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

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Atmospheric Environment 39 (2005) 4647–4657 www.elsevier.com/locate/atmosenv

Flow field measurements in the proximity of an urban intersection in London, UK A. Dobrea,, S.J. Arnolda,c, R.J. Smalleyb, J.W.D. Boddyb, J.F. Barlowa, A.S. Tomlinb, S.E. Belchera a Department of Meteorology, University of Reading, Reading RG6 6BB, UK 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 b

Received 25 January 2005; received in revised form 6 April 2005; accepted 12 April 2005

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 4-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 idealised 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 alongstreet 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. r 2005 Elsevier Ltd. All rights reserved. Keywords: Urban meteorology; Street canyon; Street flows; Ultrasonic anemometry; DAPPLE

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 Corresponding author. Tel.: +44 118 378 7392; fax: +44 118 378 8905. E-mail address: [email protected] (A. Dobre).

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. 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 ðH=W Þ 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

1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.04.015

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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 40:25 (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 (2D) 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. (2005) 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. 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 nonuniform 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 in-street 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.

2. Experimental description Measurements were taken during the 4-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 intersects Marylebone Road perpendicularly (Fig. 1). The buildings adjacent to the intersection are: 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 approximately 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 instreet ultrasonic anemometers and a roof-top reference automatic weather station are investigated. The ultrasonic anemometers were all three-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, Fig. 1). These anemometers were sampled at 20 Hz via laptops during daylight hours on 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

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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 Fig. 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 y ðy ¼ tan1 ðv=uÞÞ. 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–IV.

3. Roof-top reference conditions

Fig. 1. Experimental set-up of meteorological instruments and site layout during the DAPPLE campaign.

day, via radio communications between 16 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 Fig. 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 s averaged wind speeds and horizontal directions were provided by an 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

The probability density function (pdf) of the Westminster City Council roof-top reference wind direction ðyrt Þ and angular sector mean wind speeds measured throughout the whole campaign are shown in Figs. 2a and b. The analysis is applied to the 30 s averaged data. The roof-top wind directions were either in quadrants I ð0 oyrt p90 Þ or IV ð90 oyrt p0 Þ (Fig. 1). Moderate wind speeds were recorded during the campaign with mean winds of 3.5 m s1 in quadrant I and lighter mean winds of approximately 2 m s1 in quadrant IV (Fig. 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 eight days; day-type B with wind directions in quadrant IV for three days; and day-type C with wind directions in both quadrants I and IV for three days. Fig. 2c shows the pdf of wind direction corresponding to the three day-types described above. This classification allows 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.

4. In-street and intersection wind directions 4.1. Statistical analysis To investigate the dependency between the roof-top and in-street wind directions, pdf analysis was applied to the in-street horizontal wind directions at Sites 1, 2, 4 and 5 ðyi , i ¼ 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 pdf’s when 20 Hz data at Sites 1, 2 and 5 are resampled at 5 Hz since there is little energy between these two frequencies (see also subsection 4.2). Fig. 3 shows the corresponding wind direction pdf for

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100

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horizontal wind direction, θrt (deg)

Fig. 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) Sector-averaged mean wind speed distribution. (c) Pdfs (arbitrary units) of the horizontal wind direction for day-types A, B and C.

day-types A (Figs. 3a and b), B (Figs. 3c and d) and C (Figs. 3e and f). At Site 4, only data in day-type A were measured. For Site 1, in the intersection, the most interesting result is the double peak in the pdf of wind direction for day-types A and B. For day-type A, with roof-top reference wind directions in quadrant I ð0 oyrt p90 Þ, the two peaks are at approximately 01 and 1001 (Fig. 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 1001 due to a roof-top reference wind direction being closer to 901 than 01. For day-type B, with roof-top reference wind directions in quadrant IV ð90 o yrt p0 Þ, the peaks are at approximately 01 and 1201 (Fig. 3c) 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 (1001 and 1201) that

correspond to the north and south of Gloucester Place, respectively, can be attributed to the 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 7901, 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 Site 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. The results for Site 1 in day-type C, with rooftop reference wind directions in both quadrants I and IV, ð90 oyrt p90 Þ are shown in Fig. 3e. There are three

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Fig. 3. Pdf (arbitary units) of the in-street horizontal wind direction. +, Site 1; &, Site 2; J, Site 4; }, Site 5. (a) and (b) day-type A; (c) and (d) day-type B; (e) and (f) day-type C.

