon during hot summer weather conditions. The results of this experiment can be very useful for the understanding of the impact of the urban street microenvironment on the potential of natural and hybrid ventilation in urban buildings, as well as, for the evaluation of street canyon models that focus on this field.
Th
mean air temperature above the top of street canyon (1C) Tamb mean ambient air temperature taken from the nearest meteorological station (1C) Tasphalt mean surface temperature on the asphalt street in the centre of canyon (1C) DTasphalt-air temperature difference between asphalt street and the air layer at 3.5m above the ground ¼ Tasphalt–Ta(3.5m) (1C) Rb bulk Richardson number ¼ gh(Th–To)/ {(Ta+273)(Vh)2} s.d. standard deviation (1C) CV coefficient of variability (%) ( )z quantities at height z ( )h quantities at height h
regard to the surrounding urban scale. The canyon is oriented with its long-axis in an ESE–WNW direction (1001 from the North). The canyon walls were continuous for a 45 m block and the average wall heights were 22 m on the southern fac- ade and 18 m on the northern fac- ade (Fig. 2). The building walls are made of concrete
2. Description of field measurements In the framework of the RESHYVENT measurement campaign [20], a number of experimental procedures were organized in an urban street canyon, located at a highdensity residential area, near the centre of Athens. The experiments were performed on a 24 h basis, inside and outside the canyon during the period 19:20 LT of 19 July to 08:20 LT of 23 July 2002. The studied canyon (Fig. 1), having a height to width ratio (H/W) of 1.7, is located very near to a major high circulation axis and it is typical with
Fig. 1. A photo of the studied urban street canyon with the mobile meteorological station.
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covered with white plaster, while the centre of the canyon is an asphalt road with grey pavement tiles on each side of it. Meteorological data consisting of air and surface temperature measurements and wind velocity were re-
corded on a continuous basis. A detailed description of the field measurements is given in Table 1. The temperature measurements consisted of:
SSW Facade
22m
NNE Facade
P(1)
15.5 m
P(2)
11.5 m
P(16)
7.5 m
P(15)
P(3)
P(17)
P(4)
18m
P(14)
3.5 m P(5)
P(13)
WNW
45m P(6)
P(7)
P(8)
P(9)
P(10) P(11) P(12)
ESE 12m Surface Temperature
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Air Temperature
Fig. 2. A schematic representation of temperature measurements performed inside the urban street canyon during July 2002 in Athens, Greece.
Air temperatures in the centre of canyon: The air temperature was measured at four different heights in the centre of canyon, every 30 s. Four circular design thermometers were attached on a telescopic mast appended onto a mobile meteorological station. The miniature screen formed a housing for a range of temperature sensing elements, providing weather protection while allowing the free passage of air. The accuracy of the thermometers is 70.5 1C (max) under normal meteorological conditions, whilst at low wind speeds and radiation, it is 72 1C (max). Air temperatures near building walls: Outdoor air temperature was also measured inside the canyon and near opposite building walls, every 30 s. Small ambient temperature data loggers were shielded inside a white painted wooden cylinder that permitted air circulation. The sensors have an accuracy of 70.2 1C and they were completely protected from solar radiation. Surface temperatures at street level: Surface temperatures were conducted on the ground by an infrared thermometer equipped with a laser beam, every one hour. The surface temperatures were recorded across the canyon axis on the asphalt street and on the sidewalks on each side of the road. The emissivity value used for the measured materials was considered equal with 0.90.
Table 1 Description of the field experiments in an urban street canyon during July 2002 in Athens, Greece Air temperatures
Surface temperatures
Wind velocities
Inside canyon
At a cross section from SSW to NNE building at street level and on the building walls.
Inside canyon
In the center At 3.5, 7.5, 11.5 and 15.5 m from the ground. Near building walls At a distance of 0.5 m from the exterior walls and at a height of 14 m (near SSW fac- ade) and 10 m (near NNE fac- ade)
SSW Fac- ade Point 1 [4th floor (white plaster)] Point 2 [3rd floor (white plaster)] Point 3 [2nd floor (white plaster)] Point 4 [1st floor (white plaster)] Point 5 [ground (white plaster)]
In the center Horizontal wind speed and direction was measured at 3.5, 7.5, 11.5 and 15.5 m from the ground. Near building walls u, v, w wind velocity components were measured at a distance of 2 m from the exterior building walls and at a height of 15.5 m (near SSW fac- ade) and 11.5 m (near NNE fac- ade) from the ground. Outside canyon Horizontal wind speed and direction was measured at a distance of 24 m from the ground.
