Flow field measurements in the proximity of an urban intersection in London, UK

1 2 3 4 5 Flow field measurements in the proximity of an urban intersection in London, UK A. Dobre a,, S. J. Arnold a,c, R.J. Smalley b, J.W.D. Boddy b, J.F. Barlow a, A.S. Tomlin b and S.E. Belcher a a Department of Meteorology, University of Reading, Reading, RG6 6BB, UK b Energy and Resources Research Institute, University of Leeds, Leeds, LS2 9JT, UK c Department of Environmental Science and Technology, Imperial College, London, SW7 2AZ, UK 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Abstract Observations were made using a roof-top automatic weather station and four ultrasonic anemometers deployed in the proximity of an intersection in London, UK, during the four-week DAPPLE project field campaign in spring 2003. At the intersection the measurements show that the wind direction can switch between the different streets, suggesting that intersections are potent mechanisms for dispersion. Despite the complexity of the building geometry in the vicinity of the intersection, measurements in the adjoining streets indicate that the main large-scale features are along-street channelling and an across-street recirculating vortex, similar to those observed in idealized two-dimensional street canyons. Analysis over a relatively broad range of roof-top wind directions demonstrates that flow within the streets is the vector sum of a channelling and a recirculation vortex. Furthermore, channelling depends linearly on the along-street component of the roof-top reference wind, whilst the cross-street recirculation vortex depends linearly on the component of the roof-top reference wind perpendicular to the street. The results demonstrate that these simple ideas are robust enough to occur in streets of non-ideal geometry and are established a short distance from an intersection. Keywords: urban meteorology, street canyon, street flows, ultrasonic anemometry, DAPPLE 21 22 23 24 25 26 27 1. INTRODUCTION Dispersion in urban areas is determined by the complex transport and mixing processes within the street network, and ventilation into the boundary layer above. In addition, dispersion in urban areas has important practical applications in the prediction and response to accidental or deliberate gas releases within cities, as well as in evaluating proposed strategies for the improvement of urban air quality. Britter and Hanna (2003) provide an excellent review of both experiments and modelling relating to flow and dispersion in urban areas. Corresponding author, Tel +44 118 378 7392, Fax +44 118 378 8905 E-mail: a.dobre@rdg.ac.uk (A. Dobre) 1

28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 The most simplified geometry describing building structures and street configuration in the urban environment is the street canyon. This is characterised by a long straight street bounded by uniform parallel buildings, with a building height to street-width ratio (H/W) of order 1 (Oke, 1988). In this situation the building-street geometry forms an isolated canyon. As reviewed recently by Vardoulakis et al. (2003), field observations (e.g. Nakamura and Oke, 1988; Louka et al., 2000; Longley et al., 2004), wind tunnel measurements (e.g. Rafailidis, 1997; Kastner-Klein and Plate, 1999; Barlow and Belcher, 2002) and numerical modelling (e.g. Hunter et al., 1992; Sini et al., 1996; Cui et al., 2004) combine to reveal the elements of transport and mixing within, and ventilation from, a street canyon. When the flow above roof level is parallel to the street, the wind is efficiently channelled along the street. For flow perpendicular to the street, a shear layer is shed from the upstream building roof and one or more recirculation vortices form in the street canyon. When there are a series of parallel street canyons, such that the urban roughness sublayer does not change significantly from one canyon to the next, it is possible for recirculation vortices to span the street width for H/W > 0.25 (Leonardi et al., 2003). Even for the simplified canyon geometry there is still insufficient understanding of the in-street flow transport and mixing processes when the above-roof wind is oblique to the street. Nakamura and Oke (1988) and Johnson and Hunter (1999) showed that the in-street flow comprises both the along-street channelling and across-street recirculation. In two-dimensional street geometries, they proposed that the above-roof wind is 'reflected' from the building wall surface at low heights within the street canyon. Therefore the horizontal component perpendicular to the street is reflected, and forms a helical-type recirculation vortex. However, it is uncertain whether these concepts can be easily applied to non-idealised geometry in real cities. Wind tunnel measurements of Kastner-Klein and Rotach (2004) indicate that some of these simple ideas do carry over to real geometries, although Longley et al. (2004) show how streets bordered by buildings of different heights affect particularly the recirculation vortex. Additionally, although Boddy et al. (2004) showed that a recirculation vortex can exist for above-roof winds that are oblique to the canyon axis, in a relatively complex street canyon and under certain conditions the vortex can break down even for above-roof winds perpendicular to the canyon. Another simple building-street geometry that characterises an urban area is the street canyon intersection, where two perpendicular street canyons intersect. The wind tunnel measurements of Robins et al. (2002) suggest that street intersections play an important role in dispersing passive scalars, and that even small asymmetries in geometry or wind direction lead to very different dispersion patterns. However, little is known about the flow characteristics within street intersections and how they combine with the above-mentioned street canyon flows. The issues highlighted above are pursued further. 2

