Relating street level flows to BT Tower level flows: results from the DAPPLE 2004 campaign J. Barlow 1, A. Dobre 1, R. Smalley 2, S. Arnold 1, A. Tomlin 2, S. Belcher 1 1 Department of Meteorology, University of Reading, UK 2 Energy and Resources Research Institute, University of Leeds, UK

Street canyon flow Street level flows Street canyon, aspect ratio H/W=0.6 Perpendicular flow

Flow “rectification” means that street level flow pattern is very sensitive to outer flow direction Oblique flows

Flow “rectification” means that street level flow pattern is very sensitive to outer flow direction Oblique flows For accurate prediction of street level flow or dispersion  Need suitable reference measurement

Vegetation canopies: use windspeed or friction velocity at canopy top Choice of reference U/U H z/H 1 1 Urban canopies: Roof-top reference practical BUT local obstructions cause wakes, limited representativity

Vegetation canopies: use windspeed or friction velocity at canopy top Urban canopies: Roof-top reference practical BUT local obstructions cause wakes, limited representativity TODAY: Results from DAPPLE 2004 campaign in London, referencing street level flow Compare roof-top and upper level references Choice of reference U/U R z/H 1 1 stable Higher reference “cleaner” BUT stability can affect flow  Klein and Clark (2007) Oklahoma City, stable conditions, frequent nocturnal jets better to use rooftop ref as higher ref in “decoupled” flow

DAPPLE 2004 Field Campaign Equipment: 11 3D ultrasonic anemometers Qinetiq Zephir Doppler lidar (3 rd June) Campaign duration: 19 th April to 13 th June 2004 Site: mean building height 21m (radius of 200m) plan area index λ P ~ 0.5 frontal area index λ F ~ 0.2 (bearing ~240°) WCC LIB BT

WCC ref WCC LIB BT

LIB ref WCC LIB BT

Heights of measurement Uninterrupted flow Flow influenced by buildings Z WCC, LIB = 17m H= 21 m z BT = 190 m Z ~ 2-3H Z = H Z ~ 9H Z ~ 0.2H

Evaluating reference sites Turbulence intensity Local flow direction BT Tower: circles Small, approx. constant LIB: triangles Peaks associated with wakes

Windspeed and direction Windspeed ratio: LIB near neutral limit 0.23 Direction: LIB better correlation with BT ref

Vector decomposition model Roof top wind = channelled + perpendicular In-street wind components: u 1 =au r1 u 2 =bu r2 In-street wind direction: tan  u 2  u 1 = b/a tan  r (Dobre et al. (2005), Atmospheric Environment, 39(26), ) θrθr u r1 u r2 θ u1u1 u2u2

Evaluation using Dobre et al. model BT Tower: best fit of predicted direction to data WCC ref LIB ref

Conclusions Mean flow pattern in street is most closely related to upper level reference on BT Tower (z ~ 9H)  Develop BT Tower as centralised reference in London (5 year long ACTUAL project) Occasional “decoupling” events when stable overnight or in low wind periods Stable layers not common for London (<1% of 6 weeks) therefore upper level reference more representative than for Oklahoma / Klein and Clark 2007  Overnight urban stability depends on regional scale forcing, not just local urban energy balance

Stable “decoupling” of turbulence 2 nd May ‘04 Correlation street level TKE with U BT 2 = 0.24 (whole campaign) cf for U LIB 2 Correlation street level TKE with U BT 2 = 0.14 (overnight)

Bulk Richardson number >0.25, stable conditions Stable “decoupling” of turbulence 2 nd May ‘04

Dobre et al. model for different sites Site 3 z ~0.3HSite 4 z ~0.3H Site 11 z ~0.1H WCC LIB BT