Street level Point 6 [grey pavement tiles] Point 7 [grey pavement tiles] Point 8 [asphalt] Point 9 [asphalt] Point 10 [asphalt] Point 11 [grey pavement tiles] Point 12 [grey pavement tiles] NNE Fac- ade Point 13 [Ground (white plaster)] Point 14 [1st floor(white plaster)] Point 15 [2nd floor(white plaster)] Point 16 [3rd floor (white plaster)] Point 17 [4th floor (white plaster)]
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Surface temperatures on building walls: Surface temperatures were also measured on opposite building walls with the infrared thermometer. Measurements were performed from the ground level up to the fourth floor, every 1 h.
In order to study further, the impact of thermal effects on airflow inside the canyon, simultaneous measurements of wind velocity inside and above the street canyon were performed on a consecutive basis during the 24 h period (Table 1). 3. Analysis of surface temperatures 3.1. Street temperatures The street temperatures were measured on an hourly basis, at a number of seven points across the asphalt street and the sidewalks (Fig. 2). The maximum surface temperatures reached almost 64 1C, during midday, due to the vertical incidence of solar radiation, as shown in Fig. 3 (point 10). The pavement near the SSW facade (point 6) presented maximum temperatures up to 55 1C, which were mostly observed 1 h before the maxima on the street centre. On the contrary near the NNE fac- ade (point 12), the street temperatures were much lower and rarely exceeded 30 1C. The maximum daily temperature amplitudes across the street reached 31 1C near the south facing wall (point 6) and 35 1C on the asphalt street (point 8) due to the incident solar radiation, however near the north facing wall (point 12) these amplitudes hardly ever exceeded 6 1C. The surface temperature differences between the daily maximum values across the canyon reached 33 1C, as a function of the optical and physical properties of street materials and the variability of solar radiation. However, for the same construction materials, the corresponding maximum temperature differences ranged from 9 1C on the sidewalks, up to 28 1C on the asphalt street.
During daytime period and until 9:00 LT, the temperature distribution was uniform as during night. At noontime period and around 15:00 LT, the surface temperatures of the north part of the street showed a maximum increase of 25 1C in comparison with the corresponding temperature during the morning (9:00 LT). The highest surface temperatures were always observed near the centre and the north part of the asphalt street, while the lowest values were measured on the sidewalk near the south part of the canyon floor. This favoured the overheating of the lower air levels and thus resulted in the development of upward movements near the south facing building wall. During the night, the distribution of the surface temperatures across the canyon axis began to stabilize after 22:00 LT. The maximum surface temperatures (up to 38 1C) were observed on the asphalt street and near its centre, whilst the lowest temperatures were found on the grey pavement tiles near the NNE fac- ade. The F-Anova test [21] was applied on the means of the surface temperatures across the street and showed that the temperature differences between them are statistically significant at the significance level (s.l.) of 0.05. 3.2. Wall temperatures Surface temperatures were measured on the opposite building walls from the ground up to the fourth floor, on an hourly basis during the 24 h period. As expected, the thermal behaviour of the two opposite walls is more complex due to parameters affecting the thermal balance of building materials (physical properties and canyon geometry) and due to incident solar and emitted infrared radiation. Fig. 4 shows the distribution of wall temperatures, with the form of boxplots, during day (07:00–21:00 LT) and night (22:00–06:00 LT) period. In general, it has been observed that during day period, the SSW fac- ade presented higher temperatures than the opposite wall. The absolute maximum surface temperature amplitudes along the canyon walls during daytime were up to 13 1C on the
Surface Temperatures (°C)
65 60 55 50 45 40 35 30 25 20 0:00
Point 6 6:00
12:00
20/07/2002
18:00
Point 8 0:00
6:00
12:00
18:00
21/07/2002
Point 12 0:00
6:00
12:00
18:00
0:00
22/07/2002
Fig. 3. Hourly distribution of street temperatures (point 8: on the asphalt street and points 6 and 12: on the sidewalks near SSW and NNE facades).