58 59 60 61 62 63 64 65 This paper reports measurements made during the DAPPLE campaign of spring 2003, described in section 2. The overall aim is to identify the features of flow transport and mixing processes in real streets of non-uniform geometry and in the vicinity of an intersection. In particular we seek to assess whether results from idealised geometries carry over to complex geometries. With this is mind, section 3 describes the roof-top reference conditions and establishes the basis for analysing the data. Section 4 focuses on the relationship between the instreet wind direction relative to the above-roof wind direction. Flow decomposition and the along-street channelling are discussed in Section 5, while Section 6 considers the across-street recirculation. Section 7 describes a simplified qualitative model of the flow within urban streets. 66 67 68 69 70 71 72 2. EXPERIMENTAL DESCRIPTION Measurements were taken during the four-week DAPPLE project field campaign in the spring of 2003 between 29 April and 22 May. An overview of DAPPLE and a comprehensive description of the complete experimental set-up are presented in Arnold et al. (2004) and on the website www.dapple.org.uk. In summary the DAPPLE field site was in Westminster, London, NW1, at the intersection of Marylebone Road and Gloucester Place. Marylebone Road is approximately 38 m wide and orientated WSW-ENE. Gloucester Place is 20 m wide and 73 intersects Marylebone Road perpendicularly (Figure 1). The buildings adjacent to the intersection are: 74 75 76 77 78 79 80 81 82 83 84 85 86 87 Westminster City Council on the SW corner which is approximately 15 m in height and has a central clock tower; Marathon House on the NW corner is generally 11 m in height and also has an office tower-block section; on the NE corner buildings are approximately 30 m in height; and Bickenhall Mansions on the SE corner is 23 m in height. Within the wider study area, which is defined by a circle of radius 250 m centred on the intersection, all the buildings are less than 40 m in height and there are no uninterrupted street canyons greater than 150 m in length. Three-component velocity data acquired by four in-street ultrasonic anemometers and a roof-top reference automatic weather station are investigated. The ultrasonic anemometers were all 3-axis, Research Grade Gill Scientific Instruments. Two ultrasonic anemometers were deployed at the intersection, at 7 m in height, on two lampposts in the central reservation of Marylebone Road (Sites 1 and 2, Figure 1). These anemometers were sampled at 20 Hz via laptops during daylight hours for weekdays only. Two other ultrasonic anemometers were deployed in the street canyons at least 25 m from the nearest corners of the intersection roads. Site 4 was located on a lamppost on the south pavement of Marylebone Road, 40 m east of the intersection, 7 m from the ground and 15 m from the nearest building wall. This anemometer was sampled at 5 Hz, 24 hours a day, via 3

