Intra-urban differences in canopy layer air temperature at a mid-latitude city

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 27: (27) Published online 15 December 26 in Wiley InterScience ( Intra-urban differences in canopy layer air temperature at a mid-latitude city E. Erell a * and T. Williamson b a J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 899 Midreshet Ben-Gurion, Israel b School of Architecture, Landscape Architecture and Urban Design, The University of Adelaide, Adelaide 55, Australia Abstract: Detailed meteorological measurements were carried out in two urban street canyons in central Adelaide and at two reference sites in a suburban location and at an exposed site near the middle of the city for an extended period of nearly a year. The meteorological records revealed substantial differences between air temperature in the urban street canyons and both reference sites. The nocturnal urban heat island observed in most cities was noted in Adelaide street canyons, too. However, the frequent occurrence of a daytime cool island during summer, albeit much weaker than the night-time phenomenon, is less expected. Both phenomena are attributed at least in part to the increase in surface area participating in energy exchanges with the atmosphere in an urban street canyon compared to a typical rural site, and hence in an increase in the effective thermal mass. The presence of additional thermal mass is manifested not only in the dampening of the diurnal temperature range in the city, but also in a noticeable time lag in the maximum intensity of the daytime cool island. The measurements also demonstrate an observable difference in the daily progression of air temperature between a north south street canyon and an adjacent east west oriented street. Copyright 26 Royal Meteorological Society KEY WORDS thermal mass; nocturnal heat island; daytime cool island; field study Received 7 November 25; Revised 29 October 26; Accepted 29 October 26 INTRODUCTION From a design point of view, one function of buildings is to create favorable interior environmental conditions for the occupants, and beneficial or at least acceptable modification of microclimatic conditions in their vicinity. Seen in another way, the placement of a building on the landscape gives rise to radiative, thermal, moisture and aerodynamic modification of the surrounding environment (Oke, 1987). The environmental effect of placing a large number of buildings in close proximity, as in a city, is due not only to the sum of the effects of the individual buildings, but also to the complex interactions between them. The urban heat island (UHI) has been documented thoroughly in recent years on the basis of detailed studies of meteorological data from networks of stations in and around cities of various sizes in different climates London (Chandler, 1965); Vancouver (Runnalls and Oke, 1998, 2); Tel Aviv (Saaroni et al., 2); Melbourne (Morris and Simmonds, 21); Seoul (Kim and Baik, 22); Athens (Livada et al., 22); and New York City (Gedzelman et al., 23), to mention a few. An UHI has also been recorded in Singapore (Goh and * Correspondence to: E. Erell, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, 899 Midreshet Ben- Gurion, Israel. erell@bgumail.bgu.ac.il Chang, 1999; Chow and Roth, 26), where high atmospheric humidity and the frequent presence of low, warm clouds result in conditions that do not favor the development of rapid nocturnal cooling associated with heat islands in other climates. The development of an UHI when a new city is founded on a greenfield site, i.e. where no buildings existed previously, was monitored by Landsberg and Maisel (1972), who reported that over a period of 3 years, a heat island of up to.5 C appeared as the newly established town of Columbia, Maryland grew to a population of 1 residents. Analyses of longterm temperature records in several locations - Turkey (Tayanc and Toros, 1997); Alaska (Magee et al., 1999); Athens (Philandras et al., 1999); South Korea (Choi et al., 23); and North America (Engelhart and Douglas, 23) show an increase in the UHI effect paralleling increasing urbanisation. Over the years, a variety of explanations have been put forward for the formation of the UHI. However, it is now widely accepted that urbanization generally results in modification of all elements of the surface energy balance net radiant exchange (Q ), subsurface storage ( Q S ) and the channeling of the available heat into sensible (Q H ) and latent (Q E ) fluxes as a result of the presence of moisture together with anthropogenic heat (Q F ) release. The relative contribution of each of these factors varies among locations, but the magnitude of the UHI observed in many cities has been correlated with the Copyright 26 Royal Meteorological Society

2 12 E. ERELL AND T. WILLIAMSON density of the urban core, typically expressed as the ratio between the height of the street canyon to its width (H/W) or to the sky view factor ( s ) (Oke, 1981). The effect of urban geometry on air temperature in the canopy layer is usually explained as modification of radiant exchange: The reduced sky view factor restricts long wave losses to the sky, while multiple reflections from the building surfaces and the street result in higher overall absorption of solar radiation in the urban street canyon. Most of the field studies of the UHI have been conducted for relatively short periods of time, employing either mobile traverses or fixed stations. The expense of maintaining a large network of fully instrumented stations for extended periods, as well as issues of vandalism, among other reasons, have usually limited the duration of urban climate experiments. As a result, many recent papers cite measured data from a few well-documented studies carried out in the past, such as the landmark experiment by Nunez and Oke (1976). Clearly, however, there is much to be learned from detailed studies over extended periods. Several such experiments have been carried out in recent years, such as the Athens urban climate experiment (Santamouris, 1998; Livada et al., 22), extended heat island measurements in Göteborg (Eliasson, 1996a) and extensive studies of Vancouver (Runnalls and Oke, 2) and Singapore (Chow and Roth, 26). This paper presents the results of a monitoring study carried out in the city of Adelaide, Australia, where the primary aim was to calibrate a model capable of predicting air temperature in an urban street