local peaks at 901, 01 and +901, a combination of the other two day-types. The peaks at 901 and +901 are much smaller than that at 01 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, 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 indicates that the wind vector is always between 901 and +901 for the three day-types, with the peaks in the pdf situated at 01 for day-type A (see Fig. 3a) and at 601 for day-type B (see Fig. 3c). 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 01 for day-type B could be attributed to the presence of a carpark at the NE corner of the intersection (see Fig. 1) that deflects the flow at negative angles into the Marylebone Road. At Site 4, for day-type A, the peak in the pdf is at negative angles despite the fact that the roof-top reference angles are positive (Fig. 3b). This result can be associated with the ‘reflection’ type of phenomenon previously found in idealised 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 901 and 01 suggesting that the in-street advection is towards the east of Marylebone Road, being driven by the roof-top wind vector along Marylebone Road.

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4.2. Time series analysis

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 +901 (see Fig. 3b), whereas for daytype B (quadrant IV), the peak is at 901 (see Fig. 3d). This indicates that the flow is channelled along Gloucester Place in a direction determined by the sign of vrt , 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 (see Fig. 3f) shows a significant number of instantaneous events corresponding to the flow being driven towards the wall ðy5  1801Þ. Since the ultrasonic anemometer at Site 5 is the lowest site, z/H ¼ 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 rooftop reference winds are closer to being perpendicular to Gloucester Place, where Site 5 is located, as they vary between the two quadrants.

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 ðfEðf ÞÞ, using a filter window of 3 h, of sufficient length to include all largescale 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 Fig. 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 1–300 s (i.e. 3.3  103 to 1  101 Hz). Note that in dimensionless time scale t ¼ tU=h, these correspond to approximately 1–60, where h is the spatially averaged height ðh ¼ 22 mÞ and U is the mean roof-top wind speed. Since the spectral maxima correspond to the most energetic time scales of the flow

0.35

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Fig. 4. Frequency-compensated spectrum ðf :Eðf ÞÞ of the along-street wind velocity at Site 5 ðv5 Þ.

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2 and 5. Moreover there is temporal evidence of flow switching at Site 1. 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 1801 (i.e. reversed cross-street flow towards the upwind wall of the street). This is in close agreement with previous results in idealised 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 01 and 901 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 +901 to 901 is recorded. Fig. 5b shows a time series of the wind direction at the roof-top reference station and at Site 4. The figure shows that the in-street horizontal wind direction approxi-

(Kaimal and Finnigan, 1994), it can be inferred that an averaging period of 10 min 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 min corresponds to the time scales associated with the cross-street recirculation vortices. Fig. 5a illustrates the 10 min average of the wind direction at Sites 1, 2 and 5 and the roof-top reference for a day-type C, with reference wind from both quadrants I and IV. The horizontal line at y ¼ 01 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 01 and +901; at Site 2, close to the roof-top reference; and at Site 5, always approximately +901. These results are in close agreement with the pdf analysis confirming the channelling effect at Sites

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Fig. 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: J, Site 4.

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mately equals the negative value of the roof-top wind direction. Based on a scatter plot (not shown), the data are correlated according to y4 ¼ 1:3yrt þ 181 with an R2 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.

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 acrossstreet recirculation. This model is schematically summarised in Fig. 6. The roof-top reference wind vector is decomposed into two components: one parallel to the street, and the second perpendicular to the street. Fig. 6 shows the canyon wind at two different heights: one just below the plane of the roof-heights (Fig. 6a), and the other near the canyon floor (Fig. 6b). On dimensional grounds and with the assumption of an infinitely long street (so that there is no vertical motion associated with along-street flow), we can write u1 ¼ urt1 u^ 1 ðx2 =H; x3 =H; H=W Þ

(1a)

u2 ¼ urt2 u^ 2 ðx2 =H; x3 =H; H=W Þ

(1b)

u3 ¼ urt2 u^ 3 ðx2 =H; x3 =H; H=W Þ

(1c)

where the hatted variables are dimensionless functions of dimensionless variables, which, in general, vary with aspect ratio and position in the canyon. The coordinates u1

u2

+

=

(a)

u rt u

1

u2

+

=

u3

(b) Fig. 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.