88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 radio communications between 16 May and 22 May. Site 5 was located on a meteorological mast on the west pavement of Gloucester Place, 25 m south of the intersection at approximately 5 m in height and 2 m from the nearest building wall. Sampling was at 20 Hz via a laptop 24 hours a day. Based on preliminary tests performed at Site 5, the lowest instrument location, the ultrasonic anemometers were above the layer significantly affected by the traffic-produced turbulence, which is generally 3 m in depth (Longley et al., 2004). Roof-top reference conditions were monitored on the Westminster City Council building using an automatic weather station constructed by the School of the Environment, University of Leeds ( REF in Figure. 1, see also section 3). Data collection was via a data logger using a PCMCIA flash card, 24 hours a day, 7 days a week. Mean 30 second averaged wind speeds and horizontal directions were provided by a R.M. Young propeller anemometer and wind vane. The reference automatic weather station was located to the NW of the central clock-tower. It is acknowledged that the station is in the lee of the tower for SE winds. However the prevailing winds on the site are from the west and the station gives representative roof-top conditions for the most common synoptic flows. (Arnold et al., 2004.) Throughout this paper a right-hand Cartesian coordinate system is used, as shown in the inset to Figure 1. The u and v velocity components are aligned along Marylebone Road and Gloucester Place, respectively. Positive u is a wind from WSW to ENE and positive v is a wind from SSE to NNW. The horizontal wind-vector direction is denoted by θ ( θ = tan 1 ( v / u) ). Hereafter, it is implied that all wind directions reported relate to the Cartesian vector direction. For analysis, the data were segregated into one of the four Cartesian quadrants I, II, III and IV. 107 108 109 110 111 112 113 114 115 116 117 3. ROOF-TOP REFERENCE CONDITIONS The probability density function (pdf) of the Westminster City Council roof-top reference wind direction (θ rt ) and angular sector mean wind speeds measured throughout the whole campaign are shown in Figures 2a and 2b. The analysis is applied to the 30 second averaged data. The roof-top wind directions were either in quadrants I 0 0 0 0 ( 0 θ 90 ) or IV ( 90 < θ 0 ) (Figure 1). Moderate wind speeds were recorded during the < rt rt campaign with mean winds of 3.5 m s -1 in quadrant I and lighter mean winds of approximately 2 m s -1 in quadrant IV (Figure 2b). It was observed that three distinct types of days can be identified based on the pdf of the roof-top reference wind direction: day-type A with wind directions in quadrant I for 8 days; day-type B with wind directions in quadrant IV for 3 days; and day-type C with wind directions in both quadrants I and IV for 3 days. Figure 2c shows the 4

118 pdf of wind direction corresponding to the three day-types described above. This classification allows 119 120 investigation of the in-street characteristics based on specific roof-top flow, in particular, the sensitivity of the in-street flow to a change in roof-top wind direction. 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 4. IN-STREET AND INTERSECTION WIND DIRECTIONS 4.1. Statistical analysis To investigate the dependency between the roof-top and in-street wind directions probability density function (pdf) analysis was applied to the in-street horizontal wind directions at Sites 1, 2, 4 and 5 (θ i, i=1, 2, 4 and 5) for the three day-types defined in section 3. The analysis was applied to the instantaneous 20 Hz ultrasonic anemometer data (sites 1,2 and 5) and 5 Hz data at site 4. Note that there is little difference in pdfs when 20Hz data at sites 1, 2 and 5 are resampled at 5Hz since there is little energy between these two frequencies (see also subsection 4.2). Figures 3a, b and c show the corresponding wind direction pdf for day-types A, B and C, respectively. At Site 4 only data in day-type A were measured. For Site 1, in the intersection, a most interesting result is the double peak in the pdf of wind direction for day- 0 0 types A and B. For day-type A, with roof-top reference wind directions in quadrant I ( 0 θ 90 ), the two peaks are at approximately 0 0 and 100 0 (Figure 3a). These may be explained as a result of the in-street flow switching between the east of Marylebone Road and the north of Gloucester Place. The predominant flow represented by the larger of the two pdf peaks is at 100 0 due to a rooftop reference wind direction being closer to 90 0 than 0 0 0 0. For day-type B, with rooftop reference wind directions in quadrant IV ( 90 < θ 0 ), the peaks are at approximately 0 0 and -120 0 (Figure 3b) and can be similarly attributed to flow switching between the east of Marylebone Road and the south of Gloucester Place. In both cases the slightly off-perpendicular wind directions (100 0 and -120 0 ) that correspond to the north and south of Gloucester Place respectively can be attributed to site geometrical complexity as well as to the flow distortion introduced by the circular lamppost as in the classical flow around circular cylinders (Zdravkovich, 1997). Note that when the in-street flow at Site 1 is at ± 90 0, the sonic anemometer and the lamppost are at the same downwind location with respect to the local flow. Considering that the anemometer is placed at around 2.66 lamppost diameters from the lamppost, a deflection of the flow at the anemometer location induced by the lamppost is possible. This phenomenon of deflection is not present at Sites 2 or 4 where the flow is mainly channelled across the Marlylebone Road so that the lamppost always is downwind to the sonic. At site 5 no deflection due to the sonic anemometer holder could be considered since at this site a meteorological mast was used. < rt rt 5

