Relating street level flows to BT Tower level flows: results from the DAPPLE
2004 campaignJ. Barlow1, A. Dobre1, R. Smalley2, S. Arnold1, A. Tomlin2, S.
Belcher1
1Department of Meteorology, University of Reading, UK2Energy and Resources Research Institute, University of
Leeds, UK
• Street canyon flow
Street level flowsStreet 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/UH
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/UR
z/H
1
1stable• 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 (3rd June)
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°)
WCC
LIB BT
WCC ref
WCC
LIB BT
LIB ref
WCC
LIB BT
Heights of measurement
Uninterrupted flow
Flow influenced bybuildings
ZWCC, LIB = 17m
H= 21 m
zBT = 190 m
Z ~ 2-3H
Z = H
Z ~ 9H
Z ~ 0.2H
Evaluating reference sites
Turbulenceintensity
Local flow direction
• BT Tower: circlesSmall, approx. constant• LIB: trianglesPeaks associated with wakes
Windspeed and direction
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10 12 14 16
Ubt
Ulib
/Ub
t
• 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:
u1=aur1
u2=bur2
In-street wind direction:
tan u2u1 = b/a tanr
(Dobre et al. (2005), Atmospheric Environment, 39(26), 4647-4657 )
θr
ur1
ur2
θu1
u2
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 [email protected]
.uk
Stable “decoupling” of turbulence 2nd May ‘04
-0.5
0
0.5
1
29/4/04 0:00 30/4/04 0:00 1/5/04 0:00 2/5/04 0:00 3/5/04 0:00 4/5/04 0:00
time
norm
alis
ed
turb
ulen
t ki
netic
ene
rgy
e/
U^2
0.5
1
1.5
2
2.5
3
3.5
4
ratio
of
pote
ntia
l son
ic t
em
pera
ture
s,
BT
/roo
f
BT e/U^2
Lib e/U^2
ratio temp
Correlation street level TKE with UBT
2 = 0.24 (whole campaign) cf. 0.14 for ULIB
2
Correlation street level TKE with UBT
2 = 0.14 (overnight)
0
5
10
15
20
25
30
29/4/04 0:00 30/4/04 0:00 1/5/04 0:00 2/5/04 0:00 3/5/04 0:00 4/5/04 0:00
time
soni
c te
mpe
ratu
re (
degC
)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
stan
dard
dev
iatio
n so
nic
T (
deg
C)
BT Ts
Lib Ts
BT sigT
Lib sigT
Bulk Richardson number >0.25, stable conditions
Stable “decoupling” of turbulence 2nd May ‘04
Dobre et al. model for different sites
Site 3 z ~0.3H Site 4 z ~0.3H Site 11 z ~0.1H
WCCLIB BT