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Wall Temperatures (°C)
a
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40
35
30
25 SSW Facade
NNE Facade
20 P(1)
b
P(2)
P(3)
P(4)
P(5) P(13) P(14) P(15) P(16) P(17)
Measured Points 40
Wall Temperatures (°C)
Outliers 35
Max 75th percentile
30
Average 25th percentile
25 Min
NNE Facade
SSW Facade 20 P(1)
P(2)
P(3)
P(4)
P(5) P(13) P(14) P(15) P(16) P(17)
Measured Points Fig. 4. Boxplots of wall temperatures on SSW and NNE fac- ades (from the ground up to the fourth floor) for day (a) and night (b) during the experimental period (points are numbered as shown in Fig. 2).
SSW fac- ade (at the fourth floor) and 7 1C on the NNE fac- ade (at all floors), as a result of the incident solar radiation. During the night, these values were reduced for both building walls and were measured up to 4 1C at the ground floor mainly due to reduced wind speeds near the street level. The vertical distribution of the surface temperatures on the opposite buildings was studied during day and night, since it is important to understand the transfer phenomena between them and the adjacent air. During daytime period, the temperature differences on the SSW fac- ade reached almost 5 1C between the ground and the fourth floor. The highest temperatures occurred on the ground floor around 16:00–18:00 LT. This was due to reduced wind speeds at the lower levels and heat transfer phenomena from the asphalt heating. On the NNE fac- ade and during the day period, the temperature differences varied up to 4 1C again between the ground and the fourth floor, while the differences between the middle height surfaces were not significant, either for the SSW or NNE facade.
Table 2 Simultaneous wall temperature differences (TSSW–TNNE) from the ground level up to the forth floor during day and night period (TSSW: wall temperature on SSW fac- ade, TNNE: wall temperature on NNE fac- ade) TSSW–TNNE
Ground floor
1st floor
2nd floor
Day period Mean s.d. Max
1.46 1.42 5.00
1.15 1.44 4.00
0.50 1.15 5.00
0.50 1.68 4.00
1.00 2.16 6.00
Night period Mean s.d. Max
0.63 0.65 2.00
0.50 0.66 2.00
0.25 1.19 2.00
0.04 1.12 3.00
0.68 1.09 3.00
3rd floor
4th floor
In order to investigate the impact of canyon orientation on the wall temperatures, the differences of the instantaneous wall temperatures of the same height surfaces of the two opposite buildings, as well as, the differences of the maximum wall temperatures during day and night period
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have been compared (Table 2). This comparison analysis permits to evaluate at a certain degree the investigation of the impact of the absorbed solar radiation on the thermal balance of the measured surfaces, considering that the other terms of the thermal balance are almost similar. The simultaneous temperature differences between the opposite canyon walls during the day were higher at the fourth floor, up to 6 1C, because of the increased solar radiation and at the ground floor, up to 5 1C, as a result of the higher ground temperatures and the reduced wind effect. During the night period, these temperature differences are seriously reduced and the maximum differences were observed between the ground floors. Nevertheless, these differences reached 3 1C as a result of the direct solar heating. In almost all cases, where a significant temperature difference was observed, the SSW facing wall was warmer than the opposite building wall and especially on the ground floor. This could be explained by the fact that lower level surfaces have lower sky view factors and thus radiative losses to the sky are smaller. The comparison between the surface temperatures on the ground level and on the building walls led to the following results: during the day period, the maximum surface temperatures were higher on the street level and the absolute maximum temperature difference was 25 1C. This was expected considering that the incident solar radiation on a horizontal surface is greater during summer. However, the physical properties of the compared materials are different, since the asphalt street has a higher absorptivity than the white plaster of the building walls. During the night period, the absolute maximum surface temperature difference between the ground and the building walls was 6 1C. The positive radiative balance at the street level is directly related to the canyon geometry and the thermal capacities of the materials on the street and on the building walls. These observations will be determinant for the interpretation of air circulation inside the street canyon.
4. Analysis of air temperatures In order to understand the mechanisms that determine the distribution of air temperature inside a canyon, air temperature measurements have been performed in the centre of canyon and close to opposite building facades. The air temperature distribution has been analysed in order to investigate the impact of the street layout and orientation, as well, of the surface temperatures due to convection heat transfer phenomena. The ambient air temperature values were taken by the nearest meteorological station, namely National Observatory of Athens (NOA).