148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 The results for Site 1 in day-type C, with rooftop reference wind directions in both quadrants I and IV, 0 0 ( 90 < θ 90 ) are shown in Figure 3c. There are three local peaks at 90 0, 0 0 and +90 0, a combination of rt the other two day-types. The peaks at 90 0 and +90 0 are much smaller than that at 0 0 signifying greater advection along Marylebone Road compared with Gloucester Place, and therefore less switching between the streets. Thus, the wind switching at Site 1, at the upwind side of the intersection for winds in quadrants I and IV, depends on the precise angle of the roof-top wind vector with the street. This parameter controls the relative ratio of the peaks in the pdf function i.e. the relative time the wind spends in one street compared with the other. Further analysis is needed to clarify the origin of this phenomenon that may play an important role in understanding dispersion at intersections within cities. At Site 2 the pdf-s indicate that the wind vector is always between 90 0 and +90 0 for the three day-types, with the peaks in the pdf-s situated at 0 0 for day-type A and at 60 0 for day-type B. This result suggests that the flow at Site 2 is simply channelled towards the east of Marylebone Road due to its relatively downwind location at the intersection (compared with Site 1), at the beginning of a constraining canyon geometry, for flows from quadrants I and IV. This is unlike Site 1 that is largely unconstrained by buildings for flows from quadrants I and IV due to its relatively upwind location at the intersection and thus winds can travel unimpeded in any direction. The deviation from 0 0 for day-type B could be attributed to the presence of a carpark at the NE corner of the intersection (see Figure 1) that deflects the flow at negative angles into the Marylebone Road. At Site 5 the pdf peaks are at angles that are very sensitive to the day-type. For day-type A, (quadrant I), the peak is at around +90 0, whereas for day-type B (quadrant IV), the peak is at 90 0. This indicates that the flow is channelled along Gloucester Place in a direction determined by the sign of v rt, the component parallel to Gloucester Place. The results for day-type C also support this phenomenon, suggesting that the in-street flows have high sensitivity to changes in the roof-top wind direction. It should be noted that for day-type C the pdf at Site 5 shows a significant number of instantaneous events corresponding to the flow being driven towards the wall (θ 5 180 ). Since the ultrasonic anemometer at Site 5 is the lowest site, z/h = 1/3 (where z and H are measurement and the upwind building height, respectively), and is at the upwind wall of the street for winds from quadrants I and IV, this could be explained by the existence of a cross-street recirculation vortex that brings air from the downwind wall towards the upwind wall (see Section 6). The fact that the presence of the vortex is more significant for day-type C may be explained by the fact that the roof-top reference winds are closer to being perpendicular to Gloucester Place, where Site 5 is located, as they vary between the two quadrants. 6

178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 At Site 4, for day-type A, the peak in the pdf function is at negative angles despite the fact that the roof-top reference angles are positive (Figure 3a). This result can be associated with the reflection type of phenomenon previously found in idealized 2D street canyons (Johnson and Hunter, 1999) and will be discussed in the next section. The in-street angles at Site 4 are always between 90 0 and 0 0 suggesting that the in-street advection is towards the east of Marylebone Road, being driven by the roof- top wind vector along Marylebone Road. 4.2. Time series analysis Investigation of the time series of the in-street wind direction sensitivity relative to the roof-top reference wind direction was undertaken to complement the pdf analysis, which was applied separately to the in-street and roof- top reference winds. An important parameter for this analysis is the averaging period. Fourier-based frequency compensated spectra ( fe ( f ) ), using a filter window of 3 hours, of sufficient length to include all large-scale non-diurnal structures, were applied to all three velocity components measured at the four sites. This analysis was used to investigate the spectral energy distribution and to identify the most energetic scales in the flow. A typical spectrum is shown in Figure 4 and represents the frequency compensated spectrum in the semi-log coordinates of the along-street velocity fluctuation component at site 5. This indicates that a broad spectral maximum exists across scales corresponding to around 1 s to 300 s (i.e. to 3.3 x 10-3 Hz to 1 x 10-1 Hz). These spectral maxima correspond to the most energetic time-scales of the flow (Kaimal and Finnigan, 1994). Using the dimensionless time-scale t * = tu/h these correspond to approximately 1 to 60, where h is the spatially averaged height (h=22 m) and U is the mean roof-top wind speed). It can be inferred that an averaging period of 10 minutes will give a reasonable picture of the dynamics of the largest scales in the flow. Moreover, this also permits comparison with previous results from the literature that used similar, but not explicitly justified, time averaging (e.g. Longley et al., 2004). Also, it is shown in this paper (Section 7) that a time averaging of 10 minutes corresponds to the time scales associated with the cross-street recirculation vortices. Figure 5a illustrates the ten minutes average of the wind direction at Sites 1, 2, 5 and the roof-top reference for a day-type C, with reference wind from both quadrants I and IV. The horizontal line at θ =0 0 provides a visual aid, outlining the border between the two quadrants. The data highlights that at all the sites the in-street wind direction is sensitive to changes in roof-top wind direction. For instance, before 14:00 the roof-top reference wind direction is in quadrant I. During this period the in-street wind directions are: at Site 1 varying between 0 0 and +90 0 ; at Site 2 close to the roof-top reference; and at Site 5 always approximately +90 0. These results are in close agreement with the pdf analysis confirming the channelling effect at Sites 2 and 5. Moreover there is temporal evidence of flow switching at Site 1. 7