canyon for extended periods in a variety of weather conditions on the basis of meteorological time series recorded at an open site exposed to the same mesoscale conditions (Erell and Williamson, 26). Various meteorological parameters were recorded continuously over the course of nearly a year at two urban street canyons in the dense core of the city and at an open reference site. Data were also obtained for the same period from the Australian Bureau of Meteorology (BoM) weather station at the nearby suburb of Kent Town. This paper focuses on analysis of differences in air temperature among the sites, and discusses them in the context of established conceptual models of the UHI. A complementary interpretation is proposed based on the role of the increased surface area of the active thermal mass in a typical dense urban location. MONITORING EXPERIMENT Detailed micro-meteorological data were assembled for an extended period of nearly a year at a number of locations in the city of Adelaide, Australia, located sufficiently close to one another to be affected by uniform mesoscale environmental conditions, and yet, also affected by distinctly different micro-scale factors. The study area: Adelaide The city of Adelaide (Latitude: 3.9 S, Longitude: E) is situated at the base of the Mount Lofty Ranges (typical elevation 55 m), 1 km inland from the center of the eastern shore of the Gulf St. Vincent, on the south coast of the continent of Australia. It comprises the core of a metropolitan area that extends about 2 km east west and about 25 km from north to south, and which has a total population of almost one million residents (Figure 1). Adelaide has a Mediterranean climate with warm summers (February mean temperature 23 C), mild winters (July mean 12 C), and an average annual rainfall of about 55 mm. The elevation of the city center is about 5 m above sea level. The topography of the coastal plain is generally flat with occasional hills. However, in spite of the relatively simple topography of the majority of the populated areas of the Adelaide metropolitan area, Schwerdtfeger (1972) noted that there is a certain degree of anisotropy of meteorological characteristics: temperature differences of over 5 C and major wind parameter variations among various suburban locations at similar altitudes are not uncommon, mainly between the north and the south of the metropolitan area. The selection of a rural reference site outside of the built-up area was therefore unrealistic, as there are no means of quantifying variations that are not the result of the urban-rural differences that are the object of this study. The center of the Adelaide metropolitan area is surrounded on all sides by a belt of parklands several hundred metres wide. The area is not pristine, but much of it resembles the natural environment of the region prior to the establishment of the city. Although the parklands are in turn surrounded by suburbs extending several kilometres in all directions, a reference station located in this belt provides a fairly close approximation of conditions that would have prevailed in the area were it not for the construction of the city. Since it is very close to the center of the city, such a station is also exposed to the same mesoscale conditions, avoiding the variations noted by Schwerdtfeger (1972). The classic definition of the UHI, T u r, is based on the difference in temperature between pairs of stations one representing an urban condition, and the other a rural one. The difficulty of selecting appropriate urban-rural station pairs was highlighted by Oke (1998) with particular emphasis on soil moisture in the rural site. However, as Dewey (196) noted in a study on the sociological characteristics of different settlement types, the urban-rural dichotomy describes extreme cases of what is often in reality a continuum. In the context of climatology, too, the problem has a somewhat broader nature: the characterization of a site as being either urban or rural is too coarse, and does not allow an accurate enough description of the complexity of land use types found in and around many metropolitan areas (Stewart and Oke, 26). There are very few cities where undisturbed landscapes can be found close to the urban core. Often, the area around cities is devoted to agriculture,

3 INTRA-URBAN DIFFERENCES IN CANOPY LAYER AIR TEMPERATURE 125 Figure 1. Aerial photograph of Adelaide metropolitan area, seen from north. The study area of central Adelaide (see Figure 2) is marked by a broken line. The shoreline seen in the top right-hand corner is the eastern coast of the Gulf of St. Vincent. This figure is available in colour online at or has been so in the past. Truly rural stations are thus often located at a substantial distance from the city centre. This creates the risk that mesoscale climate effects may come into play, at least during isolated episodes (Runnalls and Oke, 2). An alternate definition of T u r may be based upon the difference between temperatures actually measured in the urban location in question and the temperature that would have been recorded at the same place in the hypothetical event that urban development in the region had not occurred. However, although this definition is conceptually sound, it is of little value in the selection of appropriate urban-rural station pairs. In order to avoid debate on whether temperature differences between the urban street canyons and the two other sites represent a true UHI according to the commonly accepted definition i.e. T u r this paper will use the term intra-urban heat island (IUHI) to describe temperature differences between the different sites. In addition, following Eliasson (1996a), it will refer explicitly to the characteristics of the sites in question: urban canyon ( c ), suburban Kent Town ( s ) and open reference ( o ). The monitoring sites Meteorological records used in this analysis were made on the basis of measurements carried out at two adjacent urban street canyons, at an open site about 1.6 km to the northeast in the belt of parklands surrounding central Adelaide, and at the Australian BoM weather station at the suburb of Kent Town, about 1.7 km to the east (Erell and Williamson, 26). Their locations are shown in Figure 2. The urban canyon sites were located in two adjacent alleys in the central business district: Chesser St., oriented approximately north south (.6 west of north), and French St,oriented atright angles to