4.1. Air temperature distribution in the centre of canyon The air temperature distribution, based on the 5 min mean air temperature values, does not seem to show significant variations with height near the centre of canyon (Fig. 5). However, the estimation of air temperature lapse rates (1C/100 m) between 3.5 and 15.5 m (Fig. 6) resulted in a number of interesting remarks. During the day, a temperature inversion was observed around 8:00–10:00 LT reaching almost 7.2 1C/100 m, which is much higher than the average normal gradient (0.6 1C/100 m). Besides, this temperature inversion coincided with the minimum of temperature differences between the asphalt street and the air layer at 3.5m above the ground (DTasphalt-air). Later in the morning, the atmosphere became unstable, as a result of the direct solar heating of asphalt street and of building walls. As shown in Fig. 6, the instability presents its maximum intensity, up to 8 1C/100 m, around noon-time period, when the maximum surface-air temperature differences reached almost 30 1C inside the canyon. Moreover, air temperature inversions were also observed during noon (around 14:00–15:00 LT), when higher air temperatures were measured above the canyon and for almost isothermal conditions near the bottom of canyon.
38
Air Temperatures (°C)
36 34 32 30 28 26 Ta (3.5m) 24 0:00
6:00
12:00 18:00 20/7/02
Ta (15.5m) 0:00
6:00
12:00 18:00 21/7/02
Ta (NOA) 0:00
6:00
12:00 18:00
0:00
22/7/02
Fig. 5. Evolution of air temperature in the centre of canyon at different heights (at 3.5 and 15.5 m) and in the ambient air (from NOA station).
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5
0
-5
Lapse Rate -10 0:00
6:00 12:00 18:00 0:00 20/7/02
6:00 12:00 18:00 0:00 Time (h) 21/7/02
Δ Tasphalt-air 6:00 12:00 18:00
35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 0:00
Asphalt-Air Temperature Differences (°C)
Lapse Rates (°C/100m)
10
1389
22/7/02
Fig. 6. Temperature differences between asphalt street and air (at 3.5 m) and lapse rates (1C/100 m, between 3.5 m and 15.5 m) in the centre of canyon.
1.2 Rb=-0.42 20:00-20:10LT
1.0
Rb=-0.48 19:00-19:10LT
z/H
0.8 0.6
Rb=-0.63 18:00-18:10LT
0.4
Rb=-0.64 16:00-16:10LT
0.2
Rb=-0.69 15:00-15:10LT
0.0
Rb=-0.85 17:00-17:10LT
0.0
0.2
0.4 0.6 0.8 (Ta-Tasphalt)/(Tamb-Tasphalt)max
1.0
1.2
Fig. 7. Normalized temperature profiles, in the centre of street canyon during 15:00–20:10 LT on 22nd of July 2002 for different instability values (Rb).
In order to decide whether the ambient temperatures from NOA station are representative of the air temperatures above the canyon, the mean air temperature values at 15.5 m inside the canyon were compared with the corresponding ambient temperatures of NOA station during day and night period. The statistical t-test of the means was applied on air temperatures and it was found that either during the day (jtj ¼ 0.629ot0.05E2) or night (jtj ¼ 0.397ot0.05E2), the temperature differences are not statistical different at the s.l. of 0.05. As a result, the ambient temperatures from NOA station can be considered equal with the air temperatures above the canyon. From the comparison analysis of the ambient temperatures of NOA station with the simultaneous air temperatures at the lowest measured level (at 3.5 m) inside the canyon during day (jtj ¼ 0.262ot0.05E2) and night (jtj ¼ 0.006ot0.05E2), it has been found that the differences are not also statistical significant. However, when the simultaneous air temperature differences (Fig. 5) are examined, it has been calculated that
ambient temperatures are higher, in almost 83%, during daytime period (with the maximum differences up to 1.9 1C), than the air temperature above the street (at 3.5 m). Similar results are reported by Georgakis and Santamouris [10], where the ambient temperatures on the top of a deep canyon in the centre of Athens were approximately 5 1C than that measured air temperatures inside the canyon. In the present canyon, the measured air temperatures during night period were found higher near the ground than in the ambient air, in almost 82% of the cases, though the maximum differences did not exceed 1 1C. This is in agreement with Santamouris et al. [13] who measured that air temperatures inside a deep pedestrian canyon were higher up to 1.5 1C compared with ambient temperatures above the canyon. 4.1.1. Air temperature profile In spite of the small vertical air temperature differences inside the canyon, the temperature profile was studied