208 209 210 211 212 213 214 215 216 217 218 219 220 221 Between 14:00 and 16:30 the roof-top wind direction switches from quadrant I to quadrant IV (corresponding to a change in sign of the roof-top reference direction). When the roof-top reference winds are perpendicular to Gloucester Place the wind direction at Site 5 approaches 180 0 (i.e. reversed cross-street flow towards the upwind wall of the street). This is in close agreement with previous results in idealized 2D street canyons that indicate the presence of a recirculating vortex which brings air from the downwind wall to the upwind wall of the street. After 16:30 the roof-top reference wind direction remains in quadrant IV. The in-street wind directions after this time are: at Site 1, varying between 0 0 and 90 0 degrees; at Site 2, close to the roof-top reference wind direction; again suggesting channelling of the flow towards the east of Marylebone Road; and at Site 5, a major change in horizontal wind direction from +90 0 to 90 0 is recorded. Figure 5b shows a time series the wind direction at the roof-top reference station and at Site 4. The figure shows that the in-street horizontal wind direction approximately equals the negative value of the roof-top wind direction. Based on a scatter plot (not shown), the data are correlated according to θ 4 = -1.3θ rt + 18 0 with an R 2 of 0.68. This provides a motivation to derive a simple model of the flow that relates the above-roof wind direction to that in the street canyon, and is applicable for all wind directions. 222 223 224 225 226 227 228 229 230 5. FLOW AT AN ANGLE TO THE STREET CANYON The analysis of the horizontal wind direction suggests that the flow within the street canyon can be written as a superposition of the along-street channelling and across-street recirculation. This model is schematically summarized in Figure 6. The roof-top reference wind vector is decomposed into two components: one parallel to the street and the second perpendicular to the street. Figure 6 shows canyon wind at two different heights: one just below the plane of the roof-heights (Figure 6a), and the other near the canyon floor (Figure 6b). On dimensional grounds and with the assumption of a infinitely long street (so that there is no vertical motion associated with along-street flow) we can write 231 232 233 u 1 = u rt1 û 1 (x 2 /H, x 3 /H, H/W) u 2 = u rt2 û 2 (x 2 /H, x 3 /H, H/W) u 3 = u rt2 û 3 (x 2 /H, x 3 /H, H/W) (1a) (1b) (1c) 234 235 236 237 where the hatted variables are dimensionless functions of dimensionless variables, which, in general vary with aspect ratio and position in the canyon. The coordinates and velocity components are denoted by subscripts 1,2,3; corresponding to the along-street (channelling), across-street and vertical velocity components respectively. The across-street and vertical velocity components are associated with the recirculation. The 8