it, approximately east west (Figure 3, left and center, respectively). Measurements were taken about 3 m north and west of the intersection between them. Both alleys are approximately 6 m wide, fully paved with asphalt. Adjacent buildings are of varying heights, including two office towers of approximately 5m height. However, the street canyon is formed primarily by lower structures either a twostorey podium on which the office block is constructed at a setback of about 8 m, or by two-three storey lowrise buildings. (The office towers probably affect airflow in the canyons monitored, but analysis of the site data failed to show any correlation between wind direction and air temperature in the streets.) The mean canyon aspect ratio near the points of measurement is approximately The surfaces comprising the canyon walls are either of clay brick or brick veneer; Glazed openings comprise a relatively small proportion of the total surface area. French St. is totally devoid of vegetation, whereas a sparse deciduous vine covers a 1-m section of Chesser St. north of the monitoring site. Vehicular traffic in both streets was very low: The total number of car trips on each street is estimated at no more than 2 3 per hour during office hours, and a negligible number at night

4 126 E. ERELL AND T. WILLIAMSON Figure 2. Aerial photograph of central Adelaide showing location of monitoring sites. This figure is available in colour online at Figure 3. Monitoring sites, showing location of sensors: North south canyon (left); east west canyon (center); and open site at City Nursery (right). This figure is available in colour online at and on weekends. The number of pedestrians is likewise very small. The streets were surveyed for possible local sources of heat, such as air-conditioning systems (window units or compressors of central systems), and none were found. Anthropogenic heat fluxes were therefore considered to be quite low, and due almost entirely to conduction through building envelopes. Net advection of anthropogenic heat from the surrounding area was considered negligible. The open site was located at the Adelaide City Nursery in the green belt of parklands surrounding the central business district of Adelaide, about 1 m north of the Torrens River and approximately 2.1 km northeast of the city center. The instrumentation was installed in an

5 INTRA-URBAN DIFFERENCES IN CANOPY LAYER AIR TEMPERATURE 127 Figure. Suburban site The Australian Bureau of Meteorology station at Kent Town. This figure is available in colour online at open plot within the premises of the Nursery, in an area covered with crushed asphalt gravel and with some potted plants arranged in rows several metres away (Figure 3, right). A dense hedge about 3 m tall surrounded the entire compound, which was in turn surrounded by grassland irrigated during the dry summer season. As the experiment progressed, more potted plants were brought close to the station, and the whole area was sprinkleirrigated on a regular basis. The Kent Town BoM station is located in a suburban location about 2.2 km east of Victoria Square, the center of Adelaide, and about 1.7 km east of the monitoring sites in Chesser St. and French St. It is also about 2 m east of the belt of parklands surrounding central Adelaide. Adjacent buildings are mostly onestorey residential houses, and the total distance between facades on opposite sides of the street is nearly 15 m, giving an aspect ratio of about.2 (Figure ). The sky view factor at the station itself is.93 (El Nahas, 1996). Instrumentation At each of the urban sites, air temperature was measured at 15-min intervals at 5 points: Two each on either side of the street, 5 cm away from the building walls, 2.5 m and 5 m above the street; and one sensor at the middle of the street, 5 m above road level. NTC thermistors (MEA model 657, nominal accuracy.2 C) were housed in radiation shields constructed especially for the experiment (Erell and Williamson, 26). Comparative tests of this type of makeshift instrument shield in conditions of very bright sunlight and low wind against a mechanically aspirated standard, confirmed that it was superior to the conventional Stevenson screen (Erell et al., 25), which has been shown to produce some error in such conditions (World Meteorological Organization, 1971; Sparks, 1972; Andersson and Mathisson, 1992; van der Meulen, 1998). However, error due to radiant load cannot be ruled out entirely when relatively large sensors are used, such as the thermistors employed in this experiment, and deviations of up to 1 C from true air temperature are nonetheless possible. Additional measurements made at the urban sites included relative humidity (RH) (one Rotronics thermo-hygrometer) and indicative wind speed and direction at a height of 6 m above the intersection of the two streets (VDO Instruments Model 65-FS anemometer/wind vane; accuracy ±2%, threshold.8 m s 1 ). Measurements taken at the open station at 15-min intervals included air temperature (DBT) and RH in a standard radiation screen 1.8 m above the ground using a combined temperature and RH sensor (UNIDATA Model 651-E by MEA, nominal accuracy ±5%); wind speed and direction 6 m above the ground (UNIDATA Model 65-F cup anemometer with a starting threshold of.8 m s 1 and accuracy of ±2%, magnetically coupled wind vane with 1% vectorial accuracy); and soil temperature at a depth of 25 cm with a stainless steel NTC thermistor (as above). To account for the possibility of a systematic error in the measurement of air temperature resulting from the use of different sensors, an additional NTC thermistor was installed at this site in a makeshift radiation shield of the type used in the urban sites. Hourly weather records obtained from the Australian BoM Kent Town site include air-dry bulb temperature and RH (both at screen height); wind speed and direction at a standard 1 m height; dew point temperature; atmospheric pressure; and rainfall. Visual observations of cloud cover were reported every 3 h. General radiation data, assumed to be representative of all sites in the experiment, were recorded at 15-min intervals on the roof of the Architecture Building at the University of Adelaide, and included global solar radiation on a horizontal surface (Middleton EP7 pyranometer, with a calibration accuracy of ±3%); diffuse radiation (Middleton EP7 pyranometer with a manually adjusted shadow band); and net radiation (Middleton CN1 net pyrradiometer).

