Velocity Adjustment and Passive Scalar Diffusion in and Above an Urban Canopy in Response to Various Approach Flows

• Published in: Boundary-layer meteorology Vol. 141; no. 3; pp. 415 - 441
• Main Authors: Kanda, Isao, Yamao, Yukio
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-12-2011; Springer; Springer Nature B.V
• Subjects: Arrays; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Boundaries; Boundary layer; Canopies; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Diffusion; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Exact sciences and technology; External geophysics; Flow velocity; Meteorology; Plumes; Roughness; Scalars; Urban areas; Wind tunnels; Wind tunnel; Adjustment; Diffusion; Roughness change; Urban canopy; models; plumes; wind tunnel; Velocity distribution; Urban atmosphere; Fetch; Drag; roughness; growth; Height; Canopy(vegetation)
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1007/s10546-011-9646-9

We used wind-tunnel experiments to investigate velocity-field adjustment and scalar diffusion behaviour in and above urban canopies located downwind of various roughness elements. Staggered arrays of rectangular blocks of various heights
H
and plan area ratios λ
p
were used to model the urban canopies. The velocity field in the roughness sublayer (height
) reached equilibrium at distances proportional to
where
L
c
is the canopy-drag length scale determined as a function of λ
p
and the block side length
L
. A distance of about
was required for adjustment at
z
 = 
H
/2 (in the canopy), and a distance of about
was required at
z
 = 2
H
(near the top of the roughness sublayer). Diffusion experiments from a ground emission source revealed that differences in upwind roughness conditions had negligible effects on the plume growth near the source (up to a few multiples of
L
from the source) if the source was located at a fetch
F
larger than about
from the upwind edge of the canopy. However, at locations farther downwind (more than several multiples of
L
from the source), upwind conditions had considerable effects on the plume growth. For a representative urban canopy, it was shown that a much larger fetch than required for velocity-field adjustment in the roughness sublayer was necessary to eliminate the effects of upwind conditions on plume widths at 24
L
downwind from the source.