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for different stability weather conditions. Stability was indicated by the bulk Richardson number [22], Rb ¼ gh(Th–T0)/{(Ta+273)(Vh)2}, where T0 was approximated by the mean surface temperature on the asphalt street, Tasphalt, that was estimated as the mean value of the surface temperatures at points 8, 9 and 10 (Fig. 2). Th was considered equal with the ambient temperature from NOA station, while Ta was estimated as the mean value of air temperatures at the four different heights in the centre of canyon. According to Oke [23], Rb number becomes negative for unstable weather conditions. During day period, unstable conditions were measured in 85% of the cases, while during night this value was reduced to 64%. Thus, it is interesting to study the vertical air temperature distribution under unstable weather conditions. Fig. 7 shows the temperature profiles in centre of street canyon, during unstable weather conditions (0.85pRbp0.42), normalized with the absolute maximum temperature difference between the ambient air and the asphalt street. It can be observed that instability increased (Rb ¼ 0.85) when the street-air (at 3.5 m) temperature differences (Tasphalt–Ta), in the central part of the street canyon increased. However, the vertical air temperature differences inside the canyon are very small for all Rb values. This was expected since when the atmosphere becomes unstable, mainly due to ground heating, the mixing inside the canyon is increased and the vertical temperature gradient is very small. 4.2. Air temperature near canyon facades The air temperature distribution across the canyon is of great interest. Roth et al. [24] and Stoll and Brazel [25] reported that in the middle of canyon and near the ground level air temperature was more dependent upon the flux divergence in air volume including that of the horizontal transport. Air temperature measurements were measured near the opposite building walls, as shown in Fig. 1. The measured air temperature differences between the two facades vary, as a function of the canyon layout and the
surface characteristics. As it was expected, the air temperature close to SSW facade was higher than near NNE facade. The mean value of the instantaneous temperature differences between the two opposite facades during the day period was close to 3 1C, while the absolute maximum air temperature difference was 5.4 1C. The highest differences were observed during the early afternoon period around 16:00–17:00 LT and the lowest in the early morning around 7:00 LT (Fig. 8). Comparison of surface and air temperatures (Table 3) during daytime shows that in almost all cases near the NNE fac- ade, the surface temperature is higher than the adjacent air temperature up to 3.3 1C. The SSW fac- ade presented higher temperature values than the air close to it, up to 3.7 1C, around 14:00 LT. However, in some cases, the air temperature is higher than the corresponding surface temperatures by almost 2.7 1C (during noon period). Similar conclusions are reported by Nakamura and Oke [15] in an east-west urban canyon when higher air than wall temperatures were observed during midday period, which was explained probably due to the warming of the air volume by the combined effects of turbulent sensible heat
Table 3 Surface and air (near the wall) temperature differences (TSSW–Tair and TNNE–Tair) during day and night period TSSWTair
TNNETair
Day period Mean s.d. CV (%) Max Min
0.87 1.41 162 3.30 2.70
2.02 1.22 60 3.70 0.40
Night period Mean s.d. CV (%)* Max Min
1.35 0.58 43 0.40 2.70
0.59 0.81 137 0.60 2.20
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Air Temperatures (°C)
36 34 32 30 28 26 Tair (SSW Facade) 24 0:00
6:00
12:00 18:00 20/7/02
0:00
6:00
Tair (NNE Facade) 12:00 18:00 21/7/02
0:00
6:00
12:00 18:00 22/7/02
Fig. 8. Evolution of air temperature near opposite building facades (SSW and NNE).