6 128 E. ERELL AND T. WILLIAMSON Quality control and data collation Weather data from all sources were read into spreadsheet files each representing a continuous month-long period. Data were then screened for completeness and faulty readings. Where short sequences of data (up to 2 h) were missing, synthetic values were generated by linear interpolation with existing data. Data recorded at short time intervals were aggregated in the form of hourly averages. Cloud cover, which is observed at the Kent Town weather station at intervals of 3 h, was converted to an hourly format by linear interpolation, and converted from oktas to tenths in accordance with the required format for the empirical correlations used to estimate long wave sky radiation in cloudy conditions. All time records were adjusted to local standard time, which precedes solar time by approximately 16 min. It was not possible to correct for sensor drift by calibration of the thermistors in the field. Records from the temperature sensors were therefore adjusted retroactively once a month on the basis of average values in each of the two streets during windy nights (wind speed at Kent Town >1 km h 1 ), conditions in which it was assumed readings of all sensors (initially five in each site) should be identical. The mean deviation of each sensor from the average for the whole group in these conditions was assumed to reflect sensor error, and a constant correction factor was therefore applied separately to each sensor for all records. The absolute magnitude of the correction factors was usually <.2 C, and was never >.5 C. Air temperature data recorded from the UNIDATA sensor were used to represent the open site, because they had fewer gaps than the NTC thermistor of the type also used at the urban sites. Comparison of the differences between the sensors showed a weak diurnal pattern approximating a sine curve with an amplitude of plus or minus.2.3 C around the average calibration error. The magnitude of this error did not justify carrying out a correction procedure to these data. No calibration was carried out with respect to the temperature data provided at a later date by the Australian BoM station at Kent Town, which was measured with a Rosemount platinum resistance thermometer (Model ST21 Mk2) in a standard Stevenson screen. RESULTS AND ANALYSIS Overview The meteorological instruments described in the preceding section were operated continuously for a period of approximately 11 months, from the beginning of May 2 to the end of March 21. Malfunctions of some of the instruments and data loggers during this extended period resulted in gaps in some of the parameters monitored. Complete (or nearly complete) data are available for the months of May, June, November, January and March, a total of 13 days. The following analysis was conducted with respect to each of these month-long periods separately and to an ensemble of the whole data. The intra-urban heat island Theory (Oke, 1981, 1982), as well as field studies (Eliasson, , 1996b; Runnalls and Oke, 2; Kim and Baik, 22), show that the nocturnal UHI is typically formed on calm clear nights when radiant cooling is strong. The UHI ( T u r ) occurs when the exposed rural surroundings cool rapidly, while the city cools down more slowly initially, because radiant loss is restricted and because of the release of excess heat stored in urban surfaces. Oke (1987) noted that maximum heat island intensity occurs about 3 5 h after sunset. However, several recent field studies found that although a substantial heat island was formed within a few hours of sunset, the maximum intensity sometimes occurred somewhat later, at about midnight (Runnalls and Oke, 2; Chow and Roth, 26). The intensity then declines marginally throughout the rest of the night, when cooling rates at rural locations are slightly lower than in the city. Figure 5 summarizes the diurnal pattern of the temperature differences between the urban street canyons and the open site ( T c o ) in Adelaide for the entire monitoring period. (The pattern of differences between the street canyons and the suburban site ( T c s ) is similar, except that the magnitude of the nocturnal heat island there is slightly smaller.) While an IUHI began to form a little before sunset in clear weather conditions, and became well established after 3 5 h, the maximum intensity in such conditions often occurred well after midnight (Figure 6). The pattern of warming/cooling rates observed in this experiment is very similar to that observed in Vancouver by Runnalls and Oke (2), but the fact that on many occasions the intensity of the IUHI increased slowly from midnight till shortly before sunrise suggests that cooling rates at the suburban and open site were in fact slightly higher than the cooling rate at the urban canyon throughout the night (Figure 7). mean T c-o ( C) min-max time of day (hour) median 25%-75% Figure 5. Hourly values of T c o, the temperature difference between the urban street canyons (average of the two sites) and the open reference site at the Adelaide City Nursery, in May, June, January and March of This figure is available in colour online at 2

7 INTRA-URBAN DIFFERENCES IN CANOPY LAYER AIR TEMPERATURE 129 sunset sunrise 7 air temperature ( C) open suburban canyon T c-s ( C) y =.122x x R 2 = time of day (hour) Figure 6. Air temperature at the urban street canyons (average of the two sites), the suburban site at the Australian BoM station at Kent Town and the open reference site on a typical clear day (May 8 9 2). The maximum temperature differences were recorded at 7 am. ( T c s = 5.1 C, T c o = 6.7 C). This figure is available in colour online at temperature change ( C h -1 ) sunset canyon suburban open time of day (hour) sunrise Figure 7. Rate of temperature change at the urban street canyons (average of the two sites), the suburban site at the Australian BoM station at Kent Town and the open reference site on a typical clear day (May 8 9 2). The most intense heat islands about 6 8 C occurred on such nights. When the maximum IUHI intensity for a given night was attained before midnight, the temperature differences between the sites were typically much smaller about 3 C only. The delay in the occurrence of maximum IUHI intensity observed in this experiment, compared to observations in most other field studies, may be attributed to the fact that neither of the open sites is in a truly rural location: They may have been affected by the presence of built-up areas in their vicinity, so that cooling in the early part of the night, while strong in absolute terms, was