238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 generalised notation is used so that equation (1) can be applied to both Site 4 and Site 5, which are perpendicular to each-another. The equations (1) express the linear dependence between the velocity vector within the street on the velocity vector above the roof, so that the channelling depends linearly on the component of the above-roof wind parallel to the street, u rt1, and both the across-street and the vertical components of the recirculation depend linearly on the component of the above-roof wind vector that is perpendicular to the street u rt2. If the component along the street is perfectly uniform (i.e. independent of x 2 /H and x 3 /H), then the decomposition in the equations (1) is dynamically consistent. For real urban flows, the across-street circulation mixes efficiently, so that the channelling along the street is probably nearly uniform over the whole street cross section. To show their validity the three equations in (1) need to be examined with three aspects of the flow. Here we examine the equivalent system of equations, namely the relationship between wind direction in the street to the above roof, the channelling and the recirculation. We first explore the usefulness of the decomposition in the equations (1) by computing the wind direction within the street, θ, given by tan u u uˆ uˆ = (2) 1 rt1 1 1 θ = = tanθrt u2 urt 2uˆ 2 uˆ 2 Notice how the relationship between the in-street wind direction and the above roof wind direction implies θ = θ rt only if u / uˆ = 1, so that an ideal reflection is a special case. For more general values of u ˆ ˆ 1 / u, the 2 ˆ1 2 wind directions have a more complex relationship as suggested by the observations of Nakamura and Oke (1988) and Hunter and Johnson (1999). Figure 7a shows a good comparison of equation 2 with observation at site 5. Similar good agreement was found with observation at site 4. These support the validity of the decomposition in the equations (1a) and (1b). 258 259 260 261 262 263 264 265 266 6. CHANNELING ALONG THE STREET We now perform quantitative analysis to test equation (1a). An averaging period of 1 hour was adopted here; long enough to include all the large energetic scales (see Figure 4) whilst of sufficiently short duration not to include diurnal uncertainties. Figure 7b shows a scatter plot of the 1 hour averaged in-street wind speed component parallel to Gloucester Place for Site 5 (v 5 ) against the magnitude of the roof-top reference wind vector parallel to Gloucester Pl (v rt ). Data in this plot corresponds to day-type A. A linear relationship can be inferred that has a relatively good fit (R 2 =0.78) and normally distributed residuals. The slope of the regression line is 0.35, whilst the intercept is 9

267 268 269 270 271 272 273 0.10, although these precise values depend on the location of the measurements and the canyon geometry. Note that the linear relationship supports equation (1a). A weaker linear relationship (R 2 = 0.47, not shown) is 2 2 0. 5 obtained if the magnitude of the resultant roof-top reference u + v ) is considered responsible for the ( rt rt channelling along the street. Moreover, the low value of the intercept in Figure 7b is evidence that the advection in the street is mainly determined by the roof-top reference component parallel to the street. When the roof-top reference wind vector component parallel to the street vanishes, i.e. there is flow only perpendicular to the street, the large time scale along-street channelling ceases. 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 7. RECIRCULATION ACROSS THE STREET According to equations (1b) and (1c) u rt2 drives the recirculation vortex across the street within idealized 2D street canyons. Figure 7c shows the 1 hour averaged cross-street resultant wind speed, 2 2 0. 5 ( u + w, which is a 5 5 ) measure of the recirculation at Site 5, plotted against the magnitude of the roof-top reference wind vector perpendicular to Gloucester Place (u rt ). Data in this plot corresponds to day-type A. A linear relationship with a reasonable fit (R 2 =0.39) can be noticed. The slope of the regression line is 0.19, whilst the intercept is 0.05, where again these values are determined by the precise location of the measurements and the street geometry. The linear dependency of the two wind speeds supports the decomposition given in equations (1b) and (1c). Notice that the scatter about the linear relationship in the plot decreases with increasing roof-top wind speed, highlighting that there is a greater variability in the velocity of the in-street recirculation vortex during light wind conditions. In fact u 5 linearly correlates with u rt with a better coefficient of fit (R 2 = 0.5) then does w 5 (R 2 = 0.35). An important question regarding to the recirculation vortex would be the level of intermittency present and the time averaging that should be involved to capture the mean recirculation vortex effects. Figure 8 shows the probability density function (pdf) of the mean vertical wind component, w5, at Site 5 for three different averaging periods: 1/20 second, one minute and ten minutes. At one minute the distribution is positively skewed, but negative vertical velocities are still present. Only for ten-minute averages do predominantly positive vertical velocities exist. Since the presence of positive vertical velocities are associated with updraft flow at the upwind wall of the street canyon, determined by the recirculation vortex, it can be inferred that an averaging time of 10 minutes is required to capture the mean vortex. This is in agreement with the time scale of the largest most energetic time scales revealed by the spectral analysis (Figure 4). Moreover, the data presented in Figure 8 is in agreement with the results of Louka et al. (2000) -see their Figure 2b. These authors showed 10