and velocity components are denoted by subscripts 1, 2 and 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 generalised notation is used so that Eq. (1) can be applied to both Sites 4 and 5 which are perpendicular to each other. Eqs. (1) express the linear dependence between the velocity vector within the street and the velocity vector above the roof, so that the channelling depends linearly on the component of the above-roof wind parallel to the street, urt1 , 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, urt2 . If the component along the street is perfectly uniform (i.e. independent of x2 =H and x3 =H), then the decomposition in Eqs. (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 Eqs. (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 Eqs. (1) by computing the wind direction within the street, y, given by tan y ¼

u1 urt1 u^ 1 u^ 1 ¼ ¼ tan yrt u2 urt2 u^ 2 u^ 2

(2)

Note how the relationship between the in-street wind direction and the above-roof wind direction implies y ¼ yrt only if u^ 1 =u^ 2 ¼ 1, so that an ideal reflection is a special case. For more general values of u^ 1 =u^ 2 , the wind directions have a more complex relationship as suggested by the observations of Nakamura and Oke (1988) and Johnson and Hunter (1999). Fig. 7a shows a good comparison of Eq. (2) with observation at Site 5. Similar good agreement was found with observation at Site 4. These support the validity of the decomposition in Eqs. (1a) and (1b).

6. Channelling along the street We now perform quantitative analysis to test Eq. (1a). An averaging period of 1 h was adopted here; long enough to include all the large energetic scales (see Fig. 4) whilst of a sufficiently short duration to not include diurnal uncertainties. Fig. 7b shows a scatter plot of the 1 h averaged instreet wind speed component parallel to Gloucester Place for Site 5 ðv5 Þagainst the magnitude of the roof-top

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Fig. 7. Roof-top versus in-street wind speed dependency at Site 5: (a) In-street wind angle y5 versus roof-top wind angle yrt : }, field data; , Eq. (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.

reference wind vector parallel to Gloucester Place ðvrt Þ. Data in this plot correspond to day-type A. A linear relationship can be inferred that has a relatively good fit ðR2 ¼ 0:78Þ and normally distributed residuals. The slope of the regression line is 0.35, whilst the intercept is 0.10, although these precise values depend on the location of the measurements and the canyon geometry. Note that the linear relationship supports Eq. (1a). A weaker linear relationship ðR2 ¼ 0:47, not shown) is obtained if the magnitude of the resultant roof-top reference ðu2rt þ v2rt Þ0:5 is considered responsible for the channelling along the street. Moreover, the low value of the intercept in Fig. 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.

7. Recirculation across the street According to Eqs. (1b) and (1c) urt2 drives the recirculation vortex across the street within idealised 2D street canyons. Fig. 7c shows the 1 h averaged crossstreet resultant wind speed, ðu25 þ w25 Þ0:5 , which is a measure of the recirculation at Site 5, plotted against the magnitude of the roof-top reference wind vector perpendicular to Gloucester Place ðurt Þ. Data in this plot correspond to day-type A. A linear relationship with a reasonable fit ðR2 ¼ 0:39Þ 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 Eqs. (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

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velocity of the in-street recirculation vortex during light wind conditions. In fact, u5 linearly correlates with urt with a better coefficient of fit ðR2 ¼ 0:5Þ then does w5 ðR2 ¼ 0:35Þ. An important question regarding 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. Fig. 8 shows the pdf of the mean vertical wind component, w5 , at Site 5 for three different averaging periods: 1/20 s, 1 and 10 min. At 1 min, the distribution is positively skewed but negative vertical velocities are still present. Only for 10 min 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 min 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 (Fig. 4). Moreover, the data presented in Fig. 8 are in agreement with the results of Louka et al. (2000), see their Fig. 2b. These authors showed that as the time averaging is increased, the signature of the recirculation vortex appears as it does in the present pdf of the 10 min averages. The presence of both positive and negative vertical velocities at time scales of less than 10 min in Fig. 8 is in agreement with their proposed assumption of vortex intermittency. Therefore, it can be inferred that the ‘reflection’ phenomenon at Site 4 (see Fig. 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

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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.

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 behaviour. However, it represents an important dispersion mechanism within urban areas that must be understood if the passage of pollutants is to be more accurately predicted both for everyday pollution episodes and emergency response to accidental and nonaccidental 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 idealised 2D street canyons. In the case of approaching oblique rooftop 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).

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Acknowledgements

pdf

0.04 0.03 0.02 0.01 0

-6

-4

-2

0

2

4

6

8

-1

w5 (m s )

Fig. 8. Pdf of the vertical velocities at Site 5 for a day-type C for different averaging times: –, 1/20 s; solid line, 60 s; - -, 600 s.

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

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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.

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