slightly lower than it might have been in a more exposed location. Theoretical analysis of the heat exchange processes near the surface indicates that calm, clear sky conditions wind speed (m s -1 ) Figure 8. Correlation of nocturnal (22 6) heat island intensity ( T c s ) with wind speed. Ensemble data for 126 h in May, June, January and March of are conducive to the formation of nocturnal canopy layer UHIs. Although many early studies of the UHI were limited to relatively short-term monitoring in such ideal conditions, e.g. Barring et al. (1985) and Eliasson (199 91), more extended field studies in a variety of weather conditions have provided statistical evidence to support this analysis (Eliasson, 1996a) and to suggest empirical relationships to quantify the combined effects of these factors on the intensity of the UHI (Runnalls and Oke, 2). Measurements in Adelaide showed that as expected, substantial nocturnal heat islands do not form in windy conditions (Figure 8). However, the probability of intense intra-urban temperature differences occurring did not exhibit a monotonic decrease as wind speed increased. Rather, as wind speed (measured at 1 m height at the suburban weather station) increased above approximately 2m s 1, a qualitative change occurred: the intensity of the heat island ( T c s ) was rarely above 2 C insuch conditions, irrespective of the effect of other factors. Conversely, temperature differences of 5 6 C ormorewere almost equally likely to occur as long as wind speeds were <2 ms 1, irrespective of other meteorological factors. The contribution of clear skies to the formation of IUHIs in Adelaide was less conclusive. The relationship between cloud cover and the nocturnal heat island ( T c s ) shows only a weak correlation (Figure 9). And while the maximum IUHI intensity for a given night tended to occur most often when cloud cover was 1 tenths, substantial heat islands of over C were also observed on some occasions when cloud cover was as high as 8 or 9 tenths, provided wind speed was <2 ms 1. There was relatively little inter-seasonal variation in the intensity of the IUHI measured with respect to the open site ( T c o ), although some of the existing variability may be masked by the fact that data for this site were not available for the month of November. The IUHI measured with respect to the suburban site ( T c s ),

8 125 E. ERELL AND T. WILLIAMSON T c-s ( C) y =.5x -.176x R 2 = cloud cover (tenths) 8 9 wind speed (ms -1 ) >3 Figure 9. Variation of maximum heat island intensity ( T c s ) with cloud cover, stratified into three classes of wind speed. Ensemble data for 1 days in May, June, January and March of The line of best fit is plotted with respect to the entire data set, and includes all wind conditions. (Note: Conversion of raw data from oktas to tenths and rounding to nearest whole number results in no values for 2 or 7 tenths). This figure is available in colour online at for which data are more complete, did in fact show some seasonal changes: it was most intense in January, and lowest in November (Table I). Differences between the air temperatures at the two stations reflect the fact that the Nursery site was more exposed, being in the belt of parklands, while the Kent Town station is on the edge of the suburban area surrounding central Adelaide, is surrounded by pavement and low-rise buildings, and has a slightly lower sky view factor. Higher values for the IUHI measured with respect to the open site therefore reflect the fact that being more exposed, it was typically colder at night. A daytime urban cool island The discussion thus far has concerned the nighttime phenomenon of the IUHI. During the daytime, temperature differences between the urban street canyon sites and the open stations were eroded, and a negative 1 heat island or intra-urban cool island (IUCI) became apparent. Although the intensity of this phenomenon was lower and its duration shorter than that of the nocturnal IUHI, the street canyons monitored in central Adelaide are nonetheless characterised by a discernible daytime urban cool island for nearly the entire year. Examination of the meteorological data shows no apparent correlation between the intensity of the daytime urban cool island and wind speed, wind direction or cloud cover at the time of maximum intensity. However, when the meteorological conditions for the entire daytime period (6 18 h) are analysed, the effect of wind speed, cloud cover and solar radiation become apparent. It is convenient to examine the data for the month of March 21 separately, since this period was characterized by changeable weather and a correspondingly broad range of maximum IUCI intensity. On a total of 8 days when the canyon was cooler than the suburban site by at least 2 C, the average daily insolation was 6.1 kw hr 1 m 2, cloud cover averaged 1.7 tenths and average wind speed (at the Kent Town BoM station) was 1.75 m s 1. During a total of 9 days in which the maximum cool island intensity was <1 C, average daily insolation was 3.2 kw hr 1 m 2, cloud cover averaged 7.6 tenths and average wind speed (at the Kent Town BoM station) was 2.56 m s 1. In other words, intense IUCI formed primarily on sunny and clear days (when direct solar radiation predominates) in association with light winds. The combined effect of these factors is reflected in air temperature: Intense cool islands were recorded only when temperature was high (and RH was correspondingly low 1 3%), whereas cooler days were characterised by weak or non-existent cool islands (Figure 1). Atmospheric moisture was not, apparently, a causative factor, however, as there was no correlation between the intensity of the daytime cool island and absolute humidity. Air temperature is a good predictor of the intensity of the IUCI not only with respect to a time scale of days, but also on a seasonal basis (Table II). As Figure 11 shows, although the average nocturnal heat island intensity showed only minor seasonal variations, daytime differences were more substantial: The IUCI was least developed in June (Southern Hemisphere winter), and most intensive in January (summer). In March, the Table I. Seasonal variations in the intensity of the nocturnal intra-urban heat island. Month T c s ( C) T c o ( C) No. of days Min Avg Max No. of days Min Avg Max May June November January March Ensemble Note: The intra-urban heat island is calculated as the difference between the average of the two urban streets and the BoM weather station at the suburb of Kent Town ( T c s ) or the Adelaide City Nursery ( T c o ), respectively.