297 298 299 300 301 302 303 304 305 that as the time averaging is increased, the signature of the recirculation vortex appears as it does in the present pdf of the 10 minutes averages. The presence of both positive and negative vertical velocities at time scales of less than 10 minutes in our Figure 8 is in agreement with their proposed assumption of vortex intermittency. Therefore, it can be inferred that the reflection phenomenon at Site 4 (see Figure 5b) is influenced by the recirculation vortex. This vortex reverses flow at the bottom of the street canyon compared to the direction of the roof-top reference wind vector component that is perpendicular to the street. The direction of the roof-top reference direction component parallel to the street is preserved by the channelling effect. These findings strengthen the idea that the large-scale recirculation vortex is a robust feature in urban streets even in the proximity (within 30 m) of intersections. 306 307 308 309 310 311 8. CONCLUSIONS Flow field measurements were taken in the proximity of a London, UK, intersection using four ultrasonic anemometers at heights free of major traffic produced turbulence, and a roof-top reference automatic weather station. A switching of the wind between the streets of the intersection was found at the upstream side of the intersection for oblique roof-top directions. Further investigation is required to identify the cause of this 312 behaviour. However, it represents an important dispersion mechanism within urban areas that must be 313 314 315 316 317 318 319 320 321 322 323 understood if the passage of pollutants is to be more accurately predicted both for everyday pollution episodes and emergency response to accidental and non-accidental releases. Despite the complexity of the intersection geometry the results show that the main features of the flow in the streets are similar to the ones found in idealized 2D street canyons. In the case of approaching oblique roof-top reference wind directions the in-street flows can be explained by a linear superposition of the parallel and perpendicular roof-top wind components. Thus the along-street channelling depends linearly on the component of the above-roof wind parallel to the street, whereas the across-street recirculation vortex depends linearly on the component of the above-roof wind perpendicular to the street. It is shown that the combination of these two effects can give a plausible physical interpretation of the main large-scale features within the urban streets, and explains the relationship between the wind directions within and above the street, as previously noted by Nakamura and Oke (1988). 324 325 ACKNOWLEDGEMENTS 11

326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 Acknowledgements are made to the EPSRC for DAPPLE funding and JIF support to the LANTERN consortium. JWDB and RJS gratefully acknowledge support from NERC. Particular thanks are given to: the other members of the DAPPLE consortium who provided support for the meteorology component within the DAPPLE fieldwork; Steve Neville and the staff at the Westminster Council House for providing a base for the fieldwork activities; Nicola Cheetham and her colleagues at Transport for London and Brian Glynn and associated personnel at Camden contactors for deploying the ultrasonic anemometers on the street furniture on Marylebone Road; the Metropolitan Police Special Events Officer and Transport for London Police for aiding working permissions; the School of the Environment, Leeds University, for lending the automatic weather station deployed as the roof-top reference; Stephen Gill, Andrew Lomas and Ken Spiers at the Department of Meteorology, University of Reading, for providing practical support with the fieldwork; and finally Surbjit Kaur and all the many field workers who helped achieve the data collection. REFERENCES Arnold, S.; ApSimon, H.; Barlow, J.; Belcher, S.; Bell, M.; Boddy, D.; Britter, R.; Cheng, H.; Clark, R.; Colvile, R.; Dimitroulopoulou, S.; Dobre, A.; Greally, B.; Kaur, S.; Knights, A.; Lawton, T.; Makepeace, A.; Martin, D.; Neophytou, M.; Neville, S.; Nieuwenhuijsen, M; Nickless, G.; Price, C.; Robins, A.; Shallcross, D.; Simmonds, P.; Smalley, R.; Tate, J., Tomlin, A.; Wang, H.; Walsh, P., 2004. Dispersion of Air Pollution and Penetration into the Local Environment, DAPPLE, Science of the Total Environment 332, 139-153. Barlow, J.F., Belcher, S.E., 2002. The rate of exchange of passive scalar between streets and the boundary layer aloft. Boundary-Layer Meteorology 104, 131-150. Boddy, J.W.D., Smalley, R.J., Dixon, N.S., Tate, J.E., Tomlin, A.S., The special variability in concentrations of a traffic-related pollutant in two street canyons in York, U.K.-Part I: The influence of background winds. Submitted to Atmospheric Environment. Britter, R.E., Hanna, S.R., 2003. Flow and dispersion in urban areas. Annual Review of Fluid Mechanics 35, 469-496 Cui, Z. Q., Cai, X. M., Baker, C. J., 2004. Large-eddy simulation of turbulent flow in a street canyon. Quarterly Journal of the Royal Meteorological Society 130, 1373-1394. Hunter, L.J., Johnson, G.T., Watson, I.D., 1992. An investigation of three-dimensional characteristics of flow regimes within an urban canyon. Atmospheric Environment 26B, 425-432. Johnson, G.T., Hunter, L.J., 1999. Some insights into typical urban canyons airflows, Atmospheric Environment 33, 3991-3999. Kaimal, J. C., Finnigan, J. J. 1994. Atmospheric boundary layer flows; their structure and measurement, pp. 37-39. Oxford University Press, Oxford, UK. Kastner-Klein, P., Plate, E.J., 1999. Wind-tunnel study of concentration fields in street canyons. Atmospheric Environment 33, 3973-3979. Kastner-Klein, P., Rotach, M.W., 2004. Mean flow and turbulence characteristics in an urban roughness sublayer. Boundary-Layer Meteorology 111, 55-84. Leonardi, S., Orlandi, P., Smalley, R.J., Djenidi, L.Antonia, R.A., 2003. Direct numerical simulations of turbulent channel flow with transverse square bars on one wall. Journal of Fluid Mechanics 491, 229 238. Longley, I.D., Gallagher, M.W., Dorsey, J.R., Flynn, M., Barlow, J.F., 2004. Short- term measurements of airflow and turbulence in two street canyons in Manchester. Atmospheric Environment 38, 69-79. 12