0:00
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transfer from the north wall and its mixing via the air circulation. The wall temperatures were lower than the adjacent air temperatures during night (Table 3). Near SSW fac- ade, the air presented higher temperatures up to 2.7 1C in comparison with the wall temperatures. Similarly, near NNE fac- ade the absolute maximum wall–air temperature differences were measured equal with 2.2 1C. The maximum surface temperature differences either on the SSW or the NNE facade were observed in the early midnight hours and around 1:00 LT. 5. Conclusions The analysis of temperature distribution in an ESE–WNW-oriented urban street canyon has been performed on a 24 h basis, during a short measurement campaign in Athens under hot summer conditions. The analysis of the field measurements has led to the following observations: The air temperature distribution in the centre of canyon was not characterized by statistical significant variations with height. A strong temperature inversion, up to 7.2 1C/100 m, was observed during the morning that coincided with the minimum of temperature differences between the asphalt street and the air layer at 3.5 m above the street. During the day the atmosphere became unstable, as a result of the direct solar heating of the asphalt street and building walls and thus the normalized temperature profile in the centre of street canyon remains the same under different unstable conditions. Generally speaking, under different weather stability conditions, the air temperature differences inside and outside the canyon were found very small. Regarding the air temperatures near the opposite building walls the highest values were measured, as it was expected, close to SSW than near NNE facade. The highest differences up to 5.4 1C, were observed during the early afternoon hours around 16:00–17:00 LT and the lowest in the early morning around 7:00 LT. Comparison of the hourly surface temperatures measured on the opposite building walls from the ground level up to the fourth floor has shown that the SSW facade presented greater temperatures than the opposite. The measured temperature differences between the opposite canyon walls during the day were higher at the fourth floor, up to 6 1C, because of the increased solar radiation and at the ground floor, up to 5 1C, as a result of the higher ground temperatures and the reduced wind effect. Analysis of the street temperatures across a section from North to South facing walls revealed the existence of maximum surface temperatures on the asphalt street up to 60 1C, during midday, due to the vertical incidence of solar radiation. The pavement near the SSW facade presented maximum temperatures equal with 55 1C, which was measured 1 h before the maxima on the street centre. On the contrary near the NNE fac- ade, the street temperatures ranged between 25 and 30 1C. The measured hourly surface
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temperature differences reached almost 30 1C between the north and south part of the street and this favoured the overheating of lower air levels and thus resulting in the development of upward movements near the south facing canyon wall. The above results indicate that the air temperature distribution inside a street canyon is function of canyon geometry and orientation, as well as, of the optical and thermal properties of building and street materials and ambient weather conditions. Furthermore, the understanding of the specific thermal characteristics is essential for the understanding of airflow inside the canyons and for the studies of natural and hybrid ventilation in the urban environment. Acknowledgments The present research has been performed in the framework of the research project RESHYVENT, which was financed by the Fifth Framework Programme of the European Commission, Directorate General for Science, Research and Technology under the Contract ENK6CT2001-00533. The contribution of the Commission is gratefully acknowledged. References [1] Vardoulakis S, Fisher BEA, Perikleous K, Gonzalez-Flesca N. Modelling air quality in street canyons: a review. Atmospheric Environment 2003;37:155–82. [2] Numez M, Oke TR. Long wave radiative flux divergence and nocturnal cooling of the urban atmosphere II. Within an urban canyon. Boundary-Layer Meteorology 1976:121–35. [3] Numez M, Oke TR. The energy balance of an urban canyon. Journal of Applied Meteorology 1977;16:11–9. [4] Niachou K, Hassid S, Santamouris M, Livada I. Comparative monitoring of natural, hybrid and mechanical ventilation systems in urban canyons. Energy and Buildings 2005;37(5):503–13. [5] Santamouris M. Energy and climate in the urban environment. London: James and James; 2001. [6] Hunter LJ, Watson ID, Johnson GT. Modelling air flow regimes in urban canyons. Energy and Buildings 1990/1991;15–16:315–24. [7] Hunter LJ, Johnson GT, Watson ID. An investigation of threedimensional characteristics of flow regimes within urban canyon. Atmospheric Environment 1992;26B:425–32. [8] Oke TR. Street design and urban canopy layer climate. Energy and Buildings 1988;11:103–13. [9] Georgakis C, Santamouris M. On the airflow in urban canyons for ventilation purposes. International Journal of Ventilation 2004; 3(1):1–9. [10] Georgakis C, Santamouris M. Experimental investigation of air flow and temperature distribution in deep urban canyons for natural ventilation purposes. Energy and Buildings 2006;38(4):367–76. [11] Longley ID, Gallagher MW, Dorsey JR, Flynn M, Barlow JF. Shortterm measurements of airflow and turbulence in two street canyons in Manchester. Atmospheric Environment 2004;38(1):69–79. [12] Louka P, Vachon G, Sini JF, Mestayer PG, Rosant JM. Thermal effects on the airflow in a street canyon—Nantes’ 99 experimental results and model simulations. Journal of Water, Air and Soil Pollution 2002;2:351–64. [13] Santamouris M, Papanikolaou N, Koronakis I, Livada I, Assimakopoulos DN. Thermal and air flow characteristics in a deep
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