9 INTRA-URBAN DIFFERENCES IN CANOPY LAYER AIR TEMPERATURE 1251 Table II. Seasonal variations in the intensity of the daytime intra-urban cool island. Month T c s ( C) T c o ( C) No. of days Min Avg Max No. of days Min Avg Max May June November January March Ensemble Note: The IUCI is calculated as the difference between the average of the two urban streets and the BoM weather station at the suburb of Kent Town ( T c s ) or the Adelaide City Nursery ( T c o ), respectively. Negative values indicate that the reference sites are warmer than the urban streets. T c-s ( C) June Jan March temperature ( C) Figure 1. Air temperature at the suburban site (Kent Town BoM) at the time of minimum daily T c s (ensemble data for June 2, January and March 21). Negative values represent a canyon cool island; positive values indicate that on the day in question, the urban canyon was always warmer than the suburban site. This figure is available in colour online at average T c-o ( C) time of day (hour) May June Jan March Figure 11. The diurnal progression of T c o at different times of the year: monthly averages for March and May (autumn), June (winter) and January (summer). IUCI was weak on cool, cloudy days and more intense on hot ones. As in the analysis of the diurnal pattern, air temperature may be considered a proxy for the combined effect of wind and solar radiation: Summer conditions in Adelaide are typically sunny with only light winds, while winters are often windy and overcast. Street orientation Much has been written about the effect of canyon geometry on the long wave radiant balance at night, but canyon geometry also has a dominant effect on energy exchange during the daytime. Several models have been developed to describe radiant exchange that deal with solar radiation in detail (Arnfield, 1982; Mills, 1997; Kondo et al., 21; Chimklai et al., 2). Some of this research was carried out to provide better estimates of urban albedo, which has been shown to be affected by street orientation (Aida, 1982). However, little has been written about the effects of street orientation on air temperature during the daytime, and the issue was not mentioned in Arnfield s exhaustive review of recent urban climate research (Arnfield, 23). Relatively deep street canyons with different orientations may be expected to exhibit small differences in air temperature because they are exposed to direct solar radiation at different times of day. Thus, a street canyon with a north south axis will be exposed to the sun near the middle of the day, when solar radiation levels are highest, while an east west canyon may be partially or entirely shaded by adjacent buildings. (It should be noted, however that in the tropics, high solar elevation results in similar penetration of direct sunlight in the middle of the day to streets of all orientations). In the absence of advection, one may therefore expect to find higher mid-day temperatures in the north south canyon. In the early morning and late afternoon, the situation is reversed. However, since the intensity of solar radiation is much lower at these hours, the effect on air temperature may be expected to be smaller. Air temperature records from Chesser Street, which runs approximately north south, and French Street, which is perpendicular to it, support the above reasoning. The two canyons are adjacent and are very similar

10 1252 E. ERELL AND T. WILLIAMSON T NS-EW ( C) time of day (hour) On sunny or partly cloudy days (cloud cover <7 tenths), the maximum difference between the two street canyons was about 1. C, compared to about.2 C in overcast conditions. The measured differences in air temperature between the two streets could also be replicated in the Canyon Air Temperature (CAT) model (Erell and Williamson, 26), which adapts data from a standard meteorological station to provide realistic site-specific air temperature in a city street exposed to the same mesoscale environment. In addition to providing further evidence for the validity of the CAT model itself, this result demonstrates that the effects of street orientation, although small, can be estimated by quantitative modeling of the surface energy balance. min-max median 25% - 75% Figure 12. Hourly differences in air temperature between north south (Chesser St.) and east west (French St.) urban street canyons, during May 2. This figure is available in colour online at mean T NS-EW ( C) time of day (hour) sunny or p. cloudy overcast Figure 13. Mean hourly differences in air temperature between north south and east west urban street canyons during May 2, stratified into two classes of sky conditions: sunny or partly cloudy, for cloud cover of 7 tenths or less; and overcast, for cloud cover in excess of 7 tenths. in geometry, building materials and traffic. Differences in temperature between them are therefore assumed to be the result of their orientation. As Figure 12 shows, the north south canyon is on average slightly warmer than the east west canyon for about 2 3 h near the middle of the day. The variance of the data during this period is also much larger than for the night-time hours, when air temperature differences between the streets are always negligible. Conversely, the east west canyon is slightly warmer than the north south street early in the morning and late in the afternoon, although this difference is typically much smaller. Filtering the data by cloud cover (Figure 13) confirms that the temperature differences are in fact driven by differential shading from solar radiation: DISCUSSION The probable existence of a daytime urban cool island was noted by Oke (1982), who proposed that the absence of a peak in warming, such as that experienced in the countryside, probably is due to the combined effects of canyon shading at low sun angles, higher thermal admittance and the lack of a capping inversion within the lowest few hundred metres of the urban atmosphere. The existence of a cool island in the middle of the day usually is attributed to canyon shading in the city center.... Givoni (1989) also suggested that cities might sometimes be cooler than their surroundings during the day, but empirical evidence is conflicting. Santamouris (21), describing an extensive monitoring project in Athens, Greece, that incorporated 2 automated weather stations, reported a very intense daytime heat island centred around the densely built business district. Other researchers have found that a daytime urban cool island does in fact exist, in certain conditions (Steinecke, 1999; Runnalls and Oke, 2; Chow and Roth, 26). However, the phenomenon has received much less attention than the nocturnal heat island. This may be because daytime cool islands, where they occur, are typically weaker than nocturnal heat islands. Also, it is more difficult to obtain sufficiently accurate temperature readings during the daytime: dissimilar exposure to sunlight among different locations may create measurement error of the same order of magnitude as the temperature differences being recorded. The mechanisms responsible for the evolution of daytime cool islands are thus less well understood, and deserve some discussion. Net radiant exchange (Q ) is dominated during the daytime by short-wave radiation. The widely used representation of the urban surface as a series of roughly similar building blocks interspaced with street canyons of equal depth leads to the qualitative assessment that absorption of solar energy depends, in addition to the albedo of individual surfaces, on the aspect ratio of the urban canyon: Deep narrow canyons trap more solar radiation than broader and shallower ones because multiple reflections between canyon surfaces mean that less solar

