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Relating street level flows to BT Tower level flows : results from the DAPPLE 2004 campaign

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.

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Relating street level flows to BT Tower level flows : results from the DAPPLE 2004 campaign

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  1. Relating street level flows to BT Tower level flows: results from the DAPPLE 2004 campaign J. Barlow1, A. Dobre1, R. Smalley2, S. Arnold1, A. Tomlin2, S. Belcher1 1Department of Meteorology, University of Reading, UK 2Energy and Resources Research Institute, University of Leeds, UK

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

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

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

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

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

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

  8. WCC BT LIB WCC ref

  9. WCC BT LIB LIB ref

  10. Uninterrupted flow Flow influenced by buildings Heights of measurement zBT = 190 m Z ~ 9H Z ~ 2-3H H= 21 m Z = H ZWCC, LIB = 17m Z ~ 0.2H

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

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

  13. Vector decomposition model Roof top wind = channelled + perpendicular In-street wind components: u1=aur1 u2=bur2 In-street wind direction: tan q = u2 /u1= b/a tan qr ur1 θr ur2 u2 θ u1 (Dobre et al. (2005), Atmospheric Environment, 39(26), 4647-4657 )

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

  15. 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 j.f.barlow@reading.ac.uk

  16. Stable “decoupling” of turbulence 2nd May ‘04 Correlation street level TKE with UBT2 = 0.14 (overnight) Correlation street level TKE with UBT2 = 0.24 (whole campaign) cf. 0.14 for ULIB2

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

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

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