366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 Louka, P., Belcher, S.E., Harrison, R.G., 2000. The coupling between air flow in-streets and the well- developed boundary layer aloft, Atmospheric Environment 34, 2613-2621. Nakamura, Y., Oke, T.R., 1988. Wind, temperature and stability conditions in an east-west oriented urban canyon. Atmospheric Environment 22, 2691-2700. Oke, T.R., 1988. Street design and the urban canopy layer climate. Energy and Buildings 11, 103-113. Rafailidis, S., 1997. Influence of building area density and roof shape on the wind characteristics above a town. Boundary Layer Meteorology 85, 255-271. Robins, A., Savory, E., Scaperdas, A., Grigoriadis, D., 2002. Spatial variability and source- receptor relations at a street intersection. Water, Air, and Soil Pollution, Focus 2, 381-393. Sini, J.F., Anquetin, S., Metsayer, P.G., 1996. Pollutant dispersion and thermal effects in urban street canyons. Atmospheric Environment 30, 2659-2677. Vardoulakis, S., Fisher, B.E.A., Pericleous, K., Gonzalez-Flesca, N., 2003. Modelling air quality in-street canyons: a review. Atmospheric Environment 37, 155-182. Zdravkovich, M.M., 1997: Flow around circular cylinders, pp 3-4, Oxford University Press, UK 13

380 381 FIGURE CAPTIONS 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 1. Experimental set-up of meteorological instruments and site layout during the DAPPLE campaign. 2. Pdf of the horizontal wind direction and the mean horizontal wind-speed distribution at the roof-top reference anemometer: a) Pdf of horizontal wind direction for the duration of the DAPPLE campaign. b) The sector-averaged mean wind speed distribution. c) Pdfs (arbitrary units) of the horizontal wind direction for day-types: A, B and C. 3. Pdf (arbitary units) of the in-street horizontal wind direction. +, Site 1;,Site 2; O, Site 4;, Site 5. a) Day-type A. b) Day-type B. c) Day-type C. 4. Frequency compensated spectrum (f.e(f)) of the along-street wind velocity at Site 5 (w 5 ). 5. Wind vector direction time series. a) Day-type C at the above-roof reference (solid line) and at: +, Site 1;,Site 2;, Site 5 b) Day-type A at the above-roof reference (solid line) and at: O, Site 4. 6. Schematic representation of a simplified qualitative model of the flow within urban streets near the building edge for a) near the roof level (dotted vector); and b) near the canyon floor, (dashed vector). A bold vector represents the above-roof reference velocity. 7. Roof-top versus in-street wind speed dependency at Site 5: a) in-street wind angle θ5 versus roof-top wind angle θ rt :, field data;. equation (2) b) in-street axial wind vector component versus roof-top reference wind vector component parallel to the Gloucester Place c) cross-street resultant wind vector at Site 5 versus roof-top reference wind vector component perpendicular to the Gloucester Place 8. The pdf of the vertical velocities at Site 5 for a day-type C for different averaging times: ---,1/20s; solid line, 60s; -.-, 600s. 14

Figure 1 Figure 2a, b and c 15

Figure 3a,b and c Figure 4 16

Figure 5a and b a) u 1 u 2 + = b) u 1 u 2 + = u rt Figure 6a and b u 3 17

Figure 7a, b and c Figure 8 18