11 INTRA-URBAN DIFFERENCES IN CANOPY LAYER AIR TEMPERATURE 1253 radiation is reflected back out of the canyon top. Many European cities fit this general scheme quite well. However, the central business districts of most United States and Australian cities, including Adelaide, consist of tall office blocks of varying height. This geometry results in the interception of a substantial proportion of the incoming solar energy on extensive wall surfaces that are many tens of metres above street level. When the energy absorbed by these surfaces is given off to the adjacent layer of air in the form of sensible heat, the excess energy is advected high above the ground, with limited effect on the temperature at street level. Even where tower blocks are constructed in a stepped section (where the lower two or three floors create the street façade, while the upper stories are set back by several meters), streets may receive very little direct sunlight, and may thus be substantially cooler during the daytime than more exposed areas. Because the interception of direct solar radiation by buildings depends both on the canyon aspect ratio and on the relative solar position, even fairly shallow canyons may be shaded by adjacent buildings if solar altitude is low. Therefore, daytime urban cool islands may be more likely to occur in high-latitude locations than in the tropics, as suggested by Steinecke (1999). Storage flux (Q s ) is related to the thermal properties of the surface and of the substrate materials. The thermal admittance of a surface (µ) is a useful indicator of its effect on the sharing of sensible heat between the substrate and the atmosphere. It combines thermal conductivity (k) and heat capacity (C) (µ = (kc) 1/2 ),and is a measure of the thermal responsiveness of a surface for a given heat flux: A surface having a high admittance accepts and releases energy easily, registering a relatively small change in temperature. The role of thermal inertia in the formation of the UHI has been the subject of some conjecture. Oke (1981) demonstrated that a heat island similar to the one found in cities could be explained not only by the effects of street canyon geometry on radiant exchange (especially long wave) but also by differences in the thermal properties of urban and rural surfaces. However, he also noted that in reality, differences in thermal admittance between urban and rural materials were too small, and were in any case affected to a great degree by rural soil moisture. A later study (Oke, 1982) suggested that differences in thermal inertia between rural and urban sites could be accounted for by considering the role of increased surface area in the city (as well as differences in moisture availability). However, the difficulty of measuring the storage flux directly in situ on a neighborhood scale has thus far limited efforts to explain this mechanism satisfactorily. Many urban experiments have shown higher daytime sensible fluxes above the urban canopy compared to an adjacent vegetated and moist rural surface. It is nonetheless useful to analyze the effect of geometry in isolation, because it has an effect on both radiant exchange and on energy storage, as well as on the complex interaction between them. A qualitative description of its contribution to the development of intra-urban temperature differences may be summarized as follows: Incoming solar radiation is absorbed over a much larger surface area in street canyons compared to open sites. A flat, unobstructed rural surface that is exposed to direct solar radiation continuously throughout the day may experience a substantial rise in temperature, unless much of the radiant flux is converted to latent heat in the presence of moisture. In a street canyon, in contrast, individual facets are exposed to direct sunlight in sequence, each for only several hours of the day. This results in reduced insolation per unit area of each canyon surface, especially if the canyon is deep in relation to its width. Although a street canyon may absorb more solar radiation overall than an unobstructed flat rural site due to multiple reflections among canyon surfaces, absorption over a larger surface area allows the thermal mass to moderate extremes of surface temperature, and hence reduces peaks of sensible heat flux. The higher overall absorption of solar radiation may, however, be reflected in a higher average air temperature in the canyon over the diurnal cycle. The release of this excess heat is typically in evidence at night: Long wave radiant loss is inhibited by a small sky view factor, so unlike exposed rural soil canyon surfaces may remain warmer than the air, resulting in a small positive sensible heat flux all night. The effect of urban thermal mass on air temperature may be seen as being analogous to the effect of thermal mass on interior temperatures of buildings (Ratti et al., 23): Irrespective of the effect of other factors, one would expect the daily amplitude of urban air temperature to be reduced compared with the surrounding environment, and the minimum and maximum temperatures to be time-lagged to a certain extent. The damping effect of urban thermal mass on air temperatures is in fact seen clearly in Figure 1, which shows the ratio between the daily temperature amplitude at the Kent Town weather station to the amplitude at the urban streets. The correlation appears to represent a linear relationship with a ratio of about.68. The equivalent ratio between the daily urban amplitude ( C) y =.68 x R 2 = suburban amplitude ( C) Figure 1. Correlation between daily amplitude of air temperature at suburban Kent Town and at the urban street canyon sites (ensemble data for June 2, January and March 21).

12 125 E. ERELL AND T. WILLIAMSON amplitude of the air temperature at the canyon site and the open site is slightly smaller, and equals about.61, reflecting the fact that the diurnal range at the open site is slightly greater than the range at the suburban location. This damping effect may be attributed to the effects of urban morphology on radiant exchange as well as to the effects of thermal mass. The fact that the relationship is strongly linear and shows no seasonal dependence, as one would expect with solar radiation effects, favors the latter explanation. The dominant role of thermal mass is also consistent with the time lag observed between the peak of the net radiant load on the surface (about noon) and the maximum intensity of the urban cool island, which, asfigure5shows,typicallyoccurred2to3hlater. The damping effect was observed both with respect to night-time differences between the street canyon and the open weather station, namely, in the IUHI, and in the daytime differences, here referred to as the IUCI. In this view, the two phenomena are mirror images of each other (Figure 15(a) and (b)). maximum daily T c-s ( C) (a) minimum daily T c-s ( C) June Jan March diurnal temp. range at suburban site ( C) June Jan March Although daytime cool islands may be more common than the relative paucity of published data on this subject suggests, the conditions in which they are likely to occur may not necessarily be found in all cities: A daytime canopy level cool island is more likely to occur: (1) if the urban structure is dense, (2) if the anthropogenic heat flux (Q F ) is small in proportion to the net radiant flux (Q ), and (3) if a substantial proportion of the incoming solar radiation is absorbed by building surfaces that are not in contact with air at street level. The size of the anthropogenic flux depends on the density of the city, as well as on climate and on building construction. In most cities it is quite small Sailor and Lu (2) report values in the range of W m 2 for a number of large US cities. Nevertheless, even a moderate mean anthropogenic flux of 5 W m 2 may be larger than Q in some cities during winter, as Offerle et al. (25) found in Lodz, Poland. In such conditions, a daytime urban cool island is unlikely to occur. However, where climate is warmer (so there is less heat loss from buildings) and insolation greater, anthropogenic heat is unlikely to dominate the urban energy balance. In such cases, the combination of factors listed above suggests a possible strategy for reduction of daytime air temperature in dense urban cores. CONCLUSIONS The monitoring experiment in Adelaide demonstrated substantial differences in air temperature between urban street canyons and more exposed open sites. The maximum intensity of the IUHI was observed, on average, shortly before sunrise. Furthermore, in the absence of substantial anthropogenic heat, a daytime urban cool island was noted, the maximum intensity of which typically occurred about 2 h after mid-day. The intensity of both the nocturnal heat island and the daytime cool island show a strong linear correlation to the diurnal range of air temperature at the weather station on the day in question. Since this correlation appears to be independent of seasonal effects, differences in daytime radiant exchange are unlikely to be main cause for both phenomena. It is suggested that since the three-dimensional geometry of the city results in much larger surface area compared with a flat rural site with the same plan area, effective thermal mass is greater in the former, which tends to reduce the diurnal temperature swing. (b) diurnal temp. range at suburban site ( C) Figure 15. Relationship between the diurnal range of air temperature at the suburban station and T c s. (a) The maximum value of the nocturnal IUHI and (b) The minimum daytime intra-urban temperature difference. Negative values represent a canyon cool island; positive values indicate that on the day in question, the urban canyon was always warmer than the suburban site. (ensemble data for June 2, January and March 21). This figure is available in colour online at ACKNOWLEDGEMENTS The authors would like to thank Mr. MacLachlan and Mr. Tormet for allowing meteorological equipment to be installed in their buildings at Chesser and French Streets for the duration of the experiment; the Adelaide City Nursery, which provided a site for the open station; and the City Council of Adelaide for their support of the project. Meteorological data for Kent Town were provided by the Australian Bureau of Meteorology. Helpful comments from anonymous reviewers are acknowledged.

13 INTRA-URBAN DIFFERENCES IN CANOPY LAYER AIR TEMPERATURE 1255 REFERENCES Aida A Urban albedo as a function of the urban structure - a model experiment. Boundary-Layer Meteorology 23: Andersson T, Mathisson I A field test of thermometer screens. Instruments and Observing Methods Reports No. 9 (WMO No. 62). World Meteorological Organization (WMO), Geneva, 36. Arnfield J An approach to the estimation of the surface radiative properties and radiation budgets of cities. Physical Geography 3(2): Arnfield J. 23. Two decades of urban climate research: a review of turbulence, exchanges of energy and water, and the urban heat island. International Journal of Climatology 23: Barring L, Mattsson JO, Lindqvist S Canyon geometry, street temperatures and urban heat island in Malmo, Sweden. Journal of Climatology 5: 33. Chandler TJ The Climate of London. Hutchinson & Co: London. Chimklai P, Hagishima A, Tanimoto J. 2. A computer system to support albedo calculation in urban areas. 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Theoretical and Applied Climatology 63: Ratti C, Raydan D, Steemers K. 23. Building form and environmental performance: archtypes, analysis and an arid climate. Energy and Buildings 35: Runnalls K, Oke TR The urban heat island of Vancouver, BC. Second Urban Environment Symposium. American Meteorological Society: Albuquerque, NM. Runnalls K, Oke TR. 2. Dynamics and controls of the near-surface heat island of Vancouver, British Columbia. Physical Geography 21(): Saaroni H, Ben-Dor E, Bitan A, Potchter O. 2. Spatial distribution and microscale characteristics of the urban heat island in Tel-Aviv, Israel. Landscape and Urban Planning 8: Sailor D, Lu L. 2. A top-down methodology for developing diurnal and seasonal anthropogenic heating profiles for urban areas. Atmospheric Environment 38: Santamouris M The Athens urban climate experiment. PLEA 98: Environmentally Friendly Cities. James & James Science Publishers: Lisbon, Portugal. Santamouris M. 21